Evaluation of Feed Efficiency Traits in Growing Bulls and Relationships with Feeding Behavior and Ultrasound Carcass Estimates

Objectives of this study were to characterize feed efficiency traits and examine phenotypic correlations with performance, ultrasound, and feeding behavior traits in growing bulls. Individual DMI and feeding behavior were measured in Angus (n = 214) and Brangus (n = 26) bulls (initial BW 368.4 ± 46.1 kg) fed a corn silage based diet (ME = 2.78 Mcal/kg) using a GrowSafe feeding system. DMI and 14-d BW were measured for 84 and 91 d in test 1 and 2, respectively, and ultrasound subcutaneous fat depth (BF) measured at start and end of each test. Residual feed intake (RFI) and residual gain efficiency (RGE) were computed as the residuals from appropriate linear regression models involving DMI, ADG and mid-test BW (MBW). Partial efficiency of growth (PEG) was computed as ADG divided by DMI for growth. Overall ADG and DMI were 1.44 (SD = 0.26) and 8.51 (SD = 1.10) kg/d in test 1 and 1.73 (SD = 0.28) and 10.07 (SD = 1.35) kg/d in test 2, respectively. RFI was correlated with DMI (0.59) and FCR (0.53), but not ADG or MBW. Bulls with low RFI (< 0.5 SD) consumed 15% less DMI and had 16% lower FCR than bulls with high RFI (> 0.5 SD). PEG was strongly correlated with DMI (-0.51) and RFI (-0.85), but moderately correlated with ADG (0.29) and MBW (0.25). Bulls with low RFI had favorable PEG (0.36 vs 0.26) compared to bulls with high RFI. RGE was correlated with RFI (-0.61), but was strongly correlated with ADG (0.74). Gain in BF was correlated with ADG (0.31) and DMI (0.29), and tended (P < 0.10) to be correlated with RFI (0.11). Feeding duration was not correlated with DMI or ADG, but was correlated with RFI (0.41) and PEG (-0.20). Feeding frequency was not correlated with DMI or ADG, but was correlated with RFI (0.17). Bulls with low RFI had lower (P < 0.01) feeding duration and frequency than bulls with high RFI. Compared to other feed efficiency traits examined in this study, RFI will facilitate selection for improved feed efficiency with minimal responses in growth or composition traits.