Effects of zilpaterol hydrochloride and virginiamycin on growth performance, carcass characteristics, and visceral organ mass in feedlot lambs

Objective: The objective was to determine the effect of dietary zilpaterol hydrochloride and virginiamycin on growth performance, dietary energetics, carcass characteristics, and visceral organ mass in feedlot lambs. Methods: Thirty-two crossbred lambs Dorper×Katahdin (34.04±4.23 kg), five months old, were used in a 30 d experiment to evaluate the inclusion of zilpaterol and virginiamycin. Treatments were randomly assigned to pens within blocks, with four replicates per treatment. Data were analyzed as a randomized complete block design arranged as 2×2 factorial, with two levels of zilpaterol (0 and 0.20 mg/kg of live weight/d, as hydrochloride form) and two levels of virginiamycin (0 and 22 mg/lamb/d). The diet was based on cracked corn (1.41 Mcal NEg/kg of dry matter and 14.1% of crude protein). Growth performance and dietary energetics variables were recorded. After the feeding trial, lambs were transported to a slaughterhouse for assessment of carcass characteristics, visceral organ mass, and primal cuts. Results: No interactions were observed between zilpaterol and virginiamycin treatments for most of the evaluated variables, except for the percentage of the empty small intestine. Lambs supplemented with both zilpaterol and virginiamycin showed improvements in live weight, feed efficiency, and energy retention. However, only zilpaterol affected dressing percentage, longissimus thoracis muscle, and body fat reduction. The combined supplementation increased final live weight without altering feed intake, thereby enhancing energy availability to promote carcass weight and dressing carcass compared to lambs treated with zilpaterol alone. Conclusion: Zilpaterol hydrochloride (0.20 mg/kg of live weight/d, equivalent to 7.55 mg/lamb/d) and virginiamycin (22 mg/lamb/d) improved growth and energy retention, but only zilpaterol improved dressing percentage and reduced fat traits of carcass and non-carcass components. The effects of virginiamycin and zilpaterol were found to be cumulative, with better responses in growth and dietary energy components observed in lambs supplemented with both additives.