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The objective of this study was to determine the metabolic, stress, hematology, organ weights, histology and cardiac anatomical features of beef heifers supplemented with zilpaterol hydrochloride (ZH) when exposed to a corticotropin-releasing hormone (CRH) and arginine vasopressin (VP) challenge. Crossbred heifers (n = 20; 556 ± 7 kg BW) were randomized into two treatment groups: 1) Control (CON): no ZH, and 2) Zilpaterol (ZIL): supplemented with ZH at 8.33 mg / kg (DM basis). The ZIL group was supplemented ZH for 20 d, with a 3-d withdrawal period. On d 24, heifers received an intravenous bolus of CRH (0.3 µg / kg BW) and VP (1.0 µg / kg BW). Blood samples were collected at 30- and 60-min intervals for serum, and 60-min intervals for plasma and whole blood, from -2 to 8 h relative to the challenge. Heifers were harvested to collect organ weights and histology samples on d 25, 26, and 27. There was a treatment (P < 0.001) effect for vaginal temperature (VT), with ZIL having a 0.1oC decrease in VT. A treatment x time effect (P = 0.002) was observed for non-esterified fatty acid such that concentrations were increased in CON heifers at 4, 6, 7, 7.5, and 8 hrs. There was a treatment effect for cortisol (P < 0.01) and epinephrine (P = 0.003) with ZIL having decreased concentrations. Glucose concentrations within the longissimus muscle were greater (P = 0.03) in CON heifers. Lactate concentrations and glycolytic potential within the biceps femoris were greater in CON heifers (P = 0.05). A treatment effect (P ≤ 0.02) was observed for calcium and potassium, with ZIL heifers having decreased concentrations. Zilpaterol heifers had increased concentrations of creatinine (P = 0.02). A treatment x time interaction (P = 0.02) was observed for phosphorus. Raw liver weights tended (P = 0.08) to be less in ZIL heifers. These data suggest some variations between treatments in terms of response to ZH supplementation and challenge; however, in the environmental conditions of this study, the data suggests that the supplementation of ZH does not negatively alter the status of homeostasis in cattle.
Advisor: Ty B. Schmidt