great progress has been made in broiler production over the past few decades , and about 85 to 90% of the increase in productivity is attributed to genetics ( chambers , 1995 ) . broilers are mainly selected by certain performance traits , such as body - weight at marketing age . however , intense selection for rapid growth has been accompanied by negative consequences , including skeletal problems and increased carcass fat deposition ( burt , 2002 ) . even though difficult and costly ( ikeobi et al . , 2002 ; , phenotype - based selection methods for reducing fat deposition have improved carcass composition by reducing overall fatness . though this has been in detriment of meat - quality ( nadaf et al . , 2007 ) . the identification of markers and genes that control phenotypic variation in quantitative traits remains a major challenge . quantitative trait locus ( qtl ) mapping is a method that has been used successfully to examine genetic contributions to certain quantitative traits , by correlating allelic variation in polymorphic genetic markers with trait variability ( tercic et al . , 2009).the identification of markers linked to genomic regions that regulate economically important traits could be useful in marker assisted selection . qtl mapping is the first stage in identifying chromosomal regions involved in the genetic control of traits that are economically important for the chicken industry . the genetic correlations between traits can provide information on how much genetic overlap exists between traits , and support qtl findings ( nones et al . , 2005 ) . the favorable progress in chicken performance is associated with an increase in fat deposition , possibly due to positive genetic correlations between body - weight and fat production traits ( zerehdaran et al . , 2004 ) . according to leenstra et al . ( 1986 ) , the application of body weight as a selection criterion has favored chickens with larger appetites . these eat more than required for muscle development , growth and maintenance , whereby this excessive energy is accumulated in the form of fat ( lin , 1981 ; leenstra et al . , 1986 ; havenstein et al . , carcass chemical composition traits , such as protein , fat and ash , are of great economic importance for the poultry segment , because they are directly related to the efficiency in meat production . genetic parameter estimates for these traits have been rarely reported , mostly due to the need for evaluating a large number of animals , so as to obtain reliable var - covariance component estimates . in this study we estimated genetic parameters for protein , fat , ash , and moisture in the carcass , as well as body - weight and carcass yield , in an f2 population designed to map quantitative trait loci ( qtl ) on a population of more than 3,000 chickens . this information , essential for a better understanding of the genetic architecture of carcass chemical composition in chickens , could possibly contribute to the development of new selection strategies in future broiler - breeding programs . two f2-chicken - resource populations ( tctc and ctct ) were developed at the embrapa swine and poultry national research center , brazil , by reciprocally crossing a broiler ( tt ) and layer ( cc ) lines , so as to produce populations with segregating qtl for performance and carcass traits . tt is a broiler male - line , with a genetic background of cornish , hampshire and white plymouth rock breeds , selected to improve body - weight , feed conversion , retail - cut yield , breast - weight , viability , fertility , hatchability , and reduced abdominal fat . cc is a pure white leghorn line selected for egg production , egg - weight , feed conversion , hatchability , sexual maturity , fertility , viability , egg quality , and reduced body weight . when the f2 population was developed , the tt and cc lines involved had already undergone multi - trait selection for six and eight generations , respectively . seven tt males were mated to seven cc females to generate seven f1 tc families , totalizing about 50 chickens , and seven cc males were mated to seven tt females , giving rise to seven f1 ct families of about 50 chickens , as well . seven tc males and 21 tc females , and seven ct males and 21 ct females ( in both cases one male and three females from each full - sib family ) , were chosen at random to be parents of the f2 population . the f1 chickens chosen to generate the f2 population were reared as broiler - breeders . each f1 male was mated to three non - related f1 females by artificial insemination to avoid inbreeding . each f1 female produced around 100 f2 offspring in 17 hatches over eight months in 1999 and 2000 , resulting in around 4,000 f2 individuals ( half of each sex and each mating  ctct and tctc ) . carcass - related traits were recorded for 3,422 ( 1,673 females and 1,749 males ) . the f2 chickens , reared as broilers , received food and water ad libitum . a commercial