Selection for feed efficiency may lead to decreased production inputs; however, associations between feed efficiency and other phenotypic traits of interest in sheep are not well understood. The objectives of this study were to 1) determine the effect of diet on feed intake traits, 2) determine the effect of dietary change on residual feed intake (RFI) and performance traits, and 3) determine relationships of RFI with carcass characteristics in lambs fed either a concentrate or forage-based diet. In Trial 1, growing wethers (initial live weight (LW) = 51.3 +/- 1.2 kg; n = 78) of Hampshire (n = 35), Rambouillet (n = 26), and Suffolk (n = 17) breed types were blocked by breed and initial LW and randomly allocated to receive a concentrate (50.2% corn, 31.0% wheat middlings, 10.0% corn gluten, 2.5% cane molasses, 1.8% calcium carbonate, 1.0% dried distillers grains with soluble, 0.8% calcium sulfate, 0.2% potassium chloride, 0.4% trace minerals and vitamins; 91.6% DM; 12.1% CP, 17.6% NDF, 3.0 Mcal/kg ME; n = 40) or forage-based (67.7% alfalfa, 27.5% wheat middlings, 2.5% cane molasses, 1.3% salt, 0.6% calcium carbonate, 0.3% trace minerals and vitamins; 92.3% DM, 16.2% CP, 36.3% NDF, 2.3 Mcal/kg ME; n = 38) pelleted diet. Individual feed intake was recorded using a GrowSafe System for 49 days and RFI was estimated for each wether. Using RFI to rank the wethers for feed efficiency, the 20% most (n = 8) and the 20% least (n = 8) efficient wethers from each diet (n = 32 total) were slaughtered and carcass data were recorded. Remaining wethers from the forage diet were subsequently changed to the concentrate diet for Trial 2; individual feed intake was again collected using the GrowSafe System for 79 days, and RFI calculated for each wether. The MIXED procedure of SAS was used to determine the effect of diet and diet change on performance traits in Trials 1 and 2, respectively, and the effects of diet, RFI class (most or least efficient), and their interaction on carcass traits using data from selected wethers (n = 32) from Trial 1; breed and pen were included as random effects. The CORR procedure of SAS was used to determine relationships of RFI with performance traits and carcass characteristics. Overall, feed intake was less variable for concentrate-fed (sigma(2) = 0.13 kg(2)) than forage-fed wethers (sigma(2) = 0.25 kg2). Feed intake and average daily gain (ADG) were greater (P <= 0.001) in forage-fed compared to concentrate-fed wethers, although the gain to feed ratio (G:F) was not affected by diet type (P = 0.23). While RFI and RFI rankings were moderately correlated (r = 0.51, r = 54, respectively; P <= 0.02) in wethers subjected to the diet change (forage to concentrate), some re-ranking was observed, especially in low efficiency lambs. Boneless cut percentage tended (P = 0.10) to be greater in forage versus concentrate wethers; there were no other effects (P >= 0.13) of diet on carcass measures. Residual feed intake and USDA quality grade tended (P = 0.10) to be positively correlated, but no other relationships (P >= 0.17) between RFI and carcass measures were observed. These data suggest that feed intake and ADG are affected by diet type, and that greater variation in feed intake is associated with a forage-based pelleted diet. Additionally, selection against low feed efficiency may be most effective in improving flock feed efficiency genetics when compared with selection for greater efficiency. Furthermore, selection for RFI should not unfavorably affect carcass traits in sheep.
