The use of β-adrenergic agonist in beef cattle diet

The aim of this study was to evaluate the use of Zilpaterol and Ractopamine Hydrochloride, in two distinct experiments on Nellore diet in feedlot system, for growth performance, serum metabolites, carcass traits, empty body weight composition, carcass gain composition, meat quality and gene expression. The Exp. 1 used 48 bulls, randomly assigned to four treatments in a 2x2 factorial arrangement; two levels of dietary CP: 100% and 120% of metabolizable protein requirement, defined as CP100 and CP120, respectively. Moreover, two levels of RH: 0 and 300 mg/animal/per day. The Exp. 2 used 72 Nellore heifers assigned to a randomized block design. Heifers were separated into 2 groups: Control and Zilpaterol (ZH). In both experiments, the animals were weighed and blood samples were taken periodically. Animals were slaughtered and HCW was recorded, muscle samples were taken to PCR analysis. After chilling, LMA and fat thickness were recorded, longissimus samples and the 911th rib section were obtained. In addition, in the Exp. 2, heifers were sub groups were slaughterer at 0, 20 and 30 days of ZH supplementation, in the last slaughterer, the carcass were debone. Supplementation with RH increased ADG, reduced DMI at CP120, improved G:F, plasma total protein at CP120, also decreased plasma glucose concentration at CP100, and increased ALP activity at CP120. Ractopamine decreased meat shear force, at day 0 of aging. Greater dietary protein increased meat shear force after 0 and 7 days of aging. Heifers fed ZH had gains in ADG, HCW, G:F ratio, increased serum creatinine, and decreased serum NEFA. Zilpaterol increased carcass dressing percentages, and also decreased kidney-pelvic fat. The EBW composition was changed after 20 d of ZH supplementation. The ZH increased EBW gain, EB protein gain, and subprimal yield. There was an increase on WBSF after 7 and 14 d of aging. The sensory panelists reported a decreased on meat tenderness by ZH supplementation, regarding aging time, and a decrease on juciness at 0 and 14 d of aging. The ZH increased the calpain and calpastatin gene expression. These results indicate the efficiency of RAC and ZH to improve the performance,feed efficiency, and muscle mass deposition in Nellore bulls and heifers. Greater CP did not further improved the RAC effect. The ZH supplementation increased the subprimal yield and changed body gain composition. Finally, the use of β-adrenergic agonist increase the beef cattle system's efficiency by increasing the lean carcass component, with greater profitability per slaughtered animal (AU)