since the herbicide glyphosate ( n-(phosphonomethyl)glycine ) 
 was commercialized in 1974 , it has become the most widely used herbicide 
 in the world , due largely to the wide scale adoption of transgenic , 
 glyphosate - resistant ( gr ) crops after their introduction in 1996 ( figure 1 ) . in gr crops , this relatively high use rate herbicide 
 ( commonly 0.5 to 2.0 kg / ha / application ) is often used multiple times 
 in a growing season . use of other herbicides declined steadily , while 
 glyphosate use increased in the three major gr crops ( figure 2 ) . the increasing incidence of evolved , gr weeds , as well as weed shifts to naturally glyphosate - tolerant 
 weed species , has resulted in increased 
 use rates and numbers of applications of glyphosate , as well as other 
 herbicides , per growing season in gr crops . since its introduction , 
 glyphosate has been considered a toxicologically and environmentally 
 safe pesticide , due to its low mammalian toxicity , relatively short 
 environmental half - life , and extremely low activity in soil due to 
 its binding to soil minerals ( reviewed by duke et al . ) . furthermore , only green plants , some fungi , and a limited 
 number of microorganisms have the target site , 5-enolpyruvylshikimic 
 acid-3-phosphate synthase ( epsps ) , of the herbicide . epsps is an enzyme 
 required for synthesis of the essential aromatic amino acids phenylalanine , 
 tyrosine , and tryptophan . adoption of the three most widely grown , 
 herbicide - resistant 
 ( hr ) crops in the united states . . data for each crop category include varieties with 
 both hr as a single and stacked trait with insect resistance . data for 20002012 are available in the 
 usda , economic research service data product , adoption of 
 genetically engineered crops in the u.s . note that adoption data for 19961999 
 include hr corn and soybeans obtained using traditional breeding methods 
 ( not transgenic ) . the more recent data ( 20002011 ) closed circles 
 = all herbicides 
 minus glyphosate , open circles = glyphosate only . department of agriculture , national agricultural statistics service 
 data and statistics site http://www.nass.usda.gov/data_and_statistics/quick_stats/ ( accessed september 12 , 2012 ) . glyphosate is a 
 nonselective herbicide , that is , it can kill all plant species , although 
 there is variation between species with regard to levels of natural 
 tolerance . glyphosate has little or no herbicidal activity in soil 
 and , thus , is used only with foliar spray applications . due to crop 
 sensitivity , its use was limited in crop production prior to the introduction 
 of gr crops , after which its use greatly expanded with the widespread 
 adoption of these crops worldwide . during its several decades 
 of use over vast areas , no significant 
 adverse secondary effects of the herbicide have been established , 
 other than the intense selection pressure that has resulted in the 
 evolution of gr weeds . in fact , its use in gr crops has been associated 
 with several environmental benefits . the topic of 
 evolution of gr weeds has been dealt with in detail in many research 
 papers and reviews . several papers have been 
 published recently that conclude that 
 glyphosate adversely affects mineral nutrition in gr crops , leading 
 to several adverse effects , including increased plant disease . others have indicated that gr crops 
 are more susceptible to plant diseases due to other mechanisms . this 
 review addresses these concerns in the context of the available literature 
 on glyphosate ( ca . 8000 peer - reviewed papers according to scifinder ) . in order to understand possible effects of glyphosate on mineral 
 nutrition of plants , it is necessary to understand the processes that 
 affect glyphosate in soil . it is also necessary to understand how 
 glyphosate interacts with minerals in soil and with soil microorganisms . once glyphosate interacts 
 with soil , whether applied directly to the soil surface , exuded from 
 a plant root , or released from decomposing plant tissue , it is subject 
 to various processes that control its environmental behavior and fate , 
 including retention ( sorption  desorption ) , transport , and degradation . 
 of these processes , sorption is arguably the most important as it 
 controls the availability for degradation , plant uptake , and offsite 
 transport . sorption of glyphosate to soil has been extensively reviewed . because glyphosate is a small polyprotic molecule ( pka1 = 2.27 , pka2 = 5.58 , pka3 = 10.25 ) with three polar functional groups and can 
 be sorbed on minerals and organic matter , its sorption on soil as 
 a whole is generally much greater compared to other pesticides , which 
 are larger molecules with fewer functional groups and are primarily 
 sorbed onto organic matter . glyphosate is primarily sorbed on 
 variable - charge surfaces such as iron and aluminum oxides , aluminum 
 silicates ( allophone and imogolite ) , and goethite ( -feooh ) , 
 and to a lesser extent on the fe - oxide coatings of permanent charge 
 minerals ( illite , smectite , and vermiculite ) and organic matter . the 
 primary mechanisms responsible for sorption are ligand exchange and 
 complex formation with mineral oxide surfaces . the magnitude of sorption 
 increases with increased surface area of the minerals and decreased 
 ph . the sorption onto the sorbent surface is fast initially for most 
 of the glyphosate added to soil , which is then followed by slower 
 sorption . further details of these processes can be found in the above 
 cited reviews and the references cited therein . the magnitude 
 of sorption has traditionally been characterized 
 as the ratio of glyphosate bound to soil to that in solution for a 
 single concentration ( kd ) or at multiple 
 concentrations ( kf and 1/n values from the freundlich sorption isotherm ) . sorption coefficients 
 are often expressed on a soil organic carbon basis ( koc , kfoc ) to normalize values 
 between different soils . , regardless of soil properties in a cultivated prairie , glyphosate 
 mean kd was 108 to133 l kg ( koc = 10  90014  900 
 l kg ) , depending on landscape position , and these 
 values were 100 greater than those for the commonly used herbicide 
 2,4-d . in a study of 20 different soils , kd ranged from 41 to 303 l kg with a median value of 97 l kg . in column leaching experiments , coarse textured soils retained 
 most all ( 8595% ) of the glyphosate applied despite the fact 
 that higher than agronomic rates , 7.414.8 mg kg , of glyphosate were used . depending on soil type , glyphosate is weakly 
 desorbed , that is , 524% of initially sorbed glyphosate . the strong adsorption of glyphosate to most 
 soils , and its low desorbability , leaves little glyphosate in soil 
 solution available for microbial degradation , interaction with trace 
 metal cations , plant uptake , or offsite transport . sorbed glyphosate 
 has been postulated to be released into soil 
 solution upon addition of phosphate ( po4 ) , which can compete with glyphosate for sorption sites on soil . however , it appears there is only limited competition 
 for sorption sites between glyphosate and po4 , even when much higher than agronomic rates of glyphosate are applied . for soils , competitive sorption studies between 
 glyphosate and po4 showed that displacement 
 of glyphosate by po4 was related to 
 the amount of clays , cec , and ph , but glyphosate was not easily displaced 
 by po4 from the clays . even when sorption competition has been shown to occur , 
 glyphosate still remains strongly sorbed . for example , increasing 
 extractable p by a factor of 10 in soil , only decreased the sorption 
 coefficient kf from 215 to 106 l kg in loamy sand and 154 to 84 l kg in coarse sand soils . as a result , solution concentration of glyphosate does not appreciably increase 
 upon the addition of po4 at environmentally 
 relevant p concentrations . also , the competition between glyphosate 
 and po4 ( if it occurs ) appears to be 
 temporary ; the same amounts of glyphosate and po4 were sorbed after 7 days whether the compounds are present 
 alone or together . therefore , it seems 
 likely that glyphosate and po4 are 
 specifically sorbed on to common as well as specific sites on various 
 soil components . glyphosate can form chelates or complexes with 
 micronutrient metal 
 ions in solution . at physiologically relevant ph levels , and ph levels 
 of most soils , cu and zn ions in solution can be relatively strongly 
 complexed with glyphosate , whereas fe , ca , mg , and mn are complexed 
 to lesser degrees . because of the ability of glyphosate 
 to complex metal ions , glyphosate has been postulated to affect plant 
 uptake of trace nutrients such as mn or zn . for plants grown in hydroponic solutions , mixed results for glyphosate 
 effects on plants have been shown . in contrast , andrade and rosolem reported that glyphosate did not affect mn absorption 
 and transport in gr soybean plants in the field . this topic is discussed 
 in more detail in the section below on the effects of glyphosate on 
 mineral nutrition of plants . it is difficult to extrapolate 
 hydroponic studies to field situations 
 where there are numerous cations at varying concentrations that can 
 form complexes with glyphosate , and where soluble metal - glyphosate 
 complexes are subject to sorption processes on soil , as are glyphosate 
 and metal ions . for example , the presence of zn increased glyphosate 
 sorption on two soils as a result of decreased solution ph resulting 
 from zn exchanging with h on the soil surface . in bioassay experiments using tomato plants 
 and white spruce seedlings , soils containing saturated solutions of 
 glyphosate - metal complexes had little or no effect on 
 the plants . in a recent study of micronutrient 
 accumulation in soybean grown using standard agronomic practices in 
 indiana , results showed that while there were differences in accumulation 
 of micronutrients between cultivars , there was no consistent effect 
 due to glyphosate treatment . for example , in five woodland sites , the mean soil 
 solution concentrations of total mn and zn were 69 and 1.8 mol 
 l , respectively , while in grassland sites , total 
 mn concentrations varied by a factor of 6 in a clay soil and a factor 
 of 2 in a sandy soil . in a soil toposequence 
 under natural vegetation , total mn concentration in soil solution 
 varied by a factor of 900 , depending on topographic soil position . in addition , the metal cations in soil solution 
 are not necessarily free ions ; they can form complexes with dissolved 
 soil organic matter and other ligands . for example , in a study of 
 15 agricultural soils , total soil solution concentrations of cu ranged 
 from 0.023 to 1.03 mol l , with free cu comprising 773% of total dissolved cu . in the same soils , total zn solution concentration 
 ranged from 0.4 to 43 mol l , with free 
 zn comprising 4799% of total dissolved zn . glyphosate 
 can only compete with other soil ligands and sorbent surfaces for 
 free metal ion activity , with most glyphosate being adsorbed by the 
 soil rather than remaining in the soil solution where it can complex 
 with metal ions . in spite of the wide range of micronutrient cation concentrations 
 in soil solutions , their concentrations are much greater than would 
 be found for glyphosate in soil solutions . using a glyphosate application 
 rate of 1 kg ha ( soil concentration = 0.75 g 
 g , assuming incorporation to a depth of 10 cm 