broiler diet containing a metabolizable energy concentration ( me ) of 3,150 kcal / kg and 21% crude protein ( 1 to 21 days ) , 3,200 kcal me / kg and 20% crude protein ( 22 to 35 days ) , and 3,200 kcal me / kg and 18.5% crude protein ( 36 to 41 days ) was provided . the body - weight of live chickens was recorded at 6 weeks of age ( bw6 ) . carcass - weight was measured after removing the feathers , feet , head , lungs , liver , kidneys , and gastrointestinal tract . whole eviscerated carcasses were ground as frozen , homogenized and then returned to 20 c until composition analysis . carcass water , protein , fat and ash contents were measured by near - infrared reflectance spectroscopy ( nirs ) . carcass chemical composition was measured in a 250 g sample ( ground and homogenized ) using a cell type coarse in an nir system 6500 ( silver spring , md , usa ) . calibration curves for nirs measurements were establisheded based on chemical analyses of 100 carcasses from various crosses and lines , as described by klein et al . dry matter , protein , fat and ash contents were determined in compliance with the standards of the association of official analytical chemists ( aoac , 1995 ) . klein et al . ( 2003 ) , on comparing laboratory results and nirs estimates from 19 carcasses , obtained coefficients of 0.88 , 0.96 and 0.95 for protein , fat and water , respectively . carcass fat ( ether extract ) , protein , ash content and moisture were estimated as percentages of sample - weight ( 250 g of ground and homogenized carcass ) . thus , carcass fat content ( in grams ) was obtained by multiplying the percentage of fat in the sample by bw6 . to avoid a possible undesirable effect of carcass water content , the fat , protein and ash contents were also expressed as dry matter ( % ) , by dividing sample fat content by carcass dry - matter content ( % ) and multiplying by 100 . these traits were reported as protein percentage of wet carcass ( ppwc ) , fat percentage of wet carcass ( fpwc ) , ash percentage of wet carcass ( apwc ) , protein percentage of dry matter ( ppdm ) , fat percentage of dry matter ( fpdm ) , and ash percentage of dry matter ( apdm ) . the normality of the residuals was verified for each trait , and records for which the standardized residual was higher than 3.5 or lower than 3.5 standard deviations were excluded . least - squares analysis using the glm procedure of sas ( sas 9.1 , sas institute , cary , nc , usa ) was used to define the fixed effects to be included in the mixed model . animals of the same sex , hatch and genetic group ( f2 tctc and f2 ctct ) were considered , when contemplating fixed effects in the contemporary group ( cg ) . the variance - covariance estimates and genetic parameters were obtained by the restricted - maximum - likelihood method , using an animal model and mtdfreml software ( boldman et al . , 1995 ) . the single - and two - trait animal models included the cg fixed - effect and random additive genetic and residual effects . the total number of animals in the numerator relationship matrix , including base animals , was 4,237 . two f2-chicken - resource populations ( tctc and ctct ) were developed at the embrapa swine and poultry national research center , brazil , by reciprocally crossing a broiler ( tt ) and layer ( cc ) lines , so as to produce populations with segregating qtl for performance and carcass traits . tt is a broiler male - line , with a genetic background of cornish , hampshire and white plymouth rock breeds , selected to improve body - weight , feed conversion , retail - cut yield , breast - weight , viability , fertility , hatchability , and reduced abdominal fat . cc is a pure white leghorn line selected for egg production , egg - weight , feed conversion , hatchability , sexual maturity , fertility , viability , egg quality , and reduced body weight . when the f2 population was developed , the tt and cc lines involved had already undergone multi - trait selection for six and eight generations , respectively . seven tt males were mated to seven cc females to generate seven f1 tc families , totalizing about 50 chickens , and seven cc males were mated to seven tt females , giving rise to seven f1 ct families of about 50 chickens , as well . seven tc males and 21 tc females , and seven ct males and 21 ct females ( in both cases one male and three females from each full - sib family ) , were chosen at random to be parents of the f2 population . the f1 chickens chosen to generate the f2 population were reared as broiler - breeders . each f1 male was mated to three non - related f1 females by artificial insemination to avoid inbreeding . each f1 female produced around 100 f2 offspring in 17 hatches over eight months in 1999 and 2000 , resulting in around 4,000 f2 individuals ( half of each sex and each mating  ctct and tctc ) . carcass - related traits were recorded for 3,422 ( 1,673 females and 1,749 males ) . the f2 chickens , reared as