Residual feed intake (RFI), defined as the difference in the observed and expected feed intake while accounting for growth and backfat, has gained much attention, but little is known about why pigs selected for reduced RFI are more efficient. To this end, a line of Yorkshire pigs selected for reduced RFI was developed. The objective of this study was to evaluate the 5th generation of this select line against a randomly selected control line for performance, carcass and chemical carcass composition, and overall efficiency toward the later part of the growth phase. Eighty barrows, 40 from each line, were paired by age (~132 d) and BW (74.8 kg) and randomly assigned to 1 of 4 feeding treatments in 10 replicates: 1) ad libitum, 2) 75% of ad libitum, 55% of ad libitum, and BW stasis, with weekly adjustments in intake to keep BW constant for each pig. Pigs were individually penned (group housing was used for selection) and on treatment for 6 wk. Initial BW did not differ between the lines. The ad libitum select pigs consumed 10% less feed than the ad libitum control with no significant difference in BW and slight differences in carcass fat composition and backfat, which resulted in significantly less carcass energy. Under restricted feeding, the select line had an increase in BW while consuming the same ration of feed as the control line with no significant difference in chemical carcass composition and lighter visceral weights, which was significant for the 75% of ad libitum treatment. Under BW stasis feeding the select line consumed 7.6% less feed overall and 18% less feed at the end of the 6 wk, to maintain static BW with no significant difference in chemical carcass composition compared with the control line. Overall, the select line had lighter visceral weight and a greater dressing percentage compared with the control line. Using regression, the select line had reduced energy retention and feed energy utilization; however, the select line appeared to have reduced maintenance requirements. In conclusion, selection for reduced RFI decreases feed intake with no significant difference in growth performance, reduced backfat, increased dressing percentage, and reduced maintenance requirements. All of these traits are appealing to the producer and result in increased profits in the production setting.

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