 and a soil bulk density of 1.33 ) , and an average soil kd = 100 , the amount of glyphosate in soil solution would 
 be 0.044 mol l , which is much smaller than 
 typical mn , zn , cu , and fe concentrations found in soil solutions from agricultural 
 soils . under agricultural production , concentrations of , mn , zn , cu , and fe in soil solutions 
 of holtville ( typic torrifluvent ) and altamont ( typic chromoxerert ) 
 soils would be on average 480 , 220 , 80 , and 310 
 greater than the glyphosate in solution , respectively . therefore , free cation concentrations , such as mn , would not be reduced appreciably by glyphosate addition to soil , 
 even at the highest recommended application rates and assuming all 
 the glyphosate in solution formed a chelate with mn . furthermore , 
 glyphosate degrades rapidly ( see degradation section below ) , whereas 
 although micronutrient concentrations in soil can fluctuate temporally 
 during the year , micronutrients do not degrade . the primary 
 degradation pathway is the cleavage of glyphosate by glyphosate oxidoreductase 
 to glyoxylate and ampa ( aminomethylphosphonic acid ) ( figure 3 ) , with the latter subsequently degraded to methylamine 
 and inorganic phosphate by c p lyase enzymes . both glyoxylate 
 and methylamine can support the growth of microorganisms . alternatively , 
 the transformation of glyphosate to ampa and glyoxylate can also be 
 performed by glycine oxidase . a second degradation pathway is the cleavage of 
 inorganic phosphate 
 from glyphosate by c sarcosine is further degraded into 
 formaldehyde and glycine , which are utilized by a wide variety of 
 soil microorganisms . soil microorganisms utilizing the sarcosine pathway 
 have been isolated and characterized , including members of the soybean root symbionts , the rhizobiaceae . soil fungi also degrade glyphosate , and ampa 
 was reported as a metabolite . since some 
 microorganisms are sensitive to glyphosate ( see this review ) , the 
 degradation of glyphosate in situ represents the activity of the glyphosate - degrading 
 microbial community modulated by relative resistance or sensitivity 
 to the herbicide . several studies 
 have utilized c - labeled glyphosate to examine the fate 
 of glyphosate in soil . these methods are useful because they provide 
 an integrated assessment of glyphosate degradation by measurement 
 of co2 production ( mineralization ) and the 
 incorporation of glyphosate and glyphosate degradation products into 
 soil organic matter and biota ( bound c residue ) . the co2 produced from microbial degradation after 
 glyphosate addition to soil is variable , ranging from 10% 
 in 332 d , to < 1040% in 96 
 d , to 5070% in 35 d , to > 70% in 140 d , and depends on soil type and c - label location ( phosphomethyl - c versus aminomethyl - c ) . the production of co2 begins after addition of glyphosate to soil 
 without a lag period , showing that microorganisms with the capacity 
 to degrade glyphosate are present in most all soils . the studies conducted 
 with c label in the phosphonomethyl carbon show degradation 
 of the ampa metabolite . the variability in the amount of co2 produced from glyphosate degradation in soil is likely 
 due to the variability in the population of glyphosate - degrading microorganisms 
 present in soil ( the glyphosate - degrading microbial community in soil 
 has not been fully characterized ) and their biological activity and 
 to competing sorption and binding processes . in addition to 
 the direct application of glyphosate to soil , any glyphosate remaining 
 in gr crop residues will be released to the soil as those crop residues 
 degrade . at 35 days after treatment , glyphosate residues within corn leaves mineralized more slowly ( 61% ) 
 than glyphosate applied directly to soil ( 77% ) . the use of gr crops allows for multiple applications 
 ( commonly 
 two or three ) of glyphosate within a field each growing season . c - labeled glyphosate has also been used to determine whether 
 repeated applications of glyphosate can affect glyphosate degradation 
 in soil . repeated applications reduced the rate of glyphosate mineralization 
 by 28% from the initial application to the fifth application in a 
 10-wk period ( figure 4 ) . however , there was 
 no difference in the rate of c - glyphosate mineralization 
 between one , two , three , or four glyphosate applications . while the rate of mineralization was decreased 
 after the fifth application relative to the first , it was not reduced 
 relative to the second , third or fourth application . in a similar 
 experiment , andra et al . found 
 that the initial , immediately mineralizable glyphosate decreased after 
 sequential applications as compared to the initial application . however , 
 after the initial mineralization , the rate of mineralization was the 
 same . for instance , the differences in the total amount of co2 detected 8 wks after treatment and the immediately 
 mineralizable glyphosate were 16 and 19% after 1 and 4 sequential 
 applications , respectively . weaver et al . reported greater mineralization ( % of applied ) from glyphosate added 
 at 1 the field rate ( 47 mg glyphosate / kg soil , assuming 0.84 
 kg / ha application distributed in the surface 2 mm of soil ) than at 
 the 3 rate of application . however , on a mass basis more glyphosate 
 was degraded in the soil receiving the 3 application . these 
 studies show that repeated glyphosate applications are unlikely to 
 severely reduce the ability of the soil microbial community to metabolize 
 glyphosate . broad spectrum measures of microbial activity ( respiration 
 and enzyme activities ) and community structure show inconsistent or 
 no response to glyphosate use ( see effects on rhizosphere 
 populations and community structure section ) . distribution of c after the ( a ) first or ( b ) fifth 
 glyphosate application to silt - loam soil the sequential glyphosate 
 applications were made at 2 week intervals . the extractable fraction 
 includes glyphosate and its transformation products extracted with 
 0.1 m naoh .. in addition to c - glyphosate being 
 mineralized , a portion 
 of the c - labeled glyphosate or its metabolites is converted 
 to microbial biomass and some remains unextractable from soil . the 
 amount of bound c - residue formed depends upon the molecular 
 location of the c - label , the soil interaction , and the 
 extraction methods used . mamy et al . compared 
 the bound residue formed from several pesticides and found that glyphosate - bound 
 residues generally accounted for less than 20% of the initial c added , equivalent to trifluralin , but less than those formed 
 after application of the c - labeled herbicides metazachlor , 
 metamitron , and sulcotrione . weaver et al . applied glyphosate equivalent to 1 or 3 recommended 
 field rates , and less than 10% of the c was present in 
 bound residues at 42 days after application . after four or five sequential 
 applications at 14-day intervals , less than 30% of the c from applied glyphosate was present as bound residue in the soil , 
 and there was no difference in bound residue formation after the first 
 or the fifth application . however , greater accumulation of bound residues 
 would occur from multiple applications than a single application . simonsen et al . assessed the bioavailability 
 of bound residues formed after a single glyphosate application to 
 plants by incubating c - glyphosate in soil for 6.5 months , 
 followed by planting canola or barley . glyphosate was not detectable 
 in soil at planting , and after 41 days of plant growth only 0.006% 
 of the applied c was detected in the plants . field dissipation rates of glyphosate are affected by soil properties , 
 method of application , and environmental conditions such as moisture 
 and temperature , and therefore are extremely variable . field studies often result in longer estimated times to 
 50% dissipation ( dt50 ) as compared to laboratory studies , 
 which are generally conducted under optimum conditions for degradation . 
 in one field study , under the application and weather conditions that 
 prevailed ( which resulted in water - saturated soils ) , no glyphosate 
 was detected 24 h after treatment , with a trace amount detected in 
 one replicate soil sample after 1 year . warm temperatures at the time of and in the season before the application 
 are thought to explain the fast degradation rate in the water - saturated 
 soil samples . in a comparison among glyphosate treatments between 
 a forest floor and mineral soil , the dt50 times for glyphosate 
 were 12 and 10 days for the forest floor matrix and mineral soil , 
 respectively . simonsen et al . measured a 9-day dt50 for glyphosate 
 and 32-day dt50 for ampa in soil . in an agronomic field study , glyphosate dissipation in the surface soil was rapid ( dt50 = 25 days ) and only 10% of applied chemical was present 
 at 34 days after application . in another 
 field study , glyphosate had an estimated dt50 of 4560 
 days , with total soil residues of glyphosate accounting for 618% 
 of initial chemical at 360 days after application . reported 
 a 110 to 151 day dt50 for glyphosate in a clay soil and 
 attributed the long persistence to adsorption ( kf > 118 ) . as a result of strong sorption and slow 
 desorption , 
 some glyphosate residues tend to stay in the surface soils through 
 the growing season . for example , of the initial amount of glyphosate 
 added to a clay soil , 59% ( glyphosate + ampa residues ) remained primarily 
 in the surface soil 748 days after application , despite large amounts 
 of precipitation after application . also , 
 only 0.009 and 0.019% of the initial amount of glyphosate added leached 
 from the sand and clay soils , respectively , during the study period . no leaching of ampa occurred in the sand , whereas 0.03 g ha leached in the clay soil . the dt50 time of glyphosate was generally < 5 months 
 in swedish railway embankments . in northern 
 climates with seasonally frozen soils , field studies have shown clear 
 overwinter persistence for glyphosate . after glyphosate applications 
 in june and july , about 1020% of applied glyphosate was detected 
 in the subsequent june in two field sites , demonstrating that the 
 time for 90% ( dt90 ) dissipation of glyphosate was about 
 11 months . similar overwinter persistence 
 was observed in agricultural fields in southeast finland . under warmer climates , glyphosate did not persist 
 past the growing season , even after 15 consecutive annual applications . after either pre - emergence or postemergence 
 applications of glyphosate , 
 the distribution of residues is nonuniform in soils and is more concentrated 
 near the surface of the soil . almost two years after application to 
 a tilled soil in an outdoor lysimeter , glyphosate accounted for 1% 
 and ampa for 19% of the applied glyphosate in the 010 cm depth 
 increment . in the 1020 cm depth increment , glyphosate was not detectable , and ampa accounted for 5% 
 of the applied herbicide . deep ( > 1 m ) leaching of glyphosate has 
 been 
 observed , but concentrations in leachate were < 0.07 g l. this was attributed to movement in macropore flow , rather than leaching 
 through the bulk soil . deep movement of glyphosate might be also expected 
 via translocation of the herbicide sprayed on to foliage of crops 
 and weeds to their roots , particularly resulting from glyphosate applications 
 later in the growing season . multiple applications of glyphosate 
