Effects of increasing supplemental zinc in beef feedlot steers administered a steroidal implant and beta agonist.

Ninety-two Angus-crossbred steers (424 ± 28.3 kg initial body weight) were used in a 98-d study to assess the effects of increasing Zn supplementation on cattle performance, liver and plasma trace mineral concentrations, blood metabolites, and carcass characteristics. All steers were implanted with a Component TE-200 (200 mg trenbolone acetate + 20 mg estradiol; Elanco Animal Health, Greenfield, IN) on d 0 and fed 300 mg‧steer -1 ‧d -1 of ractopamine hydrochloride (Zoetis, Parsippany, NJ) from d 70 to 98. Cattle were fed via GrowSafe bunks (GrowSafe Systems Ltd., Airdrie, AB, Canada), and steer served as the experimental unit ( n  = 22 or 23 steers/treatment). Supplemental Zn was administered through the diet at 0, 100, 150, or 180 mg Zn/kg on a dry matter basis from ZnSO 4 (Zn0, Zn100, Zn150, or Zn180, respectively). Cattle were weighed on d -1, 0, 9/10, 20, 41, 59, 69, 70, 78/79, 97, and 98. Blood was collected on d 0, 9/10, 69, 78/79, and 97, and liver biopsies on d 9/10 and 78/79 ( n  = 12 steers/treatment). Data were analyzed as a complete randomized design. Contrast statements were formed to test the linear, quadratic, and cubic effects of Zn supplementation and test Zn0 vs. Zn supplementation. Day 10 and 70 body weight (BW) and d 0 to 10 and 0 to 70 average daily gain were linearly increased with Zn supplementation ( P  ≤ 0.05), and greater for Zn supplemented steers ( P  ≤ 0.03). No effects of Zn supplementation were observed on final BW, dressing percentage, ribeye area, 12th rib fat, or marbling ( P  ≥ 0.11). Hot carcass weight tended to be 7 kg greater for Zn supplemented steers than Zn0 ( P  = 0.07), and yield grade linearly increased with increasing Zn supplementation ( P  = 0.02). Day 10 liver Mn concentrations tended to quadratically decrease ( P  = 0.08) with increasing Zn supplementation, though d 79 liver Mn concentrations and arginase activity were not influenced by Zn ( P  ≥ 0.28). Day 10 liver arginase activity tended to be ( r  = 0.27; P  = 0.07) and d 10 serum urea nitrogen was correlated with d 10 liver Mn ( r  = 0.55; P  < 0.0001). Zinc supplementation linearly increased d 10 liver Zn and d 10, 69, 79, and 97 plasma Zn concentrations ( P  ≤ 0.05). A cubic effect of Zn was observed on d 79 liver Zn ( P  = 0.01) with lesser liver Zn in Zn0 and Zn150 steers. These data suggest increasing dietary Zn improves growth directly following the administration of a steroidal implant and that steroidal implants and beta agonists differ in their effects on protein metabolism.