broilers , received food and water ad libitum . a commercial broiler diet containing a metabolizable energy concentration ( me ) of 3,150 kcal / kg and 21% crude protein ( 1 to 21 days ) , 3,200 kcal me / kg and 20% crude protein ( 22 to 35 days ) , and 3,200 kcal me / kg and 18.5% crude protein ( 36 to 41 days ) was provided . the body - weight of live chickens was recorded at 6 weeks of age ( bw6 ) . carcass - weight was measured after removing the feathers , feet , head , lungs , liver , kidneys , and gastrointestinal tract . whole eviscerated carcasses were ground as frozen , homogenized and then returned to 20 c until composition analysis . carcass water , protein , fat and ash contents were measured by near - infrared reflectance spectroscopy ( nirs ) . carcass chemical composition was measured in a 250 g sample ( ground and homogenized ) using a cell type coarse in an nir system 6500 ( silver spring , md , usa ) . calibration curves for nirs measurements were establisheded based on chemical analyses of 100 carcasses from various crosses and lines , as described by klein et al . dry matter , protein , fat and ash contents were determined in compliance with the standards of the association of official analytical chemists ( aoac , 1995 ) . klein et al . ( 2003 ) , on comparing laboratory results and nirs estimates from 19 carcasses , obtained coefficients of 0.88 , 0.96 and 0.95 for protein , fat and water , respectively . carcass fat ( ether extract ) , protein , ash content and moisture were estimated as percentages of sample - weight ( 250 g of ground and homogenized carcass ) . thus , carcass fat content ( in grams ) was obtained by multiplying the percentage of fat in the sample by bw6 . to avoid a possible undesirable effect of carcass water content , the fat , protein and ash contents were also expressed as dry matter ( % ) , by dividing sample fat content by carcass dry - matter content ( % ) and multiplying by 100 . these traits were reported as protein percentage of wet carcass ( ppwc ) , fat percentage of wet carcass ( fpwc ) , ash percentage of wet carcass ( apwc ) , protein percentage of dry matter ( ppdm ) , fat percentage of dry matter ( fpdm ) , and ash percentage of dry matter ( apdm ) . the normality of the residuals was verified for each trait , and records for which the standardized residual was higher than 3.5 or lower than 3.5 standard deviations were excluded . least - squares analysis using the glm procedure of sas ( sas 9.1 , sas institute , cary , nc , usa ) was used to define the fixed effects to be included in the mixed model . animals of the same sex , hatch and genetic group ( f2 tctc and f2 ctct ) were considered , when contemplating fixed effects in the contemporary group ( cg ) . the variance - covariance estimates and genetic parameters were obtained by the restricted - maximum - likelihood method , using an animal model and mtdfreml software ( boldman et al . , 1995 ) . the single - and two - trait animal models included the cg fixed - effect and random additive genetic and residual effects . the total number of animals in the numerator relationship matrix , including base animals , was 4,237 . descriptive statistics for traits evaluated in the 3,422 f2 individuals in this study are listed in table 1 . single - trait heritability estimates , as well as genetic and phenotypic correlations between the traits studied , are presented in table 2 . heritability estimates obtained from two - trait analysis ( results not shown ) were very similar to those of single - trait analysis . for all the traits , excepting cy , ppwc and apwc , everything indicated the large proportion of phenotypic variance as being attributable to additive genetic effects . the genetic association between bw6 and mp ( table 2 ) indicated that selection for higher live - weight would favor individuals with lower water accumulation in the carcass . in the case of bw6 , and both protein and ash , results were moderate and inversely proportional , whereas the genetic association between bw6 and fat , although also moderate , directly proportional , regardless of whether measurements were of the percentage of wet carcass weight or dry matter . a moderate genetic correlation was observed between cy and ppwc ( table 2 ) , but not between cy and mp , fpwc , fpdm , ppdm , apwc or apdm . thus , the genetic association of cy with others carcass components traits is expected to be low . the genetic association between mp and ppwc and between mp and ppdm was high and directly proportional ( table 2 ) . the same result was observed for ash content , with high and positive genetic correlation estimates between mp and apwc and between mp and apdm . in contrast , the genetic correlation estimates between mp and fpwc and between mp and fpdm were high and inversely proportional . genetic correlations between ppwc and ppdm , apwc and apdm , and fpwc and fpdm were high ( table 2 ) , thereby indicating that measurements