 in gr - cropping systems would 
 ( 1 ) increase the risk of carryover , especially in regions where soils 
 are seasonally frozen for extended periods and ( 2 ) increase the risk 
 of leaching to tile drains or groundwater . multiple applications of 
 glyphosate increase the time that bioavailable glyphosate is present 
 in soil . also , plant interception of glyphosate in the field may lead 
 to a delayed release of glyphosate into the soil following foliage 
 decomposition . the degree of metabolic degradation of glyphosate in 
 plants would influence how much glyphosate 
 is released into soil by degradation of plant material . both of these processes 
 have been investigated in laboratory experiments , but corresponding 
 field studies are not yet available . any increased persistence is 
 potentially important in cropping systems where glyphosate - sensitive 
 ( gs ) crops closely follow the gr - crop . the risks associated with planting 
 highly sensitive crops shortly after ( <3 days ) glyphosate applications 
 were known long before the advent of gr - crops . during the 
 course of their action on susceptible plants , herbicides 
 eventually affect almost every physiological and biochemical process , 
 including mineral nutrition . thus , glyphosate would be expected to 
 affect mineral nutrition of gs plants at herbicidal doses , but not 
 of gr plants at the same doses . recent reports of mineral deficiencies 
 in gr crops after glyphosate application were linked with claims of 
 increased susceptibility to plant diseases . there are conflicting 
 papers on the effects of glyphosate on mineral nutrition on gr crops . this is a complex topic that , for clarity , we have separated into 
 the aspects listed below . many natural metal 
 chelators such as organic and amino acids are found in plant cytoplasm 
 and xylem and phloem fluids . citrate is an important chelator of fe 
 in xylem fluid , while some amino acids chelate metals in the cytoplasm 
 where the higher ph favors chelation by amino acids compared to organic 
 acids . synthetic chelators have been 
 used in agriculture since 1950 to supply fe or zn to plants , and more 
 recently to induce phytoextraction of soil metals by plants . adding high amounts of strong chelators such 
 as edta ( ethylenediaminetetraacetate ) to soils causes sorbed metals 
 to be released from soil and metal chelates to be formed , making the 
 metal mobile . in order for added chelating agents to be effective 
 in increasing metal uptake for example , induced phytoextraction of pb required the addition of 
 10 mmol of edta kg of soil which would cost over 
 $ 30  000 ha . edta was only effective when , after binding other metals in the 
 soil , there was some free edta which attached to the root membranes , 
 causing them to become leaky . the added edta , however , also causes metal leaching , and use of edta 
 to induce phytoextraction of soil metals is not allowed in the open 
 environment . occasionally excessive fertilizer 
 rates of feedta or znedta cause phytotoxicity in the field or greenhouse . our experience with metal chelation in soils and metal chelate 
 injury of plants provides insight into whether glyphosate would be 
 expected to affect plant uptake of micronutrient cations . if low concentrations 
 of edta or similar strong chelators are added to soils , they can promote 
 uptake of strongly adsorbed metals because the dissolved metal - chelate 
 can move metals from soil particles to the root membranes , circumventing 
 the diffusion limitations of metal uptake . thus , in general , addition 
 of kg per ha levels of glyphosate might be expected to increase element 
 uptake if glyphosate were a strong chelator . however , glyphosate is 
 a relatively weak metal cation chelator compared to edta ( table 1 ) . in general , none of the research 
 on chelating agent effects on metal uptake would indicate that a weak 
 chelator such as glyphosate would reduce or increase uptake of micronutrient 
 cations from soil . although protonated and deprotonated 
 chelates also occur , the 1:1 metal - ligand chelates are listed for 
 comparison using data from the program geochem - pc . values for edta , citrate and glycine are for 0 ionic strength , 
 while the values for glyphosate are 0.1 m ionic strength . equilibria depend very strongly on solution 
 ph and the pka values for the different proton binding functional 
 groups of a chelator molecule . examination of glyphosate levels in glyphosate - treated 
 gr soybean seeds at maturity and mineral 
 levels in soybean seed shows that on 
 a molar basis the metal : glyphosate ratio can be from almost 10  000 
 times more mn to around 100  000 times more minerals such as 
 mg or ca compared to glyphosate . comparing glyphosate content of leaves 
 of glyphosate - treated gr soybean with 
 recently measured mineral content of gr soybean leaves , the ratios are smaller ( ca . 300 for ca , 30 for 
 fe , 20 for mn , and only 2 for cu ) , but the ratio of total metal atoms 
 to glyphosate molecules is close to 1000 . even if a substantial fraction 
 of the minerals in the plant tissue were unavailable to glyphosate 
 due to chelation with other compounds , sequestration , or other means , 
 the ratio of mineral cations to glyphosate anions would still be very 
 large . these large ratios do not support the view that the chelator 
 properties of glyphosate would interfere substantially with plant 
 mineral nutrition in planta . furthermore , at very 
 high in vivo concentrations of glyphosate in the 
 plant phloem , glyphosate has been calculated to be unable to effectively 
 compete for fe , fe , ca , mn , mg , cu , and zn with biological 
 chelating agents . reduction in soil 
 mn concentrations due to glyphosate use has not been demonstrated . 
 in practice , glyphosate which reaches soil is strongly adsorbed by 
 fe and mn oxides and organic matter . when 
 glyphosate is bound by soil , it can be abiotically degraded in addition to the biodegradation pathways discussed 
 earlier . other studies tested whether glyphosate reaching soil would 
 cause leaching of soil metals . barrett and mcbride tested leaching of metals in response to glyphosate application 
 for several soils and found that leaching occurred only with soils 
 highly contaminated with metals and only with high rates or repeated 
 applications of glyphosate . in contrast with some descriptions 
 of glyphosate as a strong chelator , the stability constants of glyphosate , 
 edta , and citric acid with common micronutrient ions show that glyphosate 
 is a weak chelator ( table 1 ) . the fact that the 
 relative concentrations of metal cations in soil are several orders 
 of magnitude greater ( in terms of moles of metals per ha vs moles 
 of glyphosate per ha ) than the highest concentrations of glyphosate 
 that could be expected ( discussed in detail in a previous section ) , 
 significant effects of glyphosate on soil mineral content or availability 
 to plants are highly unlikely . from 
 the earliest days of glyphosate use , it was known that using water 
 containing high levels of metal ions would significantly reduce the 
 efficacy of the herbicide , presumably because the precipitated or 
 chelated herbicide is not taken up by target plants as well as the 
 free glyphosate anion and/or precipitation of glyphosate : mineral complexes 
 ( reviewed by duke , sundaram and sundaram , and nilsson ) . if 
 metal cations are present in a tank mix solution , and ph is raised 
 by addition of microelement fertilizer or by hard water , precipitation 
 of glyphosate reduces the plant uptake of glyphosate , thereby significantly 
 reducing its herbicidal effectiveness . the solubility of 1:1 metal / glyphosate 
 complexes decreases in the order of mg  ca > mn > found ca , mn , and zn ions to reduce 
 glyphosate 
 efficacy on a variety of weeds when included in a tank mixture . several 
 researchers have shown that separate application of mn fertilizer 
 and glyphosate caused no effect or interaction , and recommend careful 
 consideration of tank mixes . edta , being a stronger chelator 
 than glyphosate , reverses the reduction of glyphosate herbicidal efficacy 
 by metal cations in spray tanks . farmers 
 have been advised to not spray glyphosate with micronutrient plant 
 nutrition supplements unless the metal is chelated with a strong chelator 
 such as edta or eddha . thus , studies on effects of glyphosate on mineral 
 nutrition of plants should not be conducted with combined spray solutions 
 of minerals and glyphosate . in short , a finding of metal ion precipitation 
 of glyphosate in a tank mix is not relevant to questions raised about 
 chemical interactions between glyphosate and micronutrients in plants 
 or soils . because glyphosate is a metal ion chelator , there was speculation 
 decades ago that this might be related to the mode of action of the 
 herbicide . however , the finding that gr crops with only a change in 
 their epsps are about 50-fold less sensitive to glyphosate than similar 
 gs crops indicated that mineral nutrition 
 is not involved in the mode of action of glyphosate . further evidence 
 of this is the recent evolution of gr palmer amaranth ( amaranthus 
 palmeri ) biotypes that have multiple copies of the gs epsps 
 gene . the greater the number of copies of the 
 gene , the more resistant these plants are . if chelating mn or any 
 other mineral was significantly involved in the mode of action of 
 glyphosate , this would not be the case . glyphosate can impede 
 absorption and translocation of calcium and magnesium in gs plants 
 ( reviewed in duke ) . nilsson found glyphosate to stimulate the accumulation 
 of fe in gs plants , while impeding movement of zn to the same sites . this result supports the finding that 
 subtoxic levels of glyphosate stimulate growth of iron - deficient wheat . nilsson found no 
 effects of glyphosate on mn , zn , or cu content of gs wheat leaves . eker et al . reported that glyphosate 
 reduced uptake and translocation of mn and fe in gs sunflower . likewise , foliar - applied glyphosate to 
 gs soybean seedlings reduced uptake and translocation of mg and ca , reduced tissue ca content , and altered cellular 
 ca distribution . found reduced levels of ca , mn , mg , and fe in 
 seeds and leaves of glyphosate - treated , gs soybean . in studies with 
 gs festuca spp . , ca , mg , mn , and fe were most reduced 
 by glyphosate treatment compared to other minerals . such effects are readily explained by the known effect 
 of glyphosate on root growth and function in gs plants . glyphosate 
 from foliar sprays is rapidly translocated to roots , where it strongly 
 inhibits root growth and other processes . nearly 
 all of the multivalent metal cations are absorbed for translocation 
 to shoots by young roots . reported reductions 
 in plant shoot fe due to glyphosate application , resulting in chlorosis 
 in both gr and gs soybean cultivars . the authors correlated the effects 
 on fe content with effects on root ferric reductase activity , however , 
 the methods used for measuring ferric reductase activity were inappropriate . roots grown in soil were removed , washed , and used in a bioassay of 
 feedta reduction . broken roots , loss of fine roots and root hairs , 
 and the presence of soil in the assay mixture confounds the measurement . ozturk et al . found inhibitory effects 
 of glyphosate on root ferric reductase in iron - deficient gs sunflower . however , no in vitro effect of the herbicide on the 
