The genetics of body weight and feed efficiency in broiler chickens

Abstract

Tremendous genetic improvement in growth rate and feed efficiency of broiler chickens has been recorded in the past five or more decades due to genetic selection. However, the increasing global human population, increasing demand for affordable animal protein, increase in feed price and the continued global environmental issue still urges the broiler industry to produce broilers that grow fast and efficiently utilize feed. In order to further improve growth rate and feed efficiency of broiler chickens, understanding the genetic background of growth rate (body weight) and feed efficiency in today’s broilers is essential. This thesis explored the genetic background of body weight (BW) and feed efficiency (FE) in broiler chickens using different statistical models and methods. Accurate estimation of genetic parameters is the primary pre-requisite to establish an efficient selection program. In this thesis, genetic parameters of BW and FE traits were estimated using different statistical models and methods. It is shown that BW and FE traits are influenced by sex by genotype and age by genotype interaction in addition to direct genetic effects, maternal permanent environmental effects and residual environmental effects. This suggests that genetic evaluation of broiler chickens should take in to account sex and age differences in order to improve accuracy of predicting breeding values and maximize genetic gain. Moreover, by studying the effect of selection on genetic parameters of BW along a selection trajectory, it is shown that the genetic variance of BW did not exhaust/diminish after several generations of selection. This is also supported by genome wide association study (GWAS) in which several quantitative trait loci (QTL) and candidate genes were identified to be associated with BW and FE in a broiler population that was pre-selected for BW and undergone several generations of selection for FE. In addition to identifying QTL regions and candidate genes for BW and FE traits, GWAS using mixed linear model and general linear model approach were compared. The results have shown that GWAS using the two approaches does not necessarily give similar results even if family structure is not a strong concern in the population suggesting that statistical models in GWAS should be carefully chosen. This thesis also explored two definitions of residual feed intake (RFI); phenotypic and genetic RFI. Phenotypic and genetic RFI were derived from a joint Bayesian analysis of body weight, feed intake (FI) and body weight gain (Gain) and genetic parameters of production (BW and Gain) and feed efficiency traits (FI and the two RFI definitions) were estimated. Moreover, genetic relationship between production and feed efficiency traits was quantified. Genetic correlations between phenotypic and genetic RFI were close to unity at a younger age and significantly different from unity at an older age suggesting that selection using either of them at that specific younger age results the same genetic response. Overall, the results of this thesis suggest that there exists considerable genetic variation in BW and FE that makes further improvement of the traits possible. The knowledge gained in this thesis will contribute to the understanding of the genetic background of BW and FE in broiler chickens. Furthermore, the statistical models and methods employed and the results reported in this thesis can be extended to other poultry species with little modification since chicken is the prime model for all avian species.