Feed Efficiency in Growing Steers: Relationships between Efficiency and Carcass Ultrasound Traits

Phenotypic correlations between feed efficiency and carcass traits were examined in growing steers. Two feed efficiency trials were conducted using 233 Santa Gertrudis steers. Steers were individually fed a roughage-based for 77days. Individual feed intake was recorded weekly and body weight was measured bi-weekly. Ultrasound measurements of the 12th rib fat thickness (back fat), longissimus muscle area (REA) and percentage intramuscular fat were obtained on day 0 and 77, Residual feed intake (RFIp) is a measure of feed efficiency that attempts to measure variation in feed intake beyond that needed for growth and maintenance. Residual feed intake was calculated as the difference between actual feed intake and expected feed intake predicted by a linear regression model of dry matter intake (DMI) on mid-test BW0.75 (MBW) and average daily gain (ADG) with trial, trial × MBW, and trial × ADG as random effects. Steers were ranked by RFI into low, medium, and high RFI groups <0.5 SD, ± 0.5, > 0.5 SD, respectively, from the mean RFIp of 0.0 ± 1.01 kg/d. Overall means for ADG and DMI were 0.38 (SD = 0.57) and 9.79 (SD = 1.03), respectively. Stepwise regression indicated that inclusion of gain in back fat (GBF) into the base model increased the r2 (0.29 vs. 31). RFIp was positively correlated (P < 0.5) with DMI (r = 0.85), and FCR (r = 0.47), but not with MBW or ADG. Feed to gain ratio (FCR) was negatively correlated (P < 0.05) with ADG (r = -0.82). Carcass adjusted for RFI (RFIc) was not correlated with ADG, DMI, MBW, or FCR. Carcass ultrasound traits were not correlated (P > 0.05) with feed efficiency traits. Gain in back fat was positively correlated with (P < 0.05) with RFIp (r = 0.17), but not correlated (P > 0.05) with FCR or RFIc. Gain in REA was negatively correlated (P < 0.05) with FCR (r = -0.21) and RFIc (r = -0.17), but not with RFIp. The Spearman rank correlation between RFIp and RFIc was high (r = 0.91). Results suggest that RFI is independent of growth rate and mature size, but related to DMI. Selecting for favorable RFIp phenotypes can potentially improve feed efficiency in cattle. Adjusting RFI for ultrasound carcass traits could also improve feed efficiency independent of growth, body size, and carcass composition.