Abstract #868

# 868
Residual feed intake in ad libitum and limit-fed steers.
Roberto D. Sainz*1, 1University of California, Davis, CA.

To determine if nutritional restriction alters the fundamental relationships between intake, growth and efficiency, 60 Angus-Hereford steers (310 ± 33 d of age, 263 ± 33 kg BW) were randomly assigned to 2 intake groups (IG): ad libitum (AL) or limit-fed (LF, 80% of ad libitum) and fed in individual pens. Intakes were monitored weekly, and steers were weighed monthly. Steers were harvested when they reached a minimum of 12.5 mm backfat, determined by ultrasound at each weighing. Carcass composition was determined from specific gravity. Residual feed intake (RFI) was calculated as the residual of the regression of DMI on average metabolic BW and ADG, with intake group included as a fixed effect; slopes were not found to be heterogeneous (P > 0.05). The model was: DMI (kg/d) = IG + 0.0737 BW0.75 + 3.296 ADG, where IG = −0.12 (AL) or −1.11 (LF). RFI groups were defined as Low (RFI < −0.5 SD from mean), Medium (−0.5 SD < RFI <0.5 SD) or High (RFI >0.5 SD). Data were analyzed by regression and ANOVA, including IG and RFI groups as main effects; no interactions were significant. (Minitab Inc., State College, PA). Days on feed were greater (P < 0.0001) in LF (198 d) than in AL (175 d) steers, however final BW were not different (455 kg; P > 0.10) between AL and LF groups. DM intakes were lower (P < 0.0001) in LF than in AL steers, both in absolute and relative terms (8.71 and 6.60 kg/d and 2.37 and 1.93% of BW, respectively). RFI groups also differed (P < 0.001) in DMI, both in absolute and relative terms (8.08, 7.91 and 6.97 kg/d and 2.29, 2.16 and 1.99% of BW, for High, Medium and Low RFI groups, respectively). Gain:feed was different among IG (0.0952 and 0.0868 for AL and LF, respectively, P = 0.003) and RFI groups (0.0806, 0.0888 and 0.1036 for High, Medium and Low RFI, respectively; P < 0.001). Carcass fat contents were not different (P > 0.10) among RFI groups, but tended to be lower (P = 0.064) in LF than in AL (28.3 and 30.3%, respectively). These results confirm that nutritional restriction limits rate and efficiency of gain and prolongs the finishing period in beef cattle, and tends to produce leaner carcasses. The lack of any difference in the slopes of the equation used to predict intake between ad libitum- vs. limit-fed steers indicate that feed restriction did not alter the fundamental relationships between intake, metabolic weight and rate of gain.

Key Words: efficiency, limit-fed, steer