 enzyme was reported to determine whether it was a primary or secondary 
 effect . high rates of phosphate fertilizer have been reported 
 to remobilize 
 small amounts of glyphosate bound to soil . these low soil solution concentrations of glyphosate were phytotoxic 
 to a gs soybean cultivar on most soil types , but stimulated plant 
 growth ( hormesis ) on one soil type . hormesis ( the stimulatory effect 
 of a toxin at subtoxic concentrations ) at low glyphosate doses is 
 a well - established phenomenon ( e.g. , velini et al . ) . however , the bott et al . experiment has no relevance to practical field environments , as 
 the researchers applied extreme rates of dissolved superphosphate 
 to the surface of glyphosate - amended soils and planted the seeds immediately . fertilizer p rates are usually applied in bands below and to the side 
 of seeds to prevent adverse effects on seed germination . considering 
 that the 240 mg p kg highest rate of p application 
 would cause the amount in the surface 12 cm of the potted 
 soil to be 1020 times higher than normally found in field 
 applications of p , one should not extrapolate from the results with 
 the high rates used in this study . it is questionable even whether 
 the low rates , where no adverse effects were observed , are relevant 
 to understanding glyphosate in the environment . the studies 
 discussed in this subsection were done on gs plants , 
 so separating secondary effects of inhibition of epsps and effects 
 via any other mechanism is impossible . it may be that some of the 
 confusion regarding glyphosate effects on mineral nutrition of gr 
 crops is due to studies on gs plants that can not be extrapolated to 
 gr plants . as mentioned above , gr crops are highly resistant to glyphosate , 
 with resistance factors ( i50 ratios between gr and susceptible 
 crops ) of about 50 for both gr canola and gr soybean . no effects on growth of gr crops are normally seen at the 
 highest recommended field rates of glyphosate . under some environmental 
 conditions with some cultivars , transient  yellow flash symptoms in gr soybeans are seen 5 to 20 days after glyphosate application 
 ( figure 5 ) . yellow flash has been attributed 
 to the rapid metabolism of glyphosate to the weakly phytotoxic ampa 
 and not to mineral nutrition effects . gr crops are not necessarily resistant to ampa , as its mode of action 
 is not the same as that of glyphosate . the yellow flash effect is 
 temporary and does not reduce yields , nor have yellow flash symptoms 
 been shown to be due to disease incidence in soybean . this yellowing 
 and interveinal chlorosis of rapidly growing young leaves in soybeans 
 experiencing yellow flash could be confused with symptoms of fe or 
 mn deficiencies . however , yellow flash symptoms are not accompanied 
 by effects on mn status of the plant or on mn uptake or distribution 
 by the plants . yellow flash symptoms 
 have not been reported in gr crops other than soybean , perhaps because 
 sufficient levels of ampa to cause such symptoms do not accumulate 
 in other gr crops or there is insufficient sensitivity of these crops 
 to ampa . little is known of ampa in gr crops , including its mechanism 
 of phytotoxicity . example of  yellow 
 flash  in gr soybeans sprayed 
 with glyphosate in illinois . huber suggested that use of glyphosate in production 
 of gr soybean leads to mn deficiencies by reduction of mn uptake and/or 
 translocation efficiency , changing soil / rhizosphere microbiology , 
 or modifying the form or availability of mn in the environment . noted that gr - soybean cultivars showed lower 
 yield , stronger yellowing symptoms , and lower foliar mn on a mn marginal 
 or deficient soil than two conventional cultivars ( non isolines ) . it now appears that they observed that the gr - cultivar was inherently 
 less able to obtain soil mn than the conventional cultivars . mn deficiency can occur in soybeans grown on 
 low mn soils such as the lake plain soils in the midwest , and the 
 coastal plain soils on the east coast of the united states . if these 
 soils are limed , mn becomes much less phytoavailable and soybeans 
 may suffer severe chlorosis and yield reduction until foliar mn sprays 
 are applied or soil ph is lowered . genetic variation for 
 susceptibility to mn deficiency exists in soybeans ( e.g. , graham et 
 al . ) . soybean cultivars for areas with 
 low phytoavailable soil mn have been developed , and farmers are advised 
 to plant more mn deficiency - resistant cultivars on such soils . as 
 breeders worked to solve this susceptibility problem ( much like the 
 case of fe chlorosis susceptibility of the early gr soybean cultivars ; 
 see below ) , improved cultivars with the gr trait were also resistant 
 to mn deficiency . this genetic variation in resistance to mn deficiency 
 among soybeans occurs because roots change the microenvironment in 
 their rhizosphere to reduce mn oxides to the soluble mn , or reduce chelated mn with fulvic acids to promote 
 uptake by the roots . local acidification of the rhizosphere may also 
 improve mn uptake by cultivars resistant to mn deficiency . plants 
 also up - regulate metal ion transporters in their young roots to better 
 absorb the free mn in the rhizosphere . experiments 
 have been conducted in the field at multiple locations over multiple 
 years which found that there was no appreciable susceptibility to 
 mn deficiency or need for mn fertilizer to grow gr - soybean cultivars . several field trials have shown that gr - soybeans are not commonly 
 experiencing mn deficiency . unfortunately , no study has been reported on soils which caused 
 clear mn deficiency in soybeans in the absence of glyphosate so that 
 any interaction with glyphosate use could be measured . there 
 are several peer - reviewed journal claims of effects of glyphosate 
 on mineral nutrition in gr soybean . reported that in the absence of glyphosate , a hydroponically 
 grown gr soybean cultivar accumulated more mn than did a gs cultivar , 
 but the two lines were not near isogenic , making interpretation of 
 the data impossible . in addition , when both types of soybean were 
 grown with low mn supply , there was no effect of glyphosate on shoot 
 concentration of mn or growth . at very high application rates of glyphosate , 
 mn concentrations in the tissue of the gr cultivar were reduced about 
 50% . there were no effects of glyphosate on mn and fe content of plant 
 tissues when the plants were grown in two different soil types , although 
 there was a reduction in insoluble foliar zn in one of the soil types . this tests whether the low molecular weight soluble chelates were 
 formed in the tissues as occurs with excessive edta . taken together , 
 the data of this study show no adverse effect of glyphosate on mn 
 uptake or translocation in gr - soybeans . zobiole et al . reported that glyphosate treatment reduced 
 essential minerals ( mg , mn , etc . ) in gr soybean tissues . they also 
 reported dramatic reductions in photosynthesis associated with these 
 reductions , a result that is difficult to 
 reconcile with the high and increasing yields of these crops ( see 
 section on yields below ) . in a more extensive study , cavalieri et 
 al . examined the effects of 0.96 kg 
 ha glyphosate from six different commercial formulations 
 on n , p , k , s , b , ca , mn , mg , fe , zn , and cu in two gr soybean cultivars 
 in the greenhouse . the results were equivocal , with both decreases 
 and increases in metals , depending on both cultivar and glyphosate 
 formulation . there was no clear pattern , other than reduced levels 
 of both metal and nonmetal elements as well as plant growth by one 
 formulation on one of the cultivars , suggesting that something other 
 than glyphosate was involved . comparing near - isolines of soybeans , 
 loecker et al . found no effect of the 
 gr transgene of gr soybean 
 on mn uptake or response to mn in the absence of glyphosate . rosolem 
 et al . found no effects of foliar application 
 of glyphosate on mn absorption , accumulation , or distribution in gr 
 soybeans . serra et al . found 
 no effect of glyphosate doses up to 2.5 kg / ha on cu , mn , and zn uptake 
 by gr soybeans , while fe uptake increased at this high dose . no effects 
 of glyphosate on translocation of these metal ions were seen up to 
 2.5 kg / ha . in this study , exogenously applied mn had no effect on 
 any responses to glyphosate . lundry et al . found no effects of glyphosate on mineral nutrition in gr soybean 
 seeds , compared to an untreated near - isogenic soybean line , indicating 
 no effect from the epsps transgene or from glyphosate . found no glyphosate - induced deficiencies in 
 macronutrients ( n , p , k , s , mg , and ca ) or micronutrients ( b , zn , 
 mn , fe , cu , and al ) in second generation gr soybeans . the application 
 of glyphosate to gr soybean had no effect on leaf mineral content 
 ( mn , fe , cu , and zn ) or yield at two different sites in brazil . there was also no effect of absorption of exogenously 
 applied mn . exogenous mn application had no effect on yield of glyphosate - treated , 
 gr soybeans , but it did enhance mn and reduce fe content in this study . no effects 0.86 kg ha glyphosate sprayed once 
 or twice on mn content of both greenhouse- and field - grown gr soybean 
 leaves ( young and old ) or seed ( figure 6 ) . the results of all of these studies indicate that glyphosate 
 does not restrict the availability of micronutrients in glyphosate - treated , 
 gr crops . thus , the results of the three research groups that have 
 reported glyphosate effects on mineral nutrition in gr crops are counter 
 to those of nine other research groups . effects of two , successive 
 glyphosate treatments ( 0.86 kg ai h at both 3 
 and 6 weeks after planting ) on the metal 
 content of mature seeds of field - grown gr soybean plants . bars respresent 
 1 se . there were no differences among any of the paired mean values 
 at the 95% confidence level .. in many locations in ia , mn , nd , and some other 
 u.s . states , soybeans may suffer iron - deficiency - chlorosis ( idc ) when 
 grown on wet calcareous soils . soybean cultivars vary widely 
 in resistance to idc , and many factors which influence soil moisture 
 and bicarbonate levels interact with severity of idc . idc can cause severe yield reduction on problem soils if cultivars 
 are not highly resistant to idc . because of the susceptibility 
 of many soybean cultivars to idc , growers with problem soils are advised 
 to select chlorosis - resistant cultivars . unfortunately , when gr soybeans 
 were first developed , the cultivars which were initially transformed 
 were not highly resistant to idc , and many of the early high yielding 
 gr soybean cultivars developed for normal soils were susceptible to 
 idc on wet calcareous soils . soybean agronomists in states where idc 
 is prevalent now screen genotypes for resistance to idc and report 
 the results to growers so they can choose cultivars to match their 
 soil idc problems . thus , gr soybean cultivars have been screened for 
 susceptibility to idc with and without glyphosate applications in 
 many locations . although this has not been reported in the literature , 
 several scientists involved in soybean idc screening confirm that 
 based on their observations in chlorosis rating field plots , glyphosate 
 causes no adverse interaction with iron deficiency in soybean ( s.r . although duke et al . found no effect 
 of two applications of glyphosate on nickel content of leaves or seed 
