Sources of biological variation in residual feed intake in growing and finishing steers

Objectives of this research were to characterize residual feed intake (RFI) in growing and finishing steers and examine phenotypic correlations between performance, feed efficiency, carcass, digestib ility, and physiological indicator traits. The research included two growing studies and one finishing study. Braunvieh-sired crossbred steers (n = 169) and Santa Gertrudis steers (n = 120) were individually fed a roughage-based diet for 77 d during the growing phase. Santa Gertrudis steers (n = 120) were individually fed a grain-based diet for 80 d during the finishing phase. Individual body weight (BW) and feed intake data were recorded. Residual feed intake was calculated as the difference between actual dry matter intake (DMI) and DMI predicted from linear regression of DMI on mid-test metabolic BW. During the growing phase, initial ultrasound measures of 12th rib fat thickness (FT) and final ultrasound measures of Longissimus muscle area (LMA), FT, and intramuscular fat (IMF) were obtained. During the finishing phase, initial and final LMA, FT, and IMF ultrasound measurements were obtained. Finishing steers were slaughtered at 1.0 cm of FT and carcass cooler traits measured. Blood samples were collected at the start and end of each feeding period and analyzed for physiological indicators. Temperament traits were also measured at the start and end of each feeding period. Growing and finishing steers with low RFI consumed 19-22% less feed than growing and finishing steers with high RFI, but did not differ in average daily gain (ADG). Consequently, steers with low RFI were also more efficient as measured by feed conversion ratio and partial efficiency of growth. Steers with low RFI had less FT compared to steers with high RFI. Initial serum IGF-I was correlated with RFI in growing steers indicating that IGF-I could be a potential indicator trait for RFI in growing cattle. Additionally, RFI was correlated with digestibility to indicate more efficient cattle had higher dry matter digestibility. Results indicate that RFI has potential to allow producers to select more efficient animals without increasing growth rate. Moreover, serum IGF-I may facilitate early detection and more accurate selection of animals that are superior for growing RFI.