Responses in production and body composition traits in ad libitum fed pigs from lines selected for high and low growth rate on a fixed ration

Abstract

Large White pigs from lines which had been divergently selected over 4 years for high and for low post-weaning daily gain on a fixed but restricted ration of approximately 80% of ad libitum, were sampled for growth testing on ad libitum individual feeding during a 6-week period starting at 50 kg. Genetic parameters and breeding values for performance and carcass composition traits were estimated on 758 pigs with pedigrees comprising a total of 5324 animals, using restricted maximum likelihood (REML) and best linear unbiased prediction (BLUP) methods applied to a multiple trait animal model. Selection resulted in significant differences between the lines. Average breeding values (± S.E.) for the high and low growth lines respectively were 24.3 and − 13.4 g/day (4.1) for test daily gain (TDG), − 0.29 and 0.50 mm (0.09) for ultrasonic P2 fat depth (UBF), − 15 and 22 g/day (5.3) for daily food intake during test (TFI), − 0.11 and 0.10 (0.01) for food conversion ratio during test (TFC), 7.2 and − 4.3 g/day (2.4) for lifetime daily gain (LDG), 5.9 and − 3.2 g/day (1.8) for carcass weight daily gain (CDG), − 0.90 and 1.25 mm (0.21) for carcass fat (CFT). Phenotypic differences between the lines were similar to these breeding value differences.

Compared with low growth selection, the correlated genetic responses to high growth selection on restricted feeding measured in these ad libitum fed pigs were economically desirable for both performance and carcass composition traits. The increase in growth and decrease in fatness, associated with a reduction in voluntary food intake in the high line pigs, suggested a reduced energy cost of the gain relative to the low growth line. It is concluded that selection for high growth rate on restricted feeding will produce pigs which perform more profitably on ad libitum individual feeding.