 ( figure 6 ) of gr soybean , another report notes 
 that glyphosate use on gr - soybeans in a brazilian study caused a significant 
 reduction in plant n - fixation and a decline in leaf ni . the foliar 
 ni levels , even in their controls , were far below normal soybean foliar 
 ni levels in other research . ni deficiency of significant 
 consequence in the field has only been observed with some low ni soils 
 of the southeastern coastal plain where pecans suffered severe deficiency 
 under some conditions of previous management which included raising 
 soil ph which reduces ni phytoavailability . legumes have a higher ni requirement than nonlegumes because ni 
 is needed for biochemical processes in nodule bacteria , as well as 
 for certain plant biochemistry . unfortunately , zobiole et al . did not test application of foliar ni fertilizer 
 to confirm that the measured yield reduction actually resulted from 
 ni deficiency induced by glyphosate . furthermore , the level of ni 
 in the brazilian soil was not reported , so whether soil ni deficiencies 
 were involved can not be determined . that glyphosate is directly toxic 
 to some strains of bradyrhizobium japonicum due the 
 fact that their epsps is also sensitive to glyphosate is well - known 
 ( see section under glyphosate effects on soil microflora below ) , and this toxicity is not related to effects on ni . studies 
 designed to address the interactions of glyphosate and ni metabolism 
 conducted on ni - deficient soils with and without ni supplementation 
 would be useful in interpreting the results of zobiole et al . there are numerous studies on the compositional ( chemical and nutritional ) 
 equivalence of gr crops with gs crops , including mineral content , 
 although the intent of these papers was to evaluate the effect of 
 the transgene(s ) on composition , rather than the effect of glyphosate 
 treatment on gr crops . in most of the published studies no mention 
 is made of whether glyphosate was used on the crop . in other studies the glyphosate application to the gr crop is not 
 completely described . , the timing of glyphosate applications in gr corn is given , 
 but not the rates . in another study with gr corn the only information provided for the glyphosate treatments 
 was that they were made according to the label . a study with gr alfalfa 
 states only that glyphosate was applied prior to each cutting . more detailed information on the glyphosate 
 applications is provided in a study with gr corn by ridley et al . for one set of trials , the gr corn received 
 an application of 1.08 kg ha , and in another ca . the purpose of these studies 
 was to provide data required by regulatory agencies to determine the 
 effect of the transgenes on the composition of the harvested crop . no effects of the gr transgenes or glyphosate application on mineral 
 content have been found in these field studies conducted under good 
 laboratory practices that usually involved multiple years and locations . however , these studies lacked comparisons of glyphosate - treated with 
 untreated crops to allow evaluation of the glyphosate effect , independent 
 of the genetic effect of the gr technology . clearly , glyphosate can have effects on mineral nutrition 
 of gs plants through its herbicidal effects on plant roots and other 
 parts of the plant . published data on the effects of glyphosate on 
 mineral nutrition of gr crops are contradictory . three groups have 
 claimed adverse effects on mineral nutrition in gr crops in peer - reviewed 
 journals  the zobiole et al . the peer - reviewed 
 results of nine laboratories show no effect of glyphosate on mineral nutrition . these seemingly 
 contradictory results could be entirely or in part due to differences 
 in the soils , climatic conditions , and/or gr cultivars used . for example , 
 one group of experiments is based almost entirely on studies with 
 low ph soils using soybean varieties developed in brazil and evaluated 
 in greenhouse studies . rigorous field studies on different soil types ( including those 
 highly susceptible to inducing mn or fe deficiency in soybeans ) are 
 needed to resolve the issue of whether glyphosate might have adverse 
 effects on mineral nutrition of gr crops . considering the available 
 data , growers are unlikely to need mn fertilizers just because they 
 use glyphosate on gr soybeans . soil microflora 
 can influence the persistence of glyphosate and 
 its metabolites in soil . evaluation of glyphosate effects 
 on soil microorganisms requires knowledge of the direct effects of 
 glyphosate and its metabolites on soil microorganisms as well as effects 
 on microorganisms through processes mediated by plants on root symbionts 
 and rhizosphere microorganisms . the determination of relevant environmental 
 exposure concentrations needs to be compared to known response factors . finally , short - term and long - term responses on processes and community 
 structure need to be evaluated . as in plants , 
 glyphosate blocks the synthesis of the aromatic amino acids phenylalanine , 
 tyrosine , and tryptophan in some bacteria and fungi through the inhibition 
 of epsps , which also causes accumulation and excretion of shikimate-3-phosphate 
 and hydroxybenzoic acids in sensitive microorganisms . the sensitivity of bacterial epsps to glyphosate varies widely . divided microbial epsps 
 into two groups : sensitive ( class i ) and relatively insensitive ( class 
 ii ) . class ii includes agrobacterium cp4 ( the source 
 of the gr - epsps transgene in most gr - cultivars ) in which the resistance 
 to glyphosate results from variations in the amino acid sequence of 
 epsps . concentrations required for 50% inhibition were 75 m 
 for e. coli , 174 m for bacillus subtilis , and 1100 m for pseudomonas aeruginosa epsps . moorman et al . reported 
 variation in susceptibility of strains of bradyrhizobium japonicum to glyphosate : 1000 m ( 169 mg l ) glyphosate 
 produced 47% inhibition for strain 110 , but only 12 and 19% inhibition 
 for strains 123 and 138 , respectively . similarly , hernandez et al . reported b. japonicum strains 
 ranging from sensitive to glyphosate ( 50% inhibition at 30 m ) 
 to insensitive ( 50% inhibition at > 1000 m ) . the full range 
 of resistance or sensitivity to glyphosate within the soil microbial 
 community is not fully known . addition of aromatic amino acids to 
 bacterial cultures can partially or fully reverse the effects of glyphosate . some fungi are also sensitive to glyphosate , with 50% inhibition of 
 growth at concentrations of 5 to 50 mg / l ( 0.848.4 m ) 
 in culture . understanding the 
 impact of glyphosate on soil microorganisms requires estimating concentrations 
 to which the microorganisms are exposed . glyphosate applied to 
 foliage is rapidly translocated to roots and other metaboically active 
 tissues . glyphosate is exuded from roots 
 of treated plants into the rhizosphere , but the resulting concentrations in the rhizosphere soil are difficult 
 to document . glyphosate applied to gs crops can be translocated to 
 the roots and released initially in exudates and later from decaying 
 tissues . as much as 15% of glyphosate applied to sensitive plants 
 could be translocated to roots laitinen et al . also showed movement 
 of glyphosate from roots of treated plants to the soil , with the concentration 
 of glyphosate reaching 0.07 mg kg soil in the 
 rhizosphere at four days after application . kremer et al . compared carbohydrate 
 and amino acid exudation from roots of gr soybeans with or without 
 glyphosate treatment in hydroponic culture . amino acid exudation was 
 increased by glyphosate , but carbohydrates ( measured by an anthrone 
 reaction ) were not different . glyphosate treatment of a gs soybean 
 variety ( williams ) also resulted in increased 
 carbohydrate exudation . the root exudation of shikimate-3-p and protochatecuic 
 acid have not been examined , but exudation of these compounds might 
 be expected from gs plants after glyphosate application , as glyphosate 
 causes marked accumulation of these compounds in sensitive plants 
 ( e.g. , lydon and duke ) . the effects of glyphosate on microorganisms in soil have been extensively 
 investigated using a variety of techniques . two techniques that investigate 
 the community level responses , microbial biomass and respiration , 
 show either no effect or a temporary inhibition of respiration due 
 to glyphosate applied at rates less than 50 mg kg . at glyphosate application rates above 50 and up to 1500 mg kg soil , the range 
 of concentrations used in these studies resulted from different assumptions 
 about the penetration of sprayed glyphosate into the soil ( see lancaster 
 et al . ) . the stimulatory effect of high 
 glyphosate concentrations on soil respiration is partly attributed 
 to microbial metabolism of glyphosate , but secondary effects due to 
 n and p mineralization could also stimulate respiration . these concentrations 
 of glyphosate seem sufficient to induce glyphosate toxicity ; a hypothetical 
 application of 50 mg kg glyphosate soil at 25% 
 gravimetric water content would result in a 1.18 mm aqueous concentration 
 in a thin layer at the surface of the soil . however , rapid adsorption 
 would reduce the concentration in the soil solution . a kd of 50 would result in approximately 2% of the applied 
 herbicide being present in the soil solution resulting in an aqueous 
 concentration of approximately 24 m glyphosate , which is sufficient 
 to affect sensitive microbial species . community level measures , such 
 as respiration or total microbial biomass , are not sufficiently sensitive 
 to detect changes in population or activity of small subpopulations . alternatively , glyphosate impacts on soil microorganisms can be 
 assessed using measures of community structure and in long - term studies 
 where cumulative impacts may be determined . hart and brookes found no difference in microbial biomass , microbial 
 respiration and n mineralization in soils after 19 years of annual 
 glyphosate application compared to an untreated control soil . busse 
 et al . compared ponderosa pine ( pinus ponderosa ) forest soils receiving glyphosate treatment 
 for understory vegetation control to control treatments ( understory 
 cover at 25100% ) . no glyphosate effects on soil respiration , 
 n mineralization , or microbial biomass were found when these plots 
 were evaluated after 9 to 13 years at each of the three sites . powell 
 et al . compared a gr - soybean to a near 
 isoline sensitive cultivar over four years in ontario . rates of soybean 
 litter decomposition of the gr and conventional cultivars were nearly 
 identical ; however , glyphosate reduced litter decomposition on the 
 soil surface , but not on buried litter . the ratios of fungal biomass 
 to bacterial biomass in the litter were only occasionally different , 
 with an increased ratio in the gr cultivars . the rhizosphere is comprised of the root surface and the immediate 
 soil layer ( 25 mm ) surrounding the root where microbial processes 
 are driven by root exudation of simple and complex substrates , which 
 include organic acids , flavonols , lignins , indole compounds , and amino 
 acids . the rhizosphere community includes 
 root symbionts , pathogens , plant growth - promoting rhizobacteria , phosphate - solublizing 
 bacteria , and microoganisms active in carbon and nitrogen cycling . significant amounts of carbon are exuded from 
