Effects of zilpaterol hydrochloride feeding time on Nellore bulls performance and carcass characteristics

Abstract Text: Nellore bulls (n = 96; initial BW = 377 ± 21.4 kg) were used in a randomized complete block design arranged in a 4 × 2 factorial to evaluate effects of zilpaterol hydrochloride (ZH; 8.33 mg/kg of diet DM basis) on performance and carcass traits. Total days on feed (DOF) were 90 and 117, and there were four periods of ZH feeding (0, 20, 30, or 40 d before slaughter) plus a 3 d ZH withdrawal period for all animals. No interactions between ZH and DOF were detected for the performance or carcass characteristics (P > 0.10). There was a linear increase for G:F (β = 0.0005; P < 0.01), HCW (β = 0.4304; P = 0.03), dressing percentage (β = 0.0541; P < 0.01), and LM area (β = 0.1318; P = 0.01) with length of ZH feeding. Additionally, there was a linear decrease for kidney, pelvic and inguinal fat (β = -0.0384; P = 0.04) when ZH feeding period was increased, however, no difference was noted for 12th-rib fat. In conclusion, Nellore bulls fed ZH had improvements in feed efficiency and carcass traits over controls and these were independent of DOF. The effects of increasing the length of ZH treatment to 30 or 40 days were relatively small, and this data can be used to find the best economic option based on feed, cattle and ZH costs.

Table 1. Effect of ZH feeding period on Nellore performance and carcass traits

	ZH feeding period				SEM	P-value
	0	20	30	40
Final BW, kg	561	572	566	571	10.60	0.68
ADG, kg/d	1.75	1.87	1.81	1.91	0.06	0.28
DMI, kg/d	10.22	10.12	9.96	9.83	0.27	0.60
G:F	0.171	0.185	0.182	0.194	0.006	< 0.01**
HCW, kg	306	320	319	324	7.06	0.03*
Dressing percentage, %	54.6	55.9	56.3	56.7	0.30	< 0.01**
Kidney, pelvic and inguinal fat, kg	13.4	11.6	11.9	11.9	0.55	0.04*
12th-rib fat, mm	5.38	4.63	4.72	5.17	0.27	0.17
LM area, cm2	76.3	79.2	81.7	81.1	1.87	0.02*

* Linear effect (P < 0.05); ** Linear effect (P < 0.01).

Keywords: β-agonist, beef cattle, feedlot