Effect of trace mineral injection and ractopamine hydrochloride on growth performance and carcass characteristics of finishing cattle

Abstract Text: The objective of this study was to determine how trace mineral (TM) injection, 96 and 28 d prior to harvest, and the β-agonist ractopamine hydrochloride impact growth and carcass characteristics of finishing beef cattle.  Two-hundred sixty-four crossbred steers (428 ± 29.6 kg, SD) consuming a common grain-based diet were assigned to one of 6 pens (n = 44 steers/pen) and randomly assigned to receive a 5-mL injection of TM (MM) or physiological saline (SAL; n = 132 per treatment; 22 per pen) on d 0. On d 68 steers were randomly assigned within treatment to receive a second injection of MM or SAL and 3 of the 6 pens were randomly assigned to receive 300 mg·steer^-1d^-1 of ractopamine hydrochloride for 28 d (RAC), or no ractopamine hydrochloride supplementation (CON). Data were analyzed as a 2×2×2 factorial, with fixed effects of initial injection, second injection, and RAC supplementation, and steer was the experimental unit.  Trace mineral injection did not affect ADG prior to RAC supplementation (P ≥ 0.78); however, there was an interaction between initial injection and RAC on ADG (P = 0.0003).  Within steers that received SAL initially, those supplemented with RAC had the greatest ADG (P < 0.001), and those without RAC had the least ADG (P = 0.03), while within those receiving MM initially, ADG was not affected by RAC supplementation (P = 0.15). The second injection did not affect ADG (P ≥ 0.22).  However, within CON cattle, MM steers had greater ADG than SAL steers (P = 0.03). An interaction between initial injection and RAC affected ribeye area (P = 0.03), where SAL+CON steers had the smallest ribeye area. Ribeye area was smaller in animals that received MM as their second injection (P = 0.04) compared with SAL. There was an interaction between initial and second injection (P = 0.01) where steers that received either MM or SAL at both time points had greater marbling scores (P ≤ 0.05) than SAL+MM steers. Another interaction between initial injection and RAC was found in yield grade (YG; P = 0.01), where SAL+CON and MM+RAC steers tended to have greater YG than SAL+RAC and MM+CON steers (P ≤ 0.10). Overall, TM injection 96 d prior to harvest may improve growth and carcass characteristics when ractopamine hydrochloride is not used, although TM injection 28 d prior to harvest appears to have no additional benefit.

Keywords: cattle, beta-agonist, trace mineral