Growth efficiency of ewe lambs classified into residual feed intake groups and pen fed a restricted amount of feed

The objective of this research was to evaluate the effects of residual feed intake (RFI) determined under ad libitum feeding conditions on DMI and performance of yearling ewes pen fed the same diet under restricted feeding conditions. In Experiment 1, 48 ewe lambs had ad libitum access to a pelleted grower diet for 49 days and individual DMI was determined using an electronic feed delivery system. Residual feed intake values were assigned to each ewe lamb as a measure of feed efficiency and ewe lambs were classified into high, medium, and low RFI groups. In Experiment 2, 12 animals (now yearling ewes) from each RFI group were then placed into pens of three (3 ewes/pen and 4 pens/treatment) and fed the same pelleted diet offered during the RFI trial under restricted feeding conditions to maintain body weight. Body weight, longissimus muscle area (LMA), and fat thickness at the 12th rib (BF) were measured at the beginning and end of both experiments. Raw fleece weight was measured at the end of Experiment 1. In Experiment 1, LMA, BF, and fleece weight were not significant sources of variation in the RFI model (P > 0.25). Ewe lamb DMI increased (linear P < 0.01) from low to high RFI group. Modeled initial and final BW, ADG and metabolic BW; initial and final LMA and BF, and fleece weight did not differ (P > 0.31) among RFI groups. In Experiment 2, DMI (kg/d) by yearling ewes did not differ (P = 0.54) among RFI groups (avg. 1.34 kg/d); however, DMI as a % BW (DMIBW) was lower (P < 0.05) by yearling ewes categorized as low than medium or high RFI (2.37 vs. 2.41% BW) due to differences in weight change during the feeding period. Yearling ewes categorized as low RFI had greater final BW and ADG (P < 0.05) than medium RFI yearling ewes and tended to have greater (P = 0.08) final BW and ADG than high RFI yearling ewes. Initial and final LMA and BF, and change in LMA of yearling ewes did not differ (P > 0.16) among RFI groups under restricted feeding conditions. Change in BF of yearling ewes tended to increase linearly (P = 0.09) from low to high RFI group under restricted feeding conditions. Ewe lambs classified as low RFI showed slightly better performance than medium and high RFI yearlings at similar DMI under restricted, group feeding conditions. These data suggest that further research into the applicability of feed efficiency of sheep as measured by RFI under commercial production settings is warranted.