Performance and net energy in high and low RFI beef cattle

Abstract Text:

The objective of this study was to relate feed efficiency to performance and net energy in beef cattle. To identify animals with greater or lesser feed efficiency, 98 weaned Angus cross beef calves (71 steers and 27 heifers) were fed individually for 56 days. Feed offered and refused were measured daily, body weights were taken at 14 day intervals, and ultrasound measures (longissimus muscle area and subcutaneous fat over the 12^th-13^th ribs) were taken at the beginning, middle and end of the trial. Feed was delivered twice a day, on an ad libitum basis. Residual feed intake (RFI) was determined as the residual of the regression of DMI on mid-test BW^0.75 and ADG. High and low RFI groups were defined as > 0.5 SD above or below zero, respectively, with intermediate animals classified as medium RFI. As expected, RFI groups had similar initial and final BW and ADG, and different DMI, gain:feed and RFI (P < 0.001). Fat gain, protein gain, and recovered energy (RE) were not different between RFI groups, although subcutaneous fat over the 12^th -13^th rib was 0.19 cm higher in high RFI than low RFI cattle (P = 0.012). Heat energy (HE), defined as the difference between metabolizable energy intake (MEI) and RE was lower in low RFI cattle (P < 0.001).  Estimated NEm requirement (Mcal/kg^0.75) was lower in low than in high RFI cattle (P = 0.001). Overall heifers gained less than steers, however there were no sex x RFI class interactions.  Low RFI cattle have similar weights and weight gains, but lower intakes and higher feed efficiencies as high RFI cattle. This may be partially due to decreased maintenance requirement and heat production.

Keywords: efficiency, net energy, residual feed intake