Supplementation of zilpaterol hydrochloride to crossbred Angus heifers does not increase stress responsiveness or homeostatic metabolic parameters following a combined corticotropin releasing hormone and vasopressin challenge
Anecdotal claims suggest that feeding zilpaterol hydrochloride (ZH) alters the stress response in cattle; however, there is no scientific data to support or refute these claims. This study was designed to determine if differences exist in the stress response of ZH-supplemented cattle when exposed to a metabolic stress challenge. Heifers (n=20; 556±7 kg BW) were randomized into two treatment groups: 1) Control: heifers received a finishing diet with no ZH, and 2) ZH: heifers received a finishing diet with ZH at 7.56 g/ton (DM basis). Three d prior to the start of ZH supplementation, heifers were fitted with indwelling vaginal temperature (VT) probes. Heifers in the ZH group were supplemented ZH for 20d, with a 3d withdraw period. On the last d of supplementation, heifers were fitted with indwelling jugular catheters. On d24, heifers received an i.v. bolus of corticotropin releasing hormone (CRH; 0.3 µg/kg BW) and vasopressin (VP; 1.0 µg/kg BW) to activate the stress axis. Blood samples were collected at 30-min intervals for serum and 60-min intervals for whole blood from -2 to 8h (0800-1600h) relative to the CRH/VP challenge at 0h (1000h). Serum was separated and stored at -80oC until analyzed for cortisol, glucose, insulin, NEFA, and blood urea nitrogen (BUN). Whole blood samples were collected in EDTA vacutainers and analyzed for complete blood cell counts. Data were analyzed with the MIXED procedure for repeated measures (SAS). There were no differences observed (P≥0.12) for glucose, insulin, red blood cells, hemoglobin change, hematocrit change, platelets, neutrophils, or monocytes between treatments. There was a treatment (P<0.001) effect for VT, with ZH heifers having a 0.1oC decrease in VT when compared to Control heifers. There was also a treatment effect (P=0.003) for cortisol; ZH heifers had decreased cortisol during the CRH/VP challenge. A treatment effect (P<0.001) was observed for BUN, with ZH heifers having decreased BUN when compared to Control heifers. A treatment x time effect (P=0.002) was observed for NEFA; NEFAs were similar between treatments prior to challenge (-2h), and remained similar until 4h post challenge. At 4, 6, 7-8h post challenge, Control heifers had greater NEFA when compared to ZH heifers. While some variations were observed between the ZH and Control heifers in terms of response to the CRH/VP challenge, none of these changes suggest that the supplementation of ZH detrimentally alters the ability of cattle to effectively respond to stressful stimuli.
Keywords: zilpaterol hydrochloride, β-agonist, stress