Physiological indicators of phenotypic variation in net efficiency of growing cattle

Abstract

Objectives of this study were to characterize residual feed intake (RFI) in steers and to examine correlated responses of RFI with performance traits, carcass composition, and physiological indicators of this trait. Braunvieh-sired crossbred steers (n = 169) were individually fed for 77 d, and weekly BW and feed intake (FI) measured. Residual feed intake was calculated as difference between actual FI and FI predicted from multiple regression of FI on mid-test BW[.75] and ADG. Ultrasonic measures of 12[th] rib (BF) and rump fat thickness (RF), longissimus muscle area (LMA) and intramuscular fat percent (IM) were obtained on d 70. Residual feed intake was not significantly correlated with final BW, ADG, LMA, or IM, but was positively correlated (P < 0.05) with FI, feed conversion ratio (FCR), and BF. High-RFI steers had 21% higher FI and 23% higher FCR, but similar ADG and final BW compared to low-RFI steers. High-RFI steers had 8.3% greater RF, but similar LMA and IM compared to low-RFI steers. Increased leanness may have contributed to enhanced feed utilization of low-RFI steers. Blood samples collected during the adaptation period and d 0 and 70 of the study were analyzed for cortisol, hematological measures, thyroid hormones (T₃ and T₄), plasma urea nitrogen (PUN), and glucose. ACTH-induced cortisol responses and hematological measures were not significantly correlated with RFI. However, cortisol concentration, red blood cell counts, hemoglobin, and hematocrit were negatively correlated with FI and ADG and positively correlated with FCR. Low- and high-RFI steers had similar T₃ concentrations on d 0 and 70, but low RFI steers had 14.2% lower T₄ concentrations on d 0 (P < 0.01). Low- and high-RFI steers had similar PUN concentrations on d 0, but high-RFI steers exhibited 13.8% higher PUN concentrations on d 70 (P < 0.001). Glucose concentrations were 16.2 and 7.9% higher in low-RFI steers on d 0 and 70 (P < 0.05), respectively. These results demonstrated that cortisol and RBC measures were correlated with ADG, FI, and FCR, but not with RFI. Thyroid hormones and blood metabolite concentrations may have the potential to be used as physiological indicators of RFI.