 growing roots , and rhizosphere populations may be exposed to glyphosate 
 through leaching of glyphosate from the soil surface and root exudation 
 of glyphosate . mijangos et al . examined glyphosate effects on gs plants ( triticale and peas ) and 
 their rhizosphere microbial communities . ammonia concentrations increased 
 in rhizosphere soil after glyphosate treatment compared to the control 
 ( no glyphosate , but clipped to remove above - ground biomass ) . functional 
 diversity of the rhizosphere microbial community was examined using 
 a multiple substrate utilization test ( biolog ecoplates ) and genetic 
 diversity by denaturing gradient gel electrophoresis of 16s - rdna after 
 pcr amplification . community diversity and richness were reduced at 
 the highest rate of glyphosate application in rhizospheres of killed 
 gs pea and gs triticale , but not in soil from triticale grown alone . the magnitude of these differences was similar to the differences 
 due to growing triticale alone or in combination with peas . several studies using different methods have examined the impact 
 of glyphosate on the rhizosphere of gr crops . glyphosate application 
 to gr - soybean cultivars in the field in two growing seasons caused 
 transient differences in dehydrogenase activity , -glucosaminidase 
 activity , -glucosidase , and respiration . these enzyme activities are broadly distributed in soil 
 microorganisms , and the results suggest that broad spectrum toxicity 
 did not result from glyphosate application . subsequent studies reported 
 increases in the ratio of mn oxidizers / mn reducers in response to 
 glyphosate and decreases in iaa - producing rhizobacteria . the magnitude of these responses increased as 
 the glyphosate application rate increased up to a rate equivalent 
 to 1.2 g ha . manganese oxidation ( mn + 1/2o2 + h2o mno2 + 2h ) reduces the solubility of manganese . the observation that 
 glyphosate affects the ratio of mn oxidizers / mn reducers in the gr 
 soybean rhizosphere led to suggestions that glyphosate reduced plant 
 available mn in soil and plant uptake of mn . however , the extent that this shift to a higher ratio of mn oxidizers 
 to mn reducers has on the availability of mn to plants was not determined . manganese is most available in soil under reduced conditions and/or 
 at low ( < 5.4 ) soil ph . a phylogenetically diverse group of both 
 bacteria and fungi are capable of mn oxidation , but the cultural methods used to assess mn oxidation or 
 reduction potential may not measure all 
 the microorganisms capable of mn transformation , or their in situ activity . also , plant roots actively regulate their 
 ability to obtain mn from soils , up - regulate mn transporters , and 
 secrete reducing materials which would release mn from bound forms 
 in the soil for plant uptake . additional research is needed to investigate 
 glyphosate - induced changes in mn bioavailability in the rhizosphere . reported reduced functional 
 diversity ( also using the multiple substrate utilization test ) in 
 response to two glyphosate applications to gr - canola . hart et al . found that rhizosphere populations of denitrifying 
 bacteria and fungi were not affected by glyphosate application to 
 gr - corn compared to gr - corn treated with conventional herbicides or 
 a gs corn isoline treated with conventional herbicides . extracted bacterial 
 dna from gr corn rhizospheres after pre - emergence treatment with no 
 herbicide , glyphosate , or gtz ( a mixture of the herbicides acetochlor 
 and terbuthylazine ) . pyrrosequencing of cloned 16s - rdna showed that 
 microbial community structure after glyphosate treatment more resembled 
 the control ( no herbicide ) than the gtz - treated community . glyphosate 
 reduced actinobacteria relative to the untreated control and proteobacteria 
 were relatively unaffected . the gtz treatment reduced microbial diversity 
 relative to the glyphosate or no - herbicide treatments . in contrast , 
 lancaster et al . showed a variable response 
 of actinobacteria populations to one or five applications of glyphosate 
 to soil without a crop , while proteobacteria were increased by glyphosate 
 applications . the concentrations of microbial fatty acid methyl - esters 
 ( fame ) from gram - negative bacteria also increased , which is consistent 
 with the increase in proteobacteria populations . longer - term 
 ( 3 year ) studies identified three microbial groups 
 dominating the gr corn rhizosphere in two fields in spain : the proteobacteria , 
 actinobacteria , and acidobacteria . glyphosate 
 was applied postemergence to gr - corn , and roots were sampled 7 days 
 after glyphosate treatment and just prior to harvest . dna extraction 
 and sequencing provided a database that was screened for 16s - rdna 
 phylogenetic sequences . the abundance of these groups indicated little 
 effect of glyphosate over three years ( figure 7 ) . analysis of the same data with a clustering procedure showed that 
 the rhizosphere community was most affected by year and field and 
 least affected by time of sampling and herbicide . acidobacteria increased 
 over time in both fields ( figure 7 ) , while 
 actinobacteria tended to decrease . eubacterial phyla ( 16s - rdna sequence abundance ) 
 recovered from 
 gr - corn rhizosphere treated with glyphosate ( g ) or without glyphosate 
 ( c ) in two fields ( upper and lower panels ) . also 
 used fame biomarkers 
 to examine the effects of two postemergent glyphosate applications 
 to gr - soybeans grown in soil with and without a history of previous 
 glyphosate use . at 7 days after application , total fame ( an indicator 
 of microbial biomass ) nonmetric multidimensional 
 scaling of the fame data showed a significant effect of the soil ( history 
 vs no - history ) on community structure , but no effect of application 
 or sampling times on community structure . the decrease in microbial biomass at 
 7 days after application does not support the conjecture that glyphosate 
 treatment increases root exudation . weaver et al . also used fame analysis to compare rhizosphere and bulk 
 soil community structures after glyphosate application to gr - soybean 
 in the field . after the second in - season glyphosate application , the 
 community structure of the bulk soil differed from that of the rhizosphere , 
 but two previous applications of glyphosate had no effect on fame . the same study included two fungal fame biomarkers ( 16:1 5c 
 and 18:2 6c ) , and these were not affected by the glyphosate 
 treatments . the 16:1 5c ( hexadecenoic acid ) content is a biomarker 
 for arbuscular mycorrhizal fungi , while 
 18:2 6c is a more broad fungal marker , including rhizoctonia 
 solani and fusarium oxysporum.(167 ) zablotowicz and reddy summarized the effects of glyphosate on soybean 
 nodulation and n fixation . gr soybeans treated with glyphosate had 
 reduced nodulation , as well as delayed n fixation , plant biomass accumulation , 
 and n fixation , but the severity of these effects was dependent upon 
 several factors . these included when glyphosate was applied to the 
 soybean , the number of glyphosate applications , the glyphosate formulation , 
 and the gr - soybean cultivar . compared nodule number and mass in six gr and three near isoline 
 gs soybean cultivars in the absence of glyphosate . significant differences 
 in nodulation were found among the cultivars , but these were not related 
 to glyphosate resistance . concentrations of glyphosate in nodules 
 and roots of soybeans were low ( < 200 ng g nodule 
 tissue ) , although shikimate and hydroxybenzoic acids were present 
 in three - to 4-fold greater concentrations , indicating inhibition of b. japonicum epsps . among 
 strains of b. japonicum , glyphosate tolerance in 
 culture multiple field studies show no effect of 
 glyphosate on gr - soybean yield . using 
 differences in natural abundance of n , bohm et al . estimated the percentage of soybean n derived 
 from fixation to be 80% for gr soybean without glyphosate , 57% for 
 the same cultivar with one glyphosate application , and 66% after two 
 applications . suggested that the glyphosate - treated soybeans obtained 
 more reduced n from the soil . these effects on 
 nodulation and n fixation may be due in part to the inhibitory effects 
 of glyphosate on b. japonicum , but may also be related 
 to gr cultivar responses to glyphosate . additional evidence of cultivar 
 variability was found in a field study using 20 gr soybean cultivars 
 with and without glyphosate applied at four combinations of rates 
 and timings . of the 20 cultivars , 9 
 showed no difference in nodule biomass compared to the unsprayed treatment . one gr 
 cultivar , brs 244 rr , which had no glyphosate effect on nodule biomass 
 in this study , was reported to have reduced nodulation after glyphosate 
 application in a subsequent study . the 
 survival of b. japonicum in soil without plants was 
 not affected by concentrations equivalent to 1x or 10x field application 
 rates of glyphosate . selection or construction 
 of gr b. japonicum would be an effective strategy 
 for alleviating negative effects on nodulation and n fixation . the glyphosate metabolite ampa can temporarily reduce chlorophyll 
 content ( causing yellowing or chlorosis ) and photosynthesis in gr 
 soybeans , particularly after foliar applications of ampa at 1.0 kg 
 ha or high rates of glyphosate . this rate was chosen to represent the complete metabolism of a glyphosate 
 application to ampa . this rate of ampa did not affect nodulation or 
 nitrogenase activity , suggesting that b. japonicum is less sensitive to ampa than soybeans . the responses of gs cultivars 
 were similar to the gr cultivars , which are explained by the fact 
 that ampa does not affect epsps . arbuscular mycorrhizal fungi 
 are obligate symbionts that transfer mineral nutrients to their plant 
 hosts . savin et al . evaluated glyphosate effects on arbuscular mycorrhizal fungi ( amf ) 
 colonization of gr cultivars of cotton , corn and soybean grown in 
 soil under greenhouse conditions . amf colonization of roots was not 
 affected by glyphosate , and neither were acid nor alkaline phosphatase 
 soil enzyme activities . other research has shown that in the tripartite 
 symbiosis of mycorrhiza , rhizobium , and soybean , no adverse effects 
 of glyphosate use on gr cultivars was observed . these studies indicate that effects of glyphosate on plant 
 mineral nutrition through effects on amf are unlikely . plants use a variety of preformed and postinfection - induced 
 defenses 
 to resist pathogens . these include phenolic compounds which are considered 
 to be major components of defense across the plant kingdom . phenolic compounds may act in defense as preformed antibiotics , 
 pathogen - induced phytoalexins , or as structural barriers in the form 
 of lignin . thus , it is not surprising that any alteration in phenolic 
 metabolism may have an impact on the expression of disease . for example , 
 treatment with inhibitors of phenylalanine ammonia lyase have been 
 shown to enhance disease susceptibility ( e.g. , holliday and keen ) , while treatment with compounds such as microbial 
 elicitors or even herbicides such as 
 the protoporphyrinogen oxidase ( ppo ) inhibitor lactofen can stimulate 
 accumulation of phenolic compounds and enhance disease resistance . because glyphosate inhibits epsps , a 
 key enzyme in the shikimic 
 acid pathway , it also inhibits the biosynthesis of phenyalanine - derived 
 phenolic compounds and should also result in lack of synthesis of 
