SELECTION FOR COMPONENTS OF EFFICIENT LEAN GROWTH-RATE IN PIGS .1. SELECTION PRESSURE APPLIED AND DIRECT RESPONSES IN A LARGE WHITE HERD

Responses to four generations of divergent selection for lean growth rate with ad-libitum feeding (LGA), for lean food conversion (LFC) and for daily food intake (DFI) in Large White pigs were studied. The LGA (LFC) selection criterion was designed to obtain equal correlated responses in growth rate (food conversion ratio) and carcass lean content, measured in phenotypic s.d. The selection criteria had phenotypic s.d. of 27, 29 and 253 units, respectively, and results are presented in s.d. units. There was a total of 3537 pigs, with an average of 40 boars and 40 gilts performance tested in each of the high, low and control lines per generation and the lines consisted of 10 sires and 20 dams. The generation interval was equal to 13.5 months. Animals were performance tested in individual pens with mean starting and finishing weights of 30 kg and 85 kg respectively. Cumulative selection differentials in the three selection groups were 5.8, 3.6 and 3.3 phenotypic s.d. for LGA, LFC and DFI respectively. Direct responses to divergent selection were 1.7, 1.3 and 1.2 (s.e. 0.17) for LGA, LFC and DFI. The correlated response in LFC (1.6 (s.e. 0.18)) with selection on LGA was greater than the direct response in LFC. Conversely, the direct response in LGA was greater than the correlated response (1.1 (s.e. 0.18)) with selection on LFC. The response in LFC (-1.1 (s.e. 0.17)) with selection on DFI was similar in size but opposite in sign to the direct response in LFC. Responses were asymmetric about the control, as the high LGA and LFC responses were proportionately smaller (0.74 and 0.58) than low line responses. In contrast, the difference between the high DFI and control was four times greater than the difference between low line and control. Heritabilities of LGA, LFC and DFI were 0.38, 0.35 and 0.29 (s.e. 0.03), when estimated by residual maximum likelihood, with common environmental effects of 0.09 (s.e. 0.02). Genetic correlations for LGA with LFC and DFI were positive, 0.76 (s.e. 0.03) and 0.23 (s.e. 0.07), but the genetic correlation between DFI and LFC was negative, -0.45 (s.e. 0.06). The experiment demonstrated that substantial responses to selection can be achieved in LGA, LFC and DFI. Selection on LGA resulted in larger direct and correlated responses than selection on LFC.