Performance and carcass traits of immunocastrated Nellore cattle fed to β-agonists

Abstract Text: Beta-adrenergic agonists (βAA) have been used to improve performance and carcass traits of feedlot cattle. The use of non-castrated males for meat production is a common practice because they grow fast, use energy more efficiently and produce leaner and high-yielding carcasses. This work was developed to evaluate the effect of βAA and immunocastration on the performance of feedlot finished Nellore cattle. Ninety-six males (409±50 kg LW; 20 mo old) were divided in two groups and half of them received two doses of immunocastration vaccine (Bopriva®) within 30 days interval. Animals were fed for 70 days a common diet containing 76% concentrate and 24% roughage (corn silage). Following they were split in 3 groups (n=32) and fed 30 more days one of the following treatments: control diet without βAA (CON); control diet plus 80mg/d zilpaterol hydrochloride (Zilmax®) (ZIL); control diet plus 300mg/d of ractopamine hydrochloride (Optaflexx®) (RAC). Individual DMI, ADG and F:G were recorded. Data was analyzed as complete block randomized design in a 2x3 factorial arrangement. No interaction was observed between sex condition and treatments. There was no difference in DMI (9.1 kg/d) but non-castrated males were heavier (551 vs 520 kg; P=0.0004), tended to have a greater ADG (1.21 vs 1.06 kg/d; P=0.1084) and F:G ratio (132 vs 115 g of DMI/kg ADG; P= 0.1102) than immunocastrated, respectively. Immunocastrated animals had smaller hot carcass weight (HCW; 304 vs 323 kg; P=0.0006) without difference in dressing percentage (DP; 58.5%). Animals fed ZIL showed highest ADG and F:G ratio (1.39 kg/d and 156 g of DMI/kg ADG, respectively; P<0.05). CON treatment showed the smallest ADG and F:G ratio (0.89 kg/d and 94 g of DMI/kg ADG, respectively; P<0.05). Treatment RAC showed intermediate results (ADG 1.1 kg/d and F:G 122 g of DMI/kg ADG, respectively; P<0.0001) compared to ZIL and CON. There was no effect of treatment on final BW, DMI and HCW. Animals fed βAA showed heavier carcasses (313 kg RAC; 320 kg ZIL) than CON (308 kg) resulting in a higher DP for ZIL (59.1%) when compared to RAC (58.3%; P=0.0351) and CON (58%; P=0.0048), respectively. Non-castrated animals have better feedlot performance and βAA improves animal efficiency in feedlot.

Keywords: Feedlot, Growth, Ruminant nutrition.