Residual feed intake and body composition in young growing cattle

Crossbred steers (n = 176), 7-8 mo of age and from the five BeefBooster strains (MI, M2, M3, M4 and TX), were used to determine the relationships between residual feed intake (RFI) and growth rate, body composition and heat production (HP), and to quantify differences in RFI independent of differences in body composition. Animals with different RFI levels were also characterized for growth, carcass and body compositional traits. Steers from each genetic strain were selected at random and serially slaughtered on 5 pre-selected days of the finishing period. Steers grew at 1.52 (SD = 0.22) kg d^-1 and had dry matter intake (DM1) of 8.5 (SD = 1.0) kg d-^ during the last 71 to 183d before slaughter. Metabolic mid-point weight, average daily gain (ADG), gain in empty body fat and gain in empty body water accounted for 67.9, 8.6, 3.9 and 1.1%, respectively, of the variation in actual feed intake. Similarly, metabolic mid-point weight (68.5%), ADG (8.2%), gain in ultrasound backfat thickness (1.8%), gain in ultrasound marbling score (1.1%) and year (1.3%) accounted for 80.9% of the variation in actual feed intake. Residual feed intake adjusted for differences in estimated composition of gain (estimated gain in empty body fat and water; RFI) ranged from -2.06 kg d^-1 to +1.61 kg d^-1 (SD = 0.60 kg d^-1). Residual feed intake adjusted for live animal measures of body composition (gain in ultrasound backfat thickness and marbling score; RFI) ranged from -2.11 kg d^-1 to +1.88 kg d^-1 (SD = 0.62 kg d^-1). Low RFI animals (efficient) had 6.0% lower metabolizable energy intake (MEI), retained 9.3% less energy and had 4.5% lower HP than medium RFI animals (P < 0.01). Low RFI animals also had 10.2% lower MEI, retained 12.0% less energy and produced 9.3% less heat than high RFI animals (P < 0.01). Liver (P <0.01), small and large intestine (P = 0.09) and stomach and intestine (P <0.01) weights were less in low and medium RFI steers compared to high RFI steers. There was a trend for low RFI steers to have less dissectible carcass fat (P = 0.08), intermuscular fat (P = 0.06), body cavity fat in the butt and loin (P = 0.01), faster accretion rate of empty body water (P = 0.04) and a slower accretion rate of empty body fat (P < 0.01) than medium and high RFI steers. A portion of the greater MEI by high RFI steer was accounted for by differences in the chemical composition of gain. However, a greater proportion was due to a disproportionate increase in the energy required for maintenance and heat increment of feeding in high RFI steers. An attempt should be made to adjust RFI for changes in the chemical composition of gain, possibly by the inclusion of ultrasound backfat thickness and marbling score into the equation for determining RFI.