Performance and net energy in High and Low RFI beef cattle on restricted intake

Abstract Text: In order to determine how beef cattle with known residual feed intake (RFI) phenotypes would perform under restricted feeding, 36 weaned Angus cross beef calves (24 steers and 12 heifers) were selected from a group of 98 calves that had been previously phenotyped for RFI. High and Low RFI animals (24 steers and 12 heifers) were subjected to a 52-d feeding trial with intake limited to 1.5% of BW. Feed offered and refused were measured daily, BW was taken at 14-d intervals, and ultrasound measures (LM area and subcutaneous fat over the 12^th-13^th ribs) were taken at the beginning, middle and end of the trial. After 52 d of intake restriction, RFI groups had similar BW, ADG, DMI, RFI and G:F. Fat gain, protein gain, and estimated recovered energy (RE) were similar between groups, although High RFI cattle had 0.26 cm more subcutaneous rib fat than Low RFI (P = 0.01). High RFI cattle also had more rib fat at the start of restricted feed trial. RFI groups did not differ in estimated heat energy (HE) or maintenance requirement (NEm; P > 0.10). Heifers had lower HE than steers (P < 0.01). All cattle had lower ADG, RE, NEm and HE in response to limited feed. Overall HE was reduced from 0.26 Mcal/kg^0.75 on ad libitum feeding to 0.16 Mcal/kg^0.75 on the restricted level of intake. The difference in HE from ad libitum to restricted feeding was -47 and -28% in High and Low RFI cattle, respectively (P < 0.01). Estimated NEm requirement was reduced from 0.095 to 0.073 Mcal/kg^0.75 overall, with a difference of -39% and +14% (P < 0.01) in High and Low RFI cattle, respectively.  Estimated NEm requirement changed by -32% and +7% (P = 0.004) in heifers and steers, respectively. These results indicate that when limited, both High and Low RFI cattle lower their maintenance requirement and heat production to similar levels, although High RFI cattle had higher HE and NEm during ad libitum feeding. Furthermore, heifers may be better equipped than steers to adapt maintenance requirement and heat production in response to limited feed.  

Keywords:  efficiency, net energy, residual feed intake