SELECTION FOR COMPONENTS OF EFFICIENT LEAN GROWTH-RATE IN PIGS .2. SELECTION PRESSURE APPLIED AND DIRECT RESPONSES IN A LANDRACE HERD

Responses to divergent selection for lean growth rate with ad-libitum feeding (LGA), for lean food conversion (LFC) and for daily food intake (DFI) in Landrace pigs were studied. Selection was practised for four generations with a generation interval of 1 year. A total of 2642 pigs were performance tested in the high, low and control lines, with an average of 37 boars and 39 gilts performance tested per selection line in each generation. The average within-line inbreeding coefficient at generation four was equal to 0.04. There was one control line for the DFI and LFC selection groups and another control line for the LGA selection group. Animals were performance tested in individual pens with mean starting and finishing weights of 30 kg and 85 kg respectively with ad-libitum feeding. The selection criteria had phenotypic s.d. of 32, 29 and 274 units, for LGA, LFC and DFI, respectively, and results are presented in phenotypic s.d. Cumulative selection differentials (CSD) were 5.1, 4.5 and 5.5 phenotypic s.d. for LGA, LFC and DFI, respectively. Direct responses to selection were 1.4, 1.1 and 0.9 (s.e. 0.20) for LGA, LFC and DFI. In each of the three selection groups, the CSD and direct responses to selection were asymmetric about the control lines. The correlated response in LFC (1.1, s.e. 0.19) with selection on LGA was equal to the direct response in LFC. In contrast, the direct response in LGA was greater than the correlated response (0.7, s.e. 0.18) with selection on LFC. There was a negative correlated response in DFI (-0.6, s.e. 0.18) with selection on LFC, but the response with selection on LGA was not significant (0.2, s.e. 0.16). Heritabilities for LGA, LFC and DFI were 0.25, 0.25 and 0.18 (s.e. 0.03), when estimated by residual maximum likelihood, with common environmental effects of 0.12 (s.e. 0.02). Genetic correlations for LFC with LGA and DFI were respectively positive (0.87, s.e. 0.02) and negative (-0.36, s.e. 0.09), while the genetic correlation between DFI and LGA was not statistically different from zero, 0.13 (s.e. 0.10). Selection on components of efficient lean growth has identified LGA as an effective selection objective for improving both LGA and LFC, without a reduction in DFI.