reported as the percentage of both dry matter and wet carcass weight , could be useful for evaluating the percentage of each trait in the carcass . however , when the breeding values for each trait , expressed as dry matter and wet weight , were plotted ( figure 1a , 1b and 1c for protein , fat and ash , respectively ) , a firm association was only observed between apwc and apdm ( figure 1b ) and between fpwc and fpdm ( figure 1c ) . the genetic association between protein with ash content was high and directly proportional , whereas the genetic correlation between protein with fat , and ash with fat were high but inversely proportional ( table 2 ) . selection for higher carcass protein content would favor individuals with lower fat content and higher ash content . as observed for genetic correlations , the estimates of phenotypic correlations between ppdm and fpdm , as well as between fpdm and apdm were high in magnitude and negative in signsign . environmental correlation estimates were only important for mp and ppwc ( 0.59  0.04 ) and mp and apwc ( 0.23  0.05 ) . these were similar in magnitude but opposite in signs to the genetic correlations observed between the same traits , this inverse response thereby indicating that inherent environmental associations should be taken into account during analysis . the lower mean body - weight at 6 weeks ( table 1 ) was expected , when compared to the results reported for pure broiler lines , with means ranging from 1.7 to 2.3 kg ( schmidt et al . , 2003 ) , and for chickens from a cross between a high - reproductive female line and a high - growth male line , as reported by zerehdaran et al . , ( 2004 ) , who studied live weights at 49 days of age ( mean of 2  0.3 kg ) . although the mean for bw6 was low ( table 1 ) , we emphasize that this is a broiler - layer cross developed for qtl mapping . despite the possible inflation of genetic parameters for the traits studied ( table 2 ) , through the use of such divergent lines , the aim was to estimate parameters for describing genetic associations between performance and carcass composition traits , as a means for understanding their genetic architecture , and not for selection . furthermore , only few heritability estimates for these traits are reported in the literature . the observed cy mean ( table 1 ) was similar to those reported by schmidt et al . ( 2004 ) , which were 69.9% and 66.1% , respectively . in this study , means for mp , ppwc , fpwc and apwc were similar to those observed by washburn et al . ( 1975 ) and pesti and bakalli ( 1997 ) , ranging from 63.9 to 67.1% for mp , 18.1  0.5% for ppwc , 11.7% for fpwc , and from 2.5  0.6% to 3.3  0.1% for apwc . means for ppdm , fpdm and apdm were close to those obtained by chambers and fortin ( 1984 ) , which were 50.4  0.7% , 41.4  0.7% and 7.4  0.2% , respectively . perreault and leeson ( 1992 ) obtained a higher mean ether extract percentage of 47.7  0.7% measured on a dry matter basis , at 43 days of age , when compared to our result for fpdm . the findings herein compiled , when compared with the literature , indicated that the use of a light - weight layer line , in the reciprocal cross under study , in no way affected carcass yield , or carcass protein , fat , moisture and ash contents , expressed as percentages of wet carcass weight and dry matter . 2004 ) reported a similar genetic correlation estimate between live weight at 7 weeks of age and cy . siegel ( 1984 ) and becker et al . ( 1984 ) reported a genetic association between live weight and carcass ether extract , similar in magnitude to that encountered herein between bw6 and carcass fat content . carcass composition ( protein , fat and ash content ) expressed on a dry matter basis would be more appropriate for studying selection based on heritability estimates than the same traits measured as the percentage of wet carcass weight , since higher heritability estimates were obtained when carcass composition traits were expressed as percentage of dry matter . carcass composition on a dry matter basis is more precise , through not being influenced by the sample , whereas measurements expressed as wet carcass weight might be influenced by the amount of water . however , besides having been used to predict carcass fatness ( taylor and shaffner , 1975 ; verstrate et al . , 1980 ) , percentages of carcass moisture may also be an effective means of accurately predicting percentages of carcass fat and protein content ( chambers and fortin , 1984 ) . similar heritability estimates , when compared to ours , were found by le bihan - duval et al . ( 2004 ) , ( live - weight 0.33  0.07 and cy at 7 weeks 0.41  0.07 ) . no heritability estimates were found in the literature for comparison with ppwc , ppdm , fpdm , fpwc , apdm and apwc data obtained herein . based on the heritability estimates found , in chickens , selection by the reduced amount of total fat ( measured as the percentage of carcass dry matter or wet - carcass weight ) would expectedly be effective in terms of genetic gain . furthermore , the small magnitude of the standard errors of the heritability estimates is an indication of their reliability . selection for increased bw6 should result in a higher percentage of fat content and a lower one of protein and ash in the carcass . the genetic correlations observed between the carcass - composition traits studied herein , indicate that selection by proteins may conduce to a higher percent of ash content and lower one of carcass - fat accumulation . the moderate phenotypic correlation ( table 2 ) between bw6 and cy indicated that higher live - weights would not necessarily lead to higher cys ( abram and goodwin , 1977 ) . 