Effects of beef production system on animal performance and carcass characteristics.

The objective of this study was to evaluate conventional (CONV) and natural (NAT) beef production systems from annual pasture through finishing through grazing. Beef steers (n = 180, initial B W = 250 ± 19 kg) were assigned randomly to 2 treatments in the pasture phase. Steers were implanted with 40 mg of trenbolone acetate (TBA), 8 mg estradiol, and 29 mg tylosin tartrate (CONV), or received no implant (NAT). Steers on the 2 treatments grazed wheat or cereal rye for 109 d. Conventional steers had an 18.5% improvement in ADG (1.22 vs. 1.03 kg/d, P < 0.01) and a heavier final BW (385 vs. 366 kg, P < 0.01) compared with NAT steers. Following the pasture phase, steers (n = 160 steers, 5 steers/pen, 8 pens/treatment) were assigned to a 2 x 2 factorial in the feedlot phase. Production system (NAT vs. CONV) was maintained from the pasture phase, and the second factor was 7 vs. 12% low-quality roughage (DM basis, LOW vs. HIGH). During finishing, CONV steers were given 120 mg of TBA and 24 mg estradiol at processing, fed monensin and tylosin, and fed zilpaterol hydrochloride for the last 20 d of the experiment. There were no program x roughage level interactions (P > 0.07). The CONV steers ate 6.9% more feed (11.8 vs. 11.0 kg/d, P < 0.01), gained 28.4% faster (1.90 vs. 1.48 kg/d, P < 0.01), and were 24.2% more efficient (0.164 vs. 0.132, P < 0.01) compared with NAT steers. The LOW steers had greater G:F (0.153 vs. 0.144, P < 0.01) compared with HIGH steers. There was a 28.3% improvement in estimated carcass weight gain (1.36 vs. 1.06 kg/d), 18.6% improvement in carcass efficiency (0.115 vs. 0.097, P < 0.01), and 21.6% improvement (1.52 vs. 1.25 Mcal/kg, P < 0.01) in calculated dietaty NEd for CONV compared with NAT steers. Hot carcass weight was increased by 62 kg (424 vs. 362 kg, P < 0.01) and LM area was increased by 16.9 cm² (100.9 vs. 84.0 cm², P < 0.01), decreasing USDA yield grade (YG, 3.09 vs. 3.54, P < 0.01) for CONV steers compared with NAT steers. Natural steers had a greater percentage of carcasses in the upper 2/3 of USDA Choice grade (48.7 vs. 18.7%, P < 0.01), a greater percentage of YG 4 and 5 carcasses (25.4 vs. 9.3%, P < 0.01), and a greater percentage of abscessed livers (39.6 vs. 10.5%, P < 0.01) compared with CONV steers. The results show that CONV production results in more rapid and efficient production that resulted in heavier carcasses with superior YG and desirable quality grades with both roughage levels. [ABSTRACT FROM AUTHOR]