The timing of tillage and seedbed preparation has large effects on the density of weeds in the subsequent crop. As explained in Chapter 2, most weed species germinate during specific periods of the year. If the tillage and seedbed preparation occur prior to the bulk of that period, most individuals will emerge after planting and potentially compete with the crop. In contrast, if planting is delayed until after most of the weeds have emerged then most can be eliminated by tillage. Three critical questions are addressed here. (1) What are the general rules regarding planting date for various phenological categories of crops and weeds.? (2) How does one determine the optimum seedbed preparation/planting date? (3) How should soil disturbance be manipulated to best exploit the seasonality of weed emergence?
Regarding question (1), the weeds that are most problematical for an annual crop are those that germinate around the time the crop is planted. Those that come up much earlier in the year will be eliminated during seedbed preparation, and those that emerge only much later in the season will not be competitive against the crop. The remaining species will be of two types: (i) those that germinate opportunistically in response to disturbance over a broad range of soil climatic conditions and (ii) seasonal species whose peak emergence occurs after the earliest practical planting date for the crop. A shift in planting date is not a viable management strategy for weeds of the first sort (Ghafar & Watson, 1983). The latter group of seasonal germinating species is thus the focus of this discussion. With regard to these, two strategies are possible.
First, if the crop is capable of emerging earlier in the season than most of the weed species in the seed bank, then planting the crop as early as possible may be a viable strategy. By the time the weeds are ready to germinate, the crop will have a competitive advantage due to its larger size, and if the crop casts sufficient shade, some weed germination and emergence may be prevented or at least further delayed. Temperate zone spring cereal grains have this sort of phenological relationship to their weeds, and although delayed planting may eliminate some spring germinating weeds (Spandl, Durgan & Forcella, 1998), it is often counterproductive (Deschenes & Dubuc, 1981; Légère, 1997). The competitive advantages of early planting of spring cereals are discussed further in Chapter 6.
A second strategy is to delay seedbed preparation and planting until after many of the weeds have germinated and are susceptible to destruction by soil disturbance. This approach appears to be most successful for spring-planted row crops, particularly maize and soybean, and for autumn-planted grains. Delaying planting from 25 April until 15 May in Wisconsin reduced in-row weed density in rotary hoed maize by an average of 55% (Mulder & Doll, 1994). However, late planting also reduced maize yield in one of two years. Similarly, late-planted soybean often has lower weed densities and lower percentage yield loss to weeds relative to early-planted soybean, but may suffer some reduction in yield due to shortened growing season (Weaver, 1986; Gunsolus, 1990; Buhler & Gunsolus, 1996). A substantial percentage of fall-germinating weeds, including Alopecurus myosuroides, Bromus secalinus, Avena ludoviciana, Veronica persica, and Lamium purpureum, can be eliminated by delaying the planting of winter wheat, but late planting often reduces yields (Moss, 1985a; Koscelny et al., 1991; Christensen, Rasmussen & Olesen, 1994; Singh et al., 1995; Cosser et al., 1997).
Delayed planting can also be useful for weed management in some spring-seeded vegetable crops (Wellbank & Witts, 1962), but will generally not be useful in summer-seeded vegetables. The latter are typically infested with weeds like Galinsoga ciliata and Portulaca oleracea that germinate over a wide range of dates in response to tillage. Note, however, that this type of weed is highly susceptible to stale and false seedbed techniques (see below).
In all the studies discussed above, the seasonal cycle was dominated by changes in the temperature regime. Whether similar planting date strategies exist for some crop-weed combinations in the wet-dry seasonal tropics remains to be determined.
Forcella and others have addressed question (2) regarding determination of the optimum planting date through a series of models that predict the percentage emergence of individual weed species based on soil temperature and moisture data (Forcella, 1993, 1998; Harvey & Forcella, 1993; King & Oliver, 1994; Wilen, Holt & McCloskey, 1996). The models predict relative weed pressure at successive potential planting dates as a proportion of the density that would occur without seedbed preparation. The time to plant occurs when the expected weed density falls below a threshold the grower believes he/she can control with the weed management tools available, or, alternatively, when the expected yield loss due to further delay in planting exceeds the expected yield loss due to weeds. A problem with these models is that they predict the percentage of ultimate emergence that has occurred by a given date rather than the actual density. The model of Reese & Forcella (1997) is presently paramat-erized for 15 weed species common in the central USA.
Forcella, Eradat-Oskoui & Wagner (1993) used a related approach to determine the optimal seeding date for maize and soybeans in the northern midwest of the USA. They computed two functions of yield versus planting date. One expressed weed-free yield as a function of growing degree-days, precipitation, and crop maturity group. The second expressed the relation of yield to weed density after planting for various planting dates. When graphed, the intersection of the two curves indicated the planting date corresponding to the maximum crop yield at a given weed density. The optimum planting date depended greatly on both weed species and density. The optimum date was early when the weed was an early-emerging species (e.g., Chenopodium album), and later when the weed emerged later in the season (Setaria spp.). In all cases examined, increase in weed density delayed the optimal seeding date. In an extreme case, increase in Setaria density in soybean from 40 to 200 m2 shifted the optimal planting date from 28 April to 7 June.
Thus, both empirical studies and models indicate that delayed planting to eliminate weeds from spring row crops and fall cereals generally reduces yields relative to the ideal situation of early planting in weed-free conditions. However, when weed control is imperfect, yield usually increases with delayed planting, and the optimum delay relative to weed-free conditions increases with the density ofger-minable seeds.
Question (3) above asked what sort of tillage systems work best with a delayed seeding approach to weed management. Note in this regard that the models of Forcella and his colleagues were developed in the context of a tillage regime with fall plowing and superficial seedbed preparation. If soil is mold-board plowed shortly before planting, then depletion of the surface seed bank by delayed tillage is largely irrelevant; seeds previously in enforced dormancy will be brought to the surface, and many of these will subsequently germinate and infest the crop. Thus, delayed planting works best when deep tillage is avoided or occurs well in advance of a shallow seedbed preparation.