We compare the effects of three different approved sources of supplemental zilpaterol on growth-performance responses and carcass characteristics of finishing lambs. Twenty four Pelibuey×Katahdin lambs (46.75±2.43 kg) were used in a 33-day feeding trial. Lambs were fed a dry rolled corn-based finishing diet. Treatments consisted of the non-supplemental basal diet (Control) versus the basal diet supplemented with 125 mg zilpaterol/kg of diet (as fed basis) from three commercial sources marketed in Mexico: Zilmax (ZIL), Grofactor, and Zipamix. Compared to controls, zilpaterol (ZH) supplementation did not affect dry matter intake (DMI), but increased carcass adjusted daily weight gain (ADG, 36.7%), gain efficiency (34.2%), and dietary net energy (26.0%), and decreased (23.4%) the ratio of observed:expected DMI. Compared to controls, supplemental ZH increased hot carcass weight (6.4%), dressing percentage (3.2%), m. longissimus thoracis (LM) area (15.6%), and shoulder muscle:fat ratio (28.7%), but decreased kidney-pelvic-heart fat, and fat thickness. Supplemental ZH increased 10.9% and 14.3% whole cut weight of loin and leg, respectively, and the proportion (as percentage of cold carcass weight) of leg (4.3%). These increases were reflected in greater forequarter and hindquarter weights. Lambs fed ZH increased (4.6%) empty body weight (EBW) and reduced (14.7%) liver/spleen weight (as g/kg EBW). Likewise, ZH supplementation tended (p = 0.08) to lower (8.9%) visceral fat. Growth performance, energetic efficiency, hot carcass weight, dressing percentage, LM area and whole cuts were not different across supplemental ZH sources. However, compared with non-supplemented controls, only ZIL appreciably decreased carcass fat distribution, including fat thickness, percentage kidney pelvic and heart fat, shoulder fat, and visceral fat. Supplemental ZH increases ADG, gain efficiency, carcass dressing percentage, and LM area. The magnitude of these responses was similar among ZH sources. Nevertheless, compared with non-supplemented controls, only ZIL appreciably decreases carcass fat. The basis for this is uncertain, but indicative that some practical differences in zilpaterol bio-equivalency may exist across commercial sources tested.

Two experiments were conducted to compare a supplemental blend of essential oils alone (EO) or combined with enzymes (EO + ENZ) versus virginiamycin (VM), on characteristics of growth performance (Exp. 1) and digestion (Exp. 2) in finishing lambs. Lambs were fed a high-energy finishing diet supplemented with: (1) no supplement (control); (2) 150 mg supplemental EO; (3) 150 mg supplemental EO plus 560 mg alpha-amylase (EO + ENZ); and 4) 25 mg VM. Compared with the control, growth performance response to EO and VM were similar, enhancing (5.7%, p < 0.05) feed efficiency and observed dietary net energy. Compared with control, supplementation with EO + ENZ tended (p = 0.09) to increase dry matter intake (6.8%), improving (p < 0.05) weight gain and feed efficiency (10.4 and 4.4%, respectively). Dietary energy utilization was greater (2.7%, p < 0.05) for EO and VM than EO + ENZ. Treatment effects on the carcass and visceral mass were small, but additive supplementation decreased (p ≤ 0.03) the relative weight of the intestines. There were no treatment effects on measures of digestion nor digestible energy of the diet. Supplemental EO may be an effective alternative to VM in high-energy finishing diets for feedlot lambs. Combination EO + ENZ may further enhance dry matter intake, promoting increased weight gain.

Ninety crossbreed bulls (349.5 ± 8.25 kg initial weight) were used in an 87day trial to compare the effects of a blend of essential oils plus 25-hydroxy-Vit-D3 (EO + HyD) versus the combination of monensin with virginiamycin (MON + VM) on feedlot growth performance and carcass characteristics. Dietary treatments (nine replicates/treatment) were supplemented with 40 mg/kg diet dry matter of MON + VM (equal parts) or with 120.12 mg/kg diet dry matter of a combination of standardized mixture of essential oils (120 mg) plus 0.12 mg of 25-hydroxy-vitamin-D3 (EO + HyD). There were no treatment effects on dry matter intake (DMI, p = 0.63). However, the coefficient of variation in day-to-day DMI was greater for EO + HyD than for MON + VM (11.4% vs. 3.88%, p = 0.04). There were no treatment effects (p ≥ 0.17) on daily weight gain, gain-to-feed ratio, and estimated dietary net energy. Cattle supplemented with EO + HyD had greater Longissimus muscle area (7.9%, p < 0.01) and estimated retail yield (1.6%, p = 0.03), and tended to have heavier (1.7%, p = 0.10) carcass weight. Differences among treatments in dressing percentage, fat thickness, kidney–pelvic–heart fat, and marbling score were not appreciable (p > 0.10). It is concluded that growth performance response and dietary energetic are similar for finishing cattle supplemented with EO + HyD vs. MON + VM. However, compared with MON + VM, supplementation with EO + HyD during the finishing phase may improve carcass Longissimus area and carcass yield.