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Effects of ractopamine or zilpaterol on physiologic and metabolic parameters in feedlot steers
The feeding of beta adrenergic agonists has recently been implicated as a potential animal welfare concern. The objective of this study was to determine the effects of ractopamine hydrochloride or zilpaterol hydrochloride on physiologic and metabolic blood parameters indicative of stress. Thirty crossbred steers (BW = 573 ± 27.4 kg) were used in a randomized complete block design. Steers were grouped into 2 blocks based on harvest date, stratified by BW within block, assigned randomly to treatment and then placed into individual pens. A basal finishing ration was delivered to each steer twice daily. Treatments were applied via premix containing 20 g ground corn at each feeding and consisted of: 1) no beta agonist (CON), 2) 300 mg/d ractopamine hydrochloride (RH), or 3) 90 mg/d zilpaterol hydrochloride (ZH). Blood samples were collected via jugular venipuncture on d -7, -1, 0, 0.25, 0.5, 1, 3, 7, 14, and 21, relative to initial delivery of treatment. Whole blood (excluding d 21) was analyzed for complete blood count using an automated hemocytometer. Serum was analyzed for cortisol and haptoglobin via ELISA, and an array of blood metabolites were evaluated using an automated analyzer. Overall, the total leukocyte concentration was greater (P = 0.01) in steers that received RH compared with CON; whereas, ZH did not differ (P ≥ 0.43). On d 1, total leukocytes were greater for both RH and ZH vs. CON (trt x d; P ≤ 0.046). Overall, neutrophil concentration was greater in RH (P = 0.01) and ZH (P < 0.01) compared with CON. Neutrophils were greater for ZH on d 1 and 3 and RH on d 7 compared to CON (trt x d; P ≤ 0.045). Although RH did not alter (P = 0.36) the neutrophil to lymphocyte ratio (NLR), NLR was greater (P = 0.03) in steers fed ZH vs. CON. Treatment did not affect any other complete blood count variables (P ≥ 0.10) and cortisol and haptoglobin were not different (P ≥ 0.19). Serum urea nitrogen was decreased (P < 0.02) and potassium and chloride were increased (P < 0.01) for both RH and ZH. Creatine kinase was increased for ZH on d 14 and 21 (P ≤ 0.017). Serum albumin, globulins, NEFA, creatinine, calcium, phosphorus, and magnesium were not different (P≥ 0.57). Under conditions of this study, feeding of either beta-agonist did not clearly affect blood parameters indicative of physiological stress.
Keywords: beta-agonists, stress