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Growth, carcass quality, and protein and energy metabolism in beef cattle with different growth potentials and residual feed intakes^1,2

F. C. P. Castro Bulle^*, P. V. Paulino^*,, A. C. Sanches^*, and R. D. Sainz^*

^* University of California, Davis 95616; and Universidade Federal de Viçosa, Viçosa-MG, Brazil; and and Universidade Católica de Goiás, Goiânia-GO, Brazil

Twenty-four beef steers (predominantly Angus x Hereford, 14 to 18 mo of age, 403 ± 3 kg of BW), were housed and fed in individual pens for about 122 d. Twelve steers came from a herd that had been selected for growth (high growth; HG) and the other 12 from a herd with no selection program (low growth; LG). Another 6 steers (3 from each group) were slaughtered at the beginning to obtain the initial composition. All steers were fed the same corn-based diet (3.06 Mcal of ME/kg of DM, 13.6% CP) on an ad libitum basis. Two weeks before slaughter, total urine was collected for 5 d for estimation of 3-methylhistidine excretion and myofibrillar protein breakdown rates. Compared with LG steers, HG steers had less initial BW but greater final BW, DMI (7.52 vs. 6.37 kg/d), ADG (1.33 vs. 0.853 kg/d), G:F (0.176 vs. 0.133 kg/kg), ME intake (0.233 vs. 0.201 Mcal · kg of BW^0.75 · d^–1), and retained energy (RE; 0.0711 vs. 0.0558 Mcal · kg of BW^0.75 · d^–1); gained more fat (676 vs. 475 g/d); and tended to gain more whole body protein (100 vs. 72 g/d), with no difference in residual feed intake (RFI). Estimated net energetic efficiency of gain (k_g) and ME for maintenance (ME_m) did not differ between the 2 groups, averaging 0.62 and 0.114, respectively. The HG steers had greater HCW (350 vs. 329 kg), backfat (16.1 vs. 11.6 mm), and yield grades (3.53 vs. 2.80), with a similar dressing percent, KPH fat, LM area, and marbling score. Skeletal muscle protein gain (70.2 vs. 57.6 g/d) and fractional protein accretion rate (0.242 vs. 0.197 %/d) tended to be greater in HG than in LG steers. Steers were classified into low (–0.367 kg/d) and high (0.380 kg/d) RFI classes. Compared with the high RFI steers, low RFI steers consumed less DM (6.61 vs. 7.52 kg/d) and ME (0.206 vs. 0.234 Mcal · kg of BW^0.75 · d^–1) and tended to gain less fat (494 vs. 719 g/d), but were similar for initial and final BW, ADG, G:F, protein gain, HCW, dressing percent, backfat, KPH fat, LM area, marbling score, and yield grade, as well as for all observations related to myofibrillar protein metabolism. Residual feed intake may be negatively correlated with ME for maintenance. The maintenance energy requirement increased by 0.0166 Mcal · kg^{– 0.75} · d^–1 for each percentage increase in fractional protein degradation rate, confirming the importance of this process in the energy economy of the animal.

Key Words: beef steer • maintenance • protein turnover • residual feed intake

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