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

¹ University of California-Davis, Davis, CA 95616
² University of California-Davis, Davis, CA 95616; Universidade Federal de Viçosa, Viçosa-MG, Brazil
³ University of California-Davis, Davis, CA 95616; Universidade Católica de Goiás, Goiânia-GO, Brazil

^* To whom correspondence should be addressed. E-mail: rdsainz{at}ucdavis.edu.

	Abstract

Twenty-four beef steers (predominantly Angus x Hereford, 14 to18 months of age, 403 ± 3 kg BW), were housed and fed in individual pens for about 122 d. Twelve steers came from a herd which has been selected for growth (HG) and the other 12 from a herd with no selection program (LG). Another 6 steers (3 from each group) were harvested at the beginning to obtain initial composition. All steers were fed the same corn-based diet (3.06 Mcal ME/kg DM, 13.6% CP) on an ad libitum basis. Two weeks before harvest, total urine was collected for five d for estimation of 3-methyhistidine excretion and myofibrillar protein breakdown rates. Compared with LG steers, HG steers had less initial BW, 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 BW^0.75·d^-1) and retained energy (RE; 0.0711 vs. 0.0558 Mcal·kg 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 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 similar dressing percentage, KPH fat, LM area, and marbling score. Skeletal muscle protein gain (70.2 vs. 57.6 g/d) and fractional protein accretion rate (FAR; 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 BW^0.75·d^-1) and tended to gain less fat (494 vs. 719 g/d), with similar initial and final BW, ADG, G:F, protein gains, HCW, dressing percentage, backfat, KPH fat, LM area, marbling score, and yield grade, as well as all observations related to myofibrillar protein metabolism. Estimated k_g and ME_m were both numerically lower (P > 0.10) in Low RFI (0.60 and 0.108 Mcal·kg BW^0.75·d^-1) than in High RFI (0.64 and 0.119 Mcal·kg BW^0.75·d^-1) steers. Residual feed intake may be negatively correlated with ME_m. Maintenance energy requirement increased by 0.0166 Mcal·kg^-0.75·d^-1 for each percentage increase in fractional degradation rate, confirming the importance of this process in the energy economy of the animal.

Key Words: Beef steers, Maintenance, Protein turnover, Residual feed intake

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