2004 ) reported phenotypic correlation estimates between live weight at 7 weeks of age and cy of 0.22  0.03 , whereas vila et al . ( 1993 ) , when studying 50-day - old chickens , found a phenotypic correlation of 0.35 between bw6 and cy , similar to that observed herein . the phenotypic association between bw6 and mp ( table 2 ) indicated the possibility of a higher live - weight resulting in lower water accumulation in the carcass . pesti and bakalli ( 1997 ) reported the relationship between these traits to be 0.22 . an increase in bw6 will necessarily result in a higher percentage of fat content , and lower protein and ash contents in the carcass . with the exception of ppwc , the phenotypic associations between bw6 and carcass composition traits , here expressed as the percentage of wet carcass weight , were , in both direction and magnitude , similar to those reported by chambers and fortin ( 1984 ) , who encountered phenotypic correlations between live weight ( at 47 to 61 days of age ) and ppwc , fpwc and apwc , of 0.18 , 0.56 and 0.25 , respectively . siegel ( 1984 ) and becker et al . ( 1984 ) reported a phenotypic association between live weight and ether extract in the carcass whose magnitude was similar to that found in the present study , between bw6 and fat content in the carcass . furthermore , these correlations , as observed between bw6 and ppdm , fpdm and apdm were also similar to those noted by chambers and fortin ( 1984 ) between live weight and ppdm , fpdm and apdm of 0.47 , 0.53 and 0.46 , respectively . based on wet and dry matter , a negative phenotypic correlation between moisture and carcass lipid percent was estimated herein , as occurred with silva et al . ( 2003 ) who obtained a highly negative estimate ( r = 0.88 ; p < 0.0001 ) between the same two factors . although the correlation between mp and ppwc was negative , this was high and positive between mp and ppdm and mp and apdm , although very low between mp and apwc . chambers and fortin ( 1984 ) reported phenotypic associations of 0.92 ( protein and moisture ) and 0.72 ( ash and moisture ) , thus in agreement with the present results . chambers and fortin ( 1984 ) reported phenotypic associations of 0.95 between protein and fat , 0.80 between fat and ash , and 0.74 between protein and ash , in carcass dry matter , thus in agreement with the present results . although no linear relationship was observed between ppwc and fpwc , washburn et al . ( 1975 ) and chambers and fortin ( 1984 ) obtained estimates of 0.46 and 0.70 , respectively . although a positive phenotypic correlation was established between apwc and ppwc ( table 2 ) , this was negative between apwc and fpwc . similar results were reported by chambers and fortin ( 1984 ) with 0.42 between the former and 0.46 between the latter . in the present study , positive relationships were observed between fpwc and fpdm , ppwc and ppdm , and apwc and apdm , thus indicating a directly proportional linear association between these traits .
the objective of this study was to estimate genetic and phenotypic correlations of body weight at 6 weeks of age ( bw6 ) , as well as final carcass yield , and moisture , protein , fat and ash contents , using data from 3,422 f2 chickens originated from reciprocal cross between a broiler and a layer line . 
 variance components were estimated by the reml method , using animal models for evaluating random additive genetic and fixed contemporary group ( sex , hatch and genetic group ) effects . 
 the heritability estimates ( h2 ) for bw6 , carcass yield and percentage of carcass moisture were 0.31  0.07 , 0.20  0.05 and 0.33  0.07 , respectively . 
 the h2 for the percentages of protein , fat and ash on a dry matter basis were 0.48  0.09 , 0.55  0.10 and 0.36  0.08 , respectively . 
 bw6 had a positive genetic correlation with fat percentage in the carcass , but a negative one with protein and ash contents . 
 carcass yield , thus , appears to have only low genetic association with carcass composition traits . 
 the genetic correlations observed between traits , measured on a dry matter basis , indicated that selection for carcass protein content may favor higher ash content and a lower percentage of carcass fat .