Residual feed intake of Angus beef cattle divergently selected for feed conversion ratio

A divergent selection experiment using feed:gain ratios as the selection criterion was conducted in the early 1980s using Angus beef cattle located at the Eastern Agricultural Research Station (EARS), Belle Valley, OH. Numbers of bulls completing the 140-d postweaning performance test in 1979 through 1983 were 35, 34, 35, 34, and 33, respectively. Each year, the 3 bulls with the highest feed:gain ratios and the 3 bulls with the lowest feed:gain ratios were selected from the individually fed bulls, and were randomly mated as 2-yr-olds to approximately 20 cows each in a test herd of Angus cows also located at EARS. The progeny of these matings were then fed to assess postweaning and carcass performance. The objective of the current study was to compare the results of the experiment based on divergent selection for feed:gain ratios with those that would have been obtained using residual feed intake (RFI) as the selection criterion. RFI was calculated using 3 approaches: (1) dry matter intake (DMI) adjusted for production (i.e., ADG and metabolic midweight) to derive RFIP; (2) DMI adjusted for ultrasonic backfat thickness at the end of the 140-d test, in addition to ADG and metabolic midweight, to obtain RFIBF; and (3) DMI estimated using the NRC net energy equations, taking into account the energy and feed required for both gain and maintenance, to derive RFINRC. RFI was calculated by subtracting expected feed intake, derived using these 3 approaches, from the observed feed intake. Low RFIP and RFIBF bulls consumed less feed than high RFI bulls, whereas no differences existed for weight traits or ADG. Both feed:gain ratio and RFINRC were significantly correlated with weight traits and ADG on a phenotypic basis. The correlation between RFIP and RFIBF was near 1.0. In addition, rankings of the bulls were very similar based on RFIP and RFIBF. However, RFIBF may be the best selection criterion for feed efficiency examined in this study, as it was phenotypically independent of backfat thickness, in addition to weights and gains, and, therefore, would be less likely to negatively impact marbling score and carcass quality grade.