 salicylic acid from isochorismate . hence , 
 this herbicide may enhance susceptibility to diseases in plants that 
 are susceptible to glyphosate . in addition to phenolic compounds , 
 glyphosate should also prevent synthesis of anthranilic acid , an intermediate 
 needed for the synthesis of the indole - based glucosinolates and phytoalexins 
 in crucifers and the avenalumin phytoanticipins and avenanthramide phytoalexins in oats . finally , many plants respond to infection 
 by strengthening their cell walls with hydroxyproline - rich glycoproteins . because these proteins contain a significant 
 amount of tyrosine , and formation of isodityrosine cross - links is 
 important in cell wall reinforcement , it would seem likely that glyphosate 
 may also impact this aspect of plant defense . however , the effect 
 of glyphosate on these nonphenylpropanoid based defenses has not been 
 reported . a summary of possible effects of glyphosate on plant defenses 
 derived from the shikimic acid pathway in shown in figure 8 . based on both known and potential effects of glyphosate 
 on disease defense compounds , it is not surprising to find reports 
 in the published literature that show the disease - enhancing effects 
 of glyphosate on gs plants ( e.g. , reviewed in johal and huber and duke et al . ) . possible effects of glyphosate treatment of glyphosate - senstive 
 plants on shikimic acid pathway metabolites considered to be important 
 in defense . products of the shikimic acid pathway involved in plant 
 defenses are outlined by boxes . metabolites and metabolic groups in 
 red have been demonstrated or are hypothesized to be reduced in gs 
 plants after glyphosate treatment . of these , only isoflavonoid phytoalexins 
 from bean and soybean and lignin deposition in 
 bean have been examined for effects of glyphosate ( and only in gs 
 plants ) . protocatechuic acid has been demonstrated to increase in 
 gs plant tissue after glyphosate treatment . * * epsps : 5-enolpyruvylshikimic acid-3-phosphate synthase , the site 
 of glyphosate action . the effect 
 of glyphosate on disease resistance of gs plants was initially reported 
 by keen and co - workers in 1982 . they 
 reported that treatment of gs soybean hypocotyls with glyphosate decreased 
 the resistance to phytophthora megsaperma and reduced 
 the accumulation of the isoflavonoid phytoalexin glyceollin . in a 
 subsequent study , ward ( 193 ) confirmed the 
 results of keen et al . and also showed 
 that glyphosate also reduced the efficacy of metalaxyl , a fungicide 
 specific for oomycetes . the resistance breaking effect of glyphosate 
 has also been tested in two gs bean pathosystems . pretreatment of 
 bean hypocotyls with glyphosate resulted in only a subset of the plants 
 becoming more susceptible to infection with an incompatible ( avirulent ) 
 race of colletotrichum lindemuthianum . the increase in susceptibility was associated 
 with a decrease in phytoalexin production , but it is important to 
 note that the response was not uniform . a much different result was 
 found in the bean  pythium interaction . in this case , this 
 change in host reaction was associated with reduced accumulation of 
 phenolic phytoalexins and deposition of lignin . although treatment of gs plants with glyphosate can 
 result in increased 
 susceptibility to pathogens , it is important to know if gr crops can 
 be predisposed to susceptibility by treatment with glyphosate . biochemically , 
 it would seem unlikely that gr plants would become more susceptible 
 after glyphosate treatment , but is that the case ? johal and huber and kremer and means reviewed glyphosate effects on gs and gr cultivars and suggested 
 that fungal root diseases were increased by the adoption of gr - cultivars 
 and the increased use of glyphosate . several 
 studies have addressed whether or not gr plants are more susceptible 
 to disease , with much of the work focusing on gr soybeans . tested two near - isogenic lines of 
 soybean gl2415 ( gs ) and gl2600rr ( gr ) for susceptibility to sclerotinia sclerotiorum , the cause of the sclerotinia stem 
 rot or white mold disease . using a detached leaf assay , there was 
 no significant difference in lesion development in nontreated gl2415 
 as compared to gl2600rr . the formulation blank for the glyphosate 
 product used in the work also had no effect on disease . most important 
 was the observation that treatment of gl2600rr with three different 
 rates of glyphosate did not increase the severity of disease in that 
 line as compared to the untreated controls for both gl2600rr and the 
 gs line gl2415 . thus , the conclusions for this work were that the 
 gr gene had no impact on disease and treatment of the gr plants with 
 glyphosate did not enhance susceptibility . nelson et al . provided further evidence that the gr trait 
 did not impact host reaction to s. sclerotiorum in 
 field studies . comparing four lines of soybean that were near - isogenic 
 for the gr trait , they found that , with one exception , the glyphosate 
 resistance trait had no effect on disease reaction . glyphosate treatment 
 of two gr lines increased disease as compared to the untreated , while 
 the opposite effect was observed with two others . however , the two 
 gr lines that showed increased disease after glyphosate treatment 
 and their near - isogenic lines were significantly more susceptible 
 to sclerotinia as compared to the two other lines 
 in which glyphosate had no effect on disease . interestingly , treatment 
 of the two most white mold susceptible gr lines with another soybean 
 herbicide ( thifensulfuron ) resulted in enhanced disease development 
 comparable to the plants treated with glyphosate . lactofen , a ppo 
 inhibitor known to induce resistance to s. sclerotiorum , was able to reduce disease severity 
 in all lines regardless of the presence or absence of the gr gene . perhaps most significant was the observation that glyphosate treatment 
 of gr lines had no effect on yield regardless of the amount of disease . also addressed the issue 
 of cultivar differences in sclerotinia stem rot susceptibility by 
 examining management options . this work further illustrated that issues 
 related to greater susceptibility of gr soybeans was not related to 
 glyphosate resistance trait , but rather to the susceptibility of the 
 cultivars that were used for transformation . a lack of impact 
 on defense responses in gr soybean was supported 
 by analysis of glyceollin accumulation in resistant as compared to 
 susceptible plants . using silver nitrate 
 as an elicitor colonization increased from 20 to 30 infections 
 per 100 cm of untreated soybean root to as little as 30 infections 
 or as much as 120 infections per 100 cm root , depending upon glyphosate 
 dose and soybean growth stage and cultivar . in the same studies , decreases in populations of pseudomonas spp . and indole acetic acid ( iaa)-producing bacteria , as well as 
 a reduction in the ratio of mn - reducing to mn oxidizing microorganisms 
 were observed . fluorescent pseudomonas populations 
 in the gr - rhizosphere were decreased by glyphosate application and 
 negatively correlated with fusarium root colonization . these fusarium infections 
 of soybeans roots developed from soil - borne inoculum , and the species 
 distribution of fusarium was not determined . the mechanisms of these glyphosate - mediated increases in fusarium root infection in gr soybeans are not established . results of studies on translocation of c - glyphosate from 
 treated leaves into roots and rhizospheres indicate that beneficial 
 microorganism - inhibiting glyphosate concentrations could occur . kremer 
 et al . showed that root exudates in 
 general increased from glyphosate - treated plants grown in soil - free 
 conditions . however , these studies did not establish that these root 
 exudates specifically stimulated the growth of fusarium spp . still , glyphosate - mediated changes in quantity and quality 
 of root exudates into the rhizosphere has not been sufficiently evaluated 
 as an influence on plant disease . the effects of glyphosate 
 and glyphosate resistance in relation 
 to sudden death syndrome ( sds ) , in soybean caused by fusarium 
 virguliforme ( formerly fusarium solani f. 
 sp . glycines ) has also been examined . sanogo et al . reported on the effects of glyphosate and two 
 other soybean herbicides ( lactofen and imazethapyr ) on sds response 
 in two gr soybean lines ( pioneer 9344 and asgrow 3701 ) and one gs 
 line ( bsr101 ) . two of the lines , pioneer 9344 and bsr 101 , are susceptible 
 to sds while the other line was noted by the authors as having  above 
 average tolerance  . in growth chamber tests , the foliar symptom 
 severity of glyphosate - treated plants was no different than the untreated 
 control or plants treated with imazethapyr . , the severity of foliar and root 
 symptoms of sds was increased by both glyphosate and imazethapyr ( with 
 the exception of foliar severity in bsr 101 in which glyphosate treatment 
 resulted in no difference from the control ) . these results suggested 
 that the glyphosate resistance trait did not impact sds response and 
 that all three lines reacted to infection by fusarium in a similar manner after herbicide treatments . later reported on the 
 field reaction of the same three soybean lines to f. virguliforme and treatment with the same three herbicides plus acifluorfen . they 
 examined both foliar symptoms and frequency of fusarium isolation from roots , and found no significant cultivar - herbicide 
 interaction . there was also a lower amount of disease in the more 
 resistant line regardless of herbicide type as compared to the two 
 susceptible lines , and treatment with glyphosate , acifluorfen and 
 imazethapyr all increased disease severity in susceptible lines as 
 compared to controls . lactofen , in general , had no effect on disease 
 severity compared to the controls . the authors concluded that there 
 was no change in host resistance to sds as a result of glyphosate 
 treatment . the effects of glyphosate treatments on sds were 
 examined in ten 
 gr soybean lines from a variety of maturity groups . in work similar that of sanogo et al . , njiti et al . reported no effect of 
 glyphosate treatment on yield , foliar symptoms , or root infection . the overall conclusion from this study is that the host genotype , 
 and not glyphosate resistance or treatment with glyphosate , was the 
 most important factor in determining the reaction of a cultivar to 
 sds . found 
 that 
 glyphosate sprayed on mixed populations of weed species caused increased fusarium spp . infection in some weed species , but not in 
 others . the number of colony - forming units of fusarium spp . per gram of dried soil increased after application of glyphosate , 
 but gs crops ( corn , pea , cucumber , and bean ) subsequently grown on 
 in the field were not affected . powell and swanton concluded that there was insufficient evidence to prove 
 a link between glyphosate and plant diseases associated with fusarium spp . harikrishnan and yang et al . examined 
 the effect of glyphosate and other herbicides on reaction of bsr 101 
 ( gs ) and pioneer 93b01 ( gr ) to rhizoctonia solani . in greenhouse studies , the severity of rhizoctonia infection in autoclaved soil was not increased by glyphosate in 
 pioneer 93b01 compared to the inoculated control . in fact , statistically 
 similar levels of severity were also observed after imazethapyr treatment . 
