B M Ortner, S L Hansen
Angus-cross steers (n = 144; 453 ± 21 kg) were used in a 3 × 2 factorial to assess influence of dietary Zn and lubabegron fumarate on performance, carcass characteristics, and circulating metabolites. Steers were assigned to two blocks (n = 72 steers/block; n = 12 pens/block) and stratified by body weight (BW) and previous growing treatment into GrowSafe (GrowSafe Systems Ltd., Airdrie, AB, Canada) equipped pens of 6 steers (experimental unit of steer; n = 24/treatment). All cattle received a high-potency implant (Synovex® Plus; 200 mg trenbolone acetate/ 28 mg estradiol benzoate; Zoetis, Parsippany, NJ) on d 0 and were assigned to a Zn treatment (ZINC; 8 pens/treatment) supplemented through the diet at 30, 60, or 90 mg Zn/kg DM as MINTREX Zn Trace Mineral (Zn30, Zn60, and Zn90, respectively; Novus International, Inc., Chesterfield, MO). Equally within ZINC, steers received a β-agonist treatment (BA; 12 pens/treatment): 1) no β-agonist (NO) or lubabegron fumarate (EXP; 36 mg⸳steer-1⸳d-1; Experior; donated by Elanco Animal Health, Greenfield, IN) from d 39 to 82, followed by a four-day withdrawal. Cattle were weighed on d -1, 0, 38, 39, 59, 84 and 85 with blood collected on d 0, 38, 59, and 84. Within Proc Mixed of SAS, linear and quadratic effects of ZINC within BA treatments and NO vs. EXP were tested, and initial values served as covariates. EXP increased BW, average daily gain, feed efficiency (G:F), hot carcass weight, and ribeye area and decreased marbling, 12th rib fat, empty body fat, and yield grade (P ≤ 0.01). Increasing Zn linearly decreased dry matter intake and quadratically increased G:F within NO during the EXP-period and overall (P ≤ 0.05), but other performance parameters were not affected (ZINC; P ≥ 0.11). Overall plasma Zn concentration was greater in Zn90 and Zn60 than Zn30 (P = 0.01). EXP treatments tended to have lesser serum glucose and insulin concentrations, and Zn90/EXP was lowest (ZINC × BA; P ≤ 0.10). During the EXP-period, EXP decreased serum glucose, urea nitrogen, non-esterified fatty acid, and insulin concentrations (BA × day; P ≤ 0.03). Greater RQUICKI values, a proxy for insulin sensitivity, were noted in EXP, and Zn90/EXP was greatest overall on d 84 (ZINC × BA × day; P = 0.09). In this study, supplementing Zn above 30 mg/kg DM did not improve growth in EXP over NO; however, in NO, Zn improved feed efficiency and altered circulating metabolites, providing evidence Zn impacts energy metabolism in finishing cattle.