Interaction of direct and social genetic effects with feeding regime in growing rabbits

Background: Most rabbit production farms apply feed restriction at fattening because of its protective effect against digestive diseases that affect growing rabbits. However, it leads to competitive behaviour between cage mates, which is not observed when animals are fed ad libitum. Our aim was to estimate the contribution of direct (d) and social (s) genetic effects (also known as indirect genetic effects) to total heritable variance of average daily gain (ADG) in rabbits on different feeding regimens (FR), and the magnitude of the interaction between genotype and FR (G x FR). Methods: A total of 6264 contemporary kits were housed in cages of eight individuals and raised on full (F) or restricted (R) feeding to 75% of the ad libitum intake. A Bayesian analysis of weekly records of ADG (from 32 to 60 days of age) in rabbits on F and R was performed with a two-trait model including d and s. Results: The ratio between total heritable variance and phenotypic variance (T 2) was low (<0.10) and did not differ significantly between FR. However, the ratio between h(2) (i.e. variance of d relative to phenotypic variance) and T 2 was similar to 0.52 and 0.86 for animals on R and F, respectively, thus s contributed more to the heritable variance of animals on R than on F. Feeding regimen also affected the sign and magnitude of the correlation between d and s, i.e. -0.5 and similar to 0 for animals on R and F, respectively. The posterior mean (posterior sd) of the correlation between estimated total breeding values (ETBV) of animals on R and F was 0.26 (0.20), indicating very strong G x FR interactions. The correlations between d and s in rabbits on F and R ranged from -0.47 (d on F and s on R) to 0.64. Conclusions: Our results suggest that selection of rabbits for ADG under F may completely fail to improve ADG in rabbits on R. Social genetic effects contribute substantially to ETBV of rabbits on R but not on F. Selection for ADG should be performed under production conditions regarding the FR, by accounting for s if the amount of food is limited.