The effects of ad libitum and restricted feeding on Yorkshire pigs selected for reduced residual feed intake

Residual feed intake (RFI), defined as the differences between observed and expected feed intake based on growth and backfat, has been used to select for improved feed efficiency in beef cattle, poultry, and now swine. However, little is known about the main biological factors that contribute to the variation in RFI in swine. The objectives of the experiments in this thesis were to compare the 5th generation of a line of pigs selected for reduced RFI (Select) against a randomly selected control (Control) line for performance parameters and to examine the biological contribution of visceral mass, carcass and chemical carcass composition, and predicted maintenance requirements on the overall efficiency during two stages of growth: the early post-weaning period (EGP) and late growth period (LGP) prior to market weight. In both experiments, Select and Control line pigs were paired based on age (~65 and 132 d for EGP and LGP, respectively) and weight (23.9y4.2 and 74.8y9.9 kg, respectively) and the pairs were randomly assigned to one of four feeding level treatments: 1) ad libitum (Ad); 2) 75% of Ad (Ad75); 3) 55% of Ad (Ad55); and 4) weight stasis to maintain a constant body weight (WS). In both experiments (EGP and LGP), pigs were individually penned and on feed treatment for 6 weeks. Overall, under Ad feeding, the Select line consumed 8 to 10% (p \u3c 0.09) less feed compared to the Control, with no significant difference in weekly BW (p \u3c 0.80). In general, the Select line under the Ad treatment had less backfat and carcass fat % but no other significant differences in carcass chemical composition. Under restricted feeding, the Select line had an increase in BW (p = 0.10) while consuming the same amount of feed as the Control, in both experiments. Furthermore, no significant differences in carcass chemical composition were found. The Select line had lower visceral weights but this was only significant (p \u3c 0.01) for the LGP experiment. In the EGP experiment, the WS treatment showed no significant differences in feed intake or BW between the Select and Control line. Conversely, for the LGP experiment, the Select line required less feed than the Control by the end of the experiment to maintain static BW (p \u3c 0.08). Furthermore, there was a trend for the Select line to have reduced maintenance energy requirements (p \u3c 0.13) for the LGP experiment, as estimated by regression of consumed on retained energy. In conclusion, selection for reduced RFI has reduced feed intake, with no significant differences in growth performance but reduced backfat, reduced carcass fat%, and lower maintenance requirements. The results of this thesis show that carcass composition and energy partitioning, primarily differences in carcass fat%, and reduced estimated maintenance requirements may significantly contributed to the differences in RFI