Evaluation of the effects of zilpaterol hydrochloride supplementation on catecholamine response and other blood metabolites following a combined corticotropin releasing hormone and vasopressin challenge

Abstract Text:

The stress response of cattle supplemented with zilpaterol hydrochloride (ZH) has become an issue due to claims of cattle responding poorly to stress.  This study was designed to determine if differences exist in the catecholamine and blood metabolite response of ZH-supplemented cattle when exposed to a metabolic stress challenge.  Crossbred heifers (n=18; 596±39 kg BW) were randomized into two treatment groups and individually fed daily with heifer serving as the experimental unit: 1) Control (CON): finishing diet with no ZH, and 2) Zilpaterol (ZIL): finishing diet with ZH (7.56 g/ton DM basis).  Zilpaterol heifers were supplemented ZH for 20d, with a 3d withdraw period.  On d 20 of supplementation, all heifers were fitted with indwelling jugular catheters.  On d24, starting at 0400h (6h prior) and 0800h (2h prior) and continuing until 1600h (8h post), blood samples were collected at 60 min intervals for plasma and serum respectively. At 1000h (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. Serum was separated and stored at -80^oC until analyzed for creatine phosphokinase (CPK), creatine, Ca, P, Na, K, and liver enzymes. Blood samples were collected in EDTA vacutainers; plasma was separated, flash frozen, and stored at -80^oC until analyzed for catecholamines. Data were analyzed using the MIXED procedure with h as a repeated measure and compound symmetry covariance structure. There was only a treatment effect (P<0.02) for Ca and K; ZIL heifers had decreased concentrations during the CRH/VP challenge. Heifers for ZIL had increased concentrations of CPK (P=0.05) and creatine (P<0.01) during CRH/VP challenge. A treatment x time effect (P=0.02) was observed for P.  Concentrations of P were similar -2h prior to 6h post-challenge.  At 7h post-challenge, CON heifers had less P compared to ZIL heifers. There were no differences observed (P≥0.22) for the liver enzymes aspartate transaminase and gamma glutamyltransferase. Heifers fed CON had greater concentrations of alkaline phosphatase and sorbitol dehydrogenase (P<0.05) compared to ZIL heifers. Heifers for ZIL had decreased concentrations of epinephrine (P=0.04). There was no difference (P=0.94) for norepinephrine. While some variations were observed between ZIL and CON heifers in terms of response to the CRH/VP challenge, these alterations appear to be minor.  In this experimental setting, supplementation of ZH did not dramatically alter the ability of cattle to respond to stress stimuli. 

Keywords: β-agonist, stress response, Zilmax