Effect of a supplemental zinc complex and ractopamine hydrochloride on blood markers of the immune and beta-adrenergic responses in beef steers

Abstract Text: Forty-one crossbred steers (380 ± 5.3 kg) were used to determine the effect of supplementing a zinc (Zn) amino-acid complex (ZnC; Availa®Zn) to steers fed diets with or without ractopamine hydrochloride (RAC) on blood markers of the immune and beta-adrenergic responses. All steers consumed a dry-rolled corn-based diet supplemented with 60 mg Zn/kg DM as ZnSO₄. Steers were assigned to one of four supplemental ZnC treatments for 86 d:  Zn0: 0 mg Zn/kg DM (n = 6), Zn30: 30 mg Zn/kg DM (n = 12), Zn60: 60 mg Zn/kg DM (n = 12), and Zn90: 90 mg Zn/kg DM (n = 11). Blood samples were taken on d -4 and 78. Data were analyzed using the MIXED procedure of SAS, steer was the experimental unit, and initial period values were used as covariates in analysis. Total white blood cell (WBC), lymphocyte, and basophil counts were lesser in Con than ZnC-supplemented steers (P < 0.05). Plasma cyclic adenosine monophosphate (cAMP) and WBC counts tended to increase linearly with increasing dietary Zn (P < 0.10). There was a quadratic response within serum interleukin-8 (IL-8) where Zn60 steers had the least concentrations (P = 0.05). On d 86 one of the two pens of supplemental ZnC steers was randomly selected to receive RAC at 300 mg⋅steer–1⋅d–1 for 29 days before harvest, creating 7 final treatments: Con) 0 mg/kg DM of Zn from ZnC, no RAC (n = 6); Zn30) 30 mg/kg DM of Zn from ZnC, no RAC (n = 6); Zn30R) 30 mg/kg DM of Zn from ZnC + RAC (n = 6); Zn60) 60 mg/kg DM of Zn from ZnC, no RAC (n = 6); Zn60R) 60 mg/kg DM of Zn from ZnC + RAC (n = 6); Zn90) 90 mg/kg DM of Zn from ZnC, no RAC (n = 5); and Zn90R) 90 mg/kg DM of Zn from ZnC + RAC (n = 6). Blood samples were taken on d 112 and d 78 values were used as covariates in analysis. Supplementation of RAC increased WBC counts (P < 0.05), specifically lymphocytes, monocytes, eosinophils, and basophils (P < 0.05). Increasing ZnC within RAC-supplemented treatments tended to linearly decrease serum haptoglobin concentrations (P = 0.07), and Zn-supplementation without RAC tended to increase plasma cAMP compared with Con (P= 0.09). Further research is required to define the potential immune response to RAC in cattle.

Keywords: cattle, zinc, ractopamine hydrochloride