 in nonautoclaved soil , glyphosate actually reduced the severity of rhizoctonia as compared to plants that were inoculated but 
 not treated with a herbicide . in two years of field trials , glyphosate 
 treatment of pioneer 9344 resulted in no difference in response to rhizoctonia infection based on shoot dry weight , rhizoctonia severity and plant stand . gr soybeans 
 are a rotation crop with cereals , and recent reports have suggested 
 that glyphosate treatment of soybeans increases the occurrence of 
 fusarium head blight of wheat and barley . because of these observations , brub et al . tested the effects of tillage and glyphosate 
 treatment of gr soybean on fusarium head blight development in a subsequent 
 planting of wheat and barley . there was no measurable effect of treating 
 gr soybeans with glyphosate on development of head blight and accumulation 
 of mycotoxins in barley or wheat . in sugar beet , gr varieties were tested 
 for the effects of glyphosate treatment on expression of disease caused 
 by rhizoctonia solani and fusarium oxysporum f. sp . betae . inoculation 
 with r. solani isolate r-1411 ( ag-4 ) resulted in 
 comparable amounts of disease in both b4rr and h16 whether or not 
 they were treated with glyphosate or a surfactant . however , inoculation 
 with r. solani r-9 ( ag-2 - 2 ) revealed that glyphosate 
 treatment resulted in increased disease in b4rr as compared to h16 . inoculation with f. oxysporum isolate fob13 
 resulted in increased disease in glyphosate - treated b4rr and h16 as 
 compared to nontreated controls . there was no effect of glyphosate 
 treatment on infection of the two sugar beet lines by f. oxysporum isolate f19 . in a two year field study with gr sugar beet , barnett 
 et al . reported that glyphosate had 
 no effect on expression of rhizoctonia crown and root rot in four 
 gr lines ( hilleshg 9027rr , hilleshg 9029rr , hilleshg 
 9028rr and crystal ) . they also reported that glyphosate treatments 
 did not impact efficacy of the fungicide azoxystrobin . using field 
 and greenhouse evaluations , a follow - up study by barnett et al . confirmed that glyphosate treatment of gr lines 
 had no effect on reaction to rhizoctonia . their recommendation to 
 growers was to use gr sugar beet varieties with the greatest amount 
 of rhizoctonia resistance . two wheat lines 
 that were near - isogenic for glyphosate resistance were tested for 
 the effect of glyphosate on disease caused by rhizoctonia 
 oryzae , r. solani , pythium ultimum and gaeumannomyces graminis var . the gr lines were not more susceptible 
 to any of these pathogens than the lines from which they were derived . however , this study reported that volunteer gs wheat , 
 if killed by a foliar treatment with glyphosate resulted in increased 
 infection by r. solani and g. graminis var . tritici , possibly as a result of increased 
 amounts of pathogen inoculum produced in the crop residue . the 
 reaction of gr cotton seedlings to rhizoctonia solani after treatment with several pre- emergent herbicides and glyphosate 
 as a foliar treatment was tested in field and greenhouse experiments . glyphosate applied at the cotyledon or four 
 leaf stage of gr cotton reduced rhizoctonia infection of hypocotyls 
 in the field . in greenhouse studies , several pre - emergent herbicides 
 predisposed cotton seedlings to greater hypocotyl infection by r. solani , but subsequent application of glyphosate did 
 not increase severity of the disease . baird et al . found that four varieties of gr cotton ( pm 1220 , dpl 5690 , 
 dpl 5415 , and dpl 50 ) had similar seedling stand count , height , and 
 dry weight when compared to gs varieties from the same lineage group , 
 regardless of glyphosate application . when differences did occur , 
 no consistent trends could be determined within the lineage groups 
 tested . glyphosate was shown 
 to have both preventive and curative activities against both stripe 
 rust ( puccinia striiformis f. sp . tritici ) and leaf rust ( puccinia triticina ) on gr wheat . in these cases , it appears that 
 glyphosate is acting directly as a fungicide . some efficacy against phakopsora pachyrhizi , the cause of asian soybean rust , 
 was reported in both greenhouse and 
 in the field on gr soybeans . tuffi santos 
 at al . showed that glyphosate reduced 
 the severity of rust caused by puccinia pdisii on eucalyptus grandis . they found that there was a systemic 
 effect of glyphosate on rust development as illustrated by reduced 
 urediniospore germination and appressorium formation on tissues that 
 were not directly treated with the herbicide . similar to soybean , 
 glyphosate has recently been reported to protect gr alfalfa against 
 the rust uromyces striatus when applied prior to 
 or up to 10 days after inoculation . in 
 this study , these latter two results are interesting as these pathogens , unlike 
 biotrophic rusts , are hemibiotrophic and necrotophic in their attack 
 of their hosts . holliday and keen examined the effect of glyphosate on the response 
 of gs soybean leaves to the bacterial pathogen pseudomonas 
 syringae pv glycinea . in this case , the 
 effect of glyphosate on resistance was less conclusive . although glyphosate 
 treatment significantly decreased glyceollin accumulation , it had 
 no effect on the expression of the hypersensitive response . glyphosate 
 treatment also resulted in only a relatively small increase in bacterial 
 growth in the treated plants . this suggests that in gs plants resistance 
 to bacterial blight is not greatly reduced after treatment with glyphosate . several hundred gr 
 soybean lines were screened for resistance to bacterial pustule , caused 
 by xanthomonas axonopodis pv glycines . the authors report that resistance 
 to the disease occurs in gr soybeans , but that not all genotypes were 
 resistant . although they did not test the effect of glyphosate on 
 the host response to xanthomonas , the authors did recommend that growers 
 assess the risk for this disease and plant resistant cultivars when 
 the disease is likely to occur . goss s wilt and 
 leaf blight of corn , caused by the gram positive bacterium clavibacter michiganensis subsp . nebraskensis , has increased over the last 5 years in corn - producing states , as has increased planting of gr corn ( figure 1 ) . there are some logical explanations for the recent increase in 
 the occurrence of goss s wilt and leaf blight in corn growing 
 areas that do not implicate the use of gr corn or glyphosate . . 
 both of these practices will allow the buildup of pathogen inoculum 
 over time , and reduced tillage practices allow the pathogen to survive . reduced tillage practices that reduce residue decomposition will also 
 increase pathogen inoculum , although one of the perceived benefits 
 of gr crops has been the ability to manage weeds in reduced tillage . another factor that may contribute to increased disease development 
 are reduced efforts to select for resistant hybrids and/or failure 
 to promote resistant hybrids by seed companies . finally , weather events 
 ( such as early season hail damage ) will promote infection in even 
 the most resistant hybrids . nebraskensis genotypes 
 might also be a factor , but further work is needed to determine if 
 this has occurred . there was no mention 
 in any of the recently published reports that the gr trait or glyphosate 
 application is a contributing factor to the increase in goss s 
 wilt . the report by ruhl et al . noted 
 that the first indiana finds were on both field corn and popcorn . since popcorn is not gr , this would further implicate other factors 
 in the recent outbreaks of the disease . considering that most corn 
 produced in the us is now gr ( figure 1 ) , it is likely that inoculum buildup and use of 
 gr corn that is not resistant to goss s wilt are the reasons 
 for increases in this disease . in addition , there are no reports in 
 the published literature that suggest that glyphosate resistance or 
 treatment of gr varieties with the herbicide will increase the risk 
 of other diseases in this crop . yang et al . examined the effect of glyphosate on soybean cyst nematode 
 ( scn , heterodora glycines ) infection of the gr and 
 scn - resistant variety countrymark 316 . greenhouse tests demonstrated 
 no effect of glyphosate on scn development on this genotype as compared 
 to untreated controls . noel and wax compared 
 the reactions of gr soybean lines dr 320 ( scn susceptible ) and dsr 
 327 ( scr resistant ) to glyphosate treatment and inoculation with h. glycines . they reported that glyphosate did result in 
 increased numbers of the nematode on the susceptible line , but not 
 the resistant line . even with the increase in nematode populations , 
 there was no impact on yield . this study , like those with other soybean 
 diseases , suggests that genotypic resistance or susceptibility , rather 
 than glyphosate resistance , is the most important factor related to 
 disease severity . although 
 it is clear that glyphosate does increase severity of disease on gs 
 plants , the published evidence for its effects on gr plants presents 
 a different story . overall , it appears that in gr crops the baseline 
 disease resistance or susceptibility of the host plant , not the presence 
 of the glyphosate resistance gene or treatment with glyphosate , is 
 the major contributor to susceptibility . in the u.s . , gr 
 soybeans , cotton , and corn were introduced in 1996 , 
 1997 , and 1998 , respectively . adoption of the crops has been rapid 
 and overwhelming , with more than 90% of soybeans , ca . 80% of cotton , 
 and about 70% of corn currently grown being gr ( figure 1 ) . after the introduction of gr sugar beets in 2008 , the adoption 
 rate was essentially 100% in 2009 . thus , one might expect that 
 if there were any significant mineral 
 nutrition and/or disease problems with these crops , the problems would 
 be manifested in yield reductions and farmer dissatisfaction . yield 
 data from the years before introduction of gr crops , continuing to 
 the present show that the same yield trends before introduction continued 
 after introduction ( e.g. , figure 9 ) . while 
 there could be isolated pockets of adverse effects of glyphosate on 
 gr crops that would be masked by their general success , such cases 
 have not been conclusively documented . there were initial concerns 
 with transgenic crops in general that there would be  yield 
 drags  due to factors not associated with disease or mineral 
 nutrition , but to suboptimal cultivars and potential pleiotrophic 
 effects of the transgenes . these problems 
 have not materialized . to summarize , yield data for crops that are 
 now predominantly gr cultivars do not support the view that there 
 are significant mineral nutrition or disease problems with gr crops . yields 
 of the three crops over the past 30 years that are 
 now grown mostly as gr cultivars . data are from the usda , national 
 agricultural statistics service data and statistics web site : http://www.nass.usda.gov/data_and_statistics/quick_stats/ ( accessed 
 september 12 , 2012 ) . scientific accounts about increased plant disease 
 and mineral nutrition 
 problems in gr crops are based on publications from a limited number 
 of researchers . in the context of the entire body of relevant science still , considering 
 the enormous importance of and reliance on gr crops and glyphosate , 
 there has been a paucity of publically funded research into potential 
 problems with this weed management technology . farmers have generally 
 embraced this technology , so that there has been no widespread call 
 for studies of potential problems with gr crops other than those associated 
 with gr weeds , a growing problem that is well documented . furthermore , 
 publication of negative ( no effect ) results is generally unattractive 
 to journals , and , therefore , to scientists whose success depends on 
 publications . so , the  no effect  papers that have been 
 published may not represent all such data that have been generated . reports of significant adverse effects of glyphosate on mineral nutrition 
 and diseases of gr crops are perplexing in light of the considerable 
 body of literature and yield data that contradict such claims . nevertheless , 
 there might be effects of glyphosate in gr crops on mineral nutrition 
 and/or disease under particular but uncommon conditions ( e.g. , specific 
 soil , environmental conditions , particular gr crop cultivars , and/or 
 glyphosate formulations ) .
claims have been made recently that glyphosate - resistant 
 ( gr ) crops 
 sometimes have mineral deficiencies and increased plant disease . 
 this 
 review evaluates the literature that is germane to these claims . 
 our 
 conclusions are : ( 1 ) although there is conflicting literature on the 
 effects of glyphosate on mineral nutrition on gr crops , most of the 
 literature indicates that mineral nutrition in gr crops is not affected 
 by either the gr trait or by application of glyphosate ; ( 2 ) most of 
 the available data support the view that neither the gr transgenes 
 nor glyphosate use in gr crops increases crop disease ; and ( 3 ) yield 
 data on gr crops do not support the hypotheses that there are substantive 
 mineral nutrition or disease problems that are specific to gr crops .