Exploring the interaction of vitamin A and zinc in finishing cattle

Angus-cross steers (n = 95; BW = 473 ± 20 kg) were used to explore the interaction between vitamin A and zinc (Zn) and their influence on performance and carcass quality. Steers were stratified by BW to pens (n = 16; four pens/treatment), each equipped with a GrowSafe bunk (Vytelle, LLC., Lenexa, KS). Treatments were arranged as a 2 × 2 factorial; vitamin A treatments (VA) were: 0 IU supplemental vitamin A/kg DM (0VA) or 2,200 IU supplemental vitamin A/kg DM (2200VA; retinyl acetate, DSM Nutritional Products, Pendergrass, GA); Zn treatments (ZN) were: 0 mg/kg Zn (0ZN) or 120 mg/kg Zn (120ZN; ZnSO4). Steers were implanted with Synovex Plus (200 mg trenbolone acetate, 28 mg estradiol benzoate; Zoetis, Parsippany, NJ) at trial initiation. Individual BW was collected on days -1, 0, 28, 56, 83, and 84. Blood (serum, plasma) was collected on days 0, 56, and 79. Liver was collected on days 3 and 79. Blood and liver were analyzed for retinol and trace mineral concentrations. Carcass data were collected after a 48-hr chill. Data were analyzed in Proc MIXED or Proc GLIMMIX (SAS 9.4, SAS Inst. Inc., Cary, NC) with fixed effects of ZN and VA and the interaction; blood was analyzed via repeated measures (n = 23-24 per treatment, steer as experimental unit). There were no differences due to ZN, VA, or ZN × VA for live BW, overall ADG, or overall dry matter intake (P ≥ 0.21). Overall G:F was lesser in 0VA compared to 2200VA (VA; P = 0.05). No differences were observed in HCW, marbling, or REA due to ZN, VA, or ZN × VA (P ≥ 0.14). There was a tendency for rib fat to be greater in 0ZN-2200VA compared to other treatments (ZN × VA, P = 0.08). Dressing percentage was greatest in 0ZN-0VA and 120ZN-2200VA and least in 120ZN-0VA and 0ZN-2200VA, with 0ZN-0VA being not different from 120ZN-0VA (ZN × VA, P < 0.01). A greater percentage of carcasses graded average Choice or higher in 0VA compared to 2200VA (VA, P = 0.03). Liver retinol concentrations were lesser in 0VA compared to 2200VA (P = 0.01) and were unaffected by ZN (P ≥ 0.31). Serum retinol in 0VA declined upon removing vitamin A supplementation, 2200VA remained constant and was greater than that of 0VA from day 56 until harvest (VA × Day; P < 0.01). Liver Zn was unaffected by any treatment (P ≥ 0.16). Plasma Zn concentration was affected by ZN × VA (P = 0.02), where it was greatest in 120ZN (0VA and 2200VA), least in 0ZN-0VA, and intermediate in 0ZN-2200VA. Plasma Cu remained constant in 0VA until d 56 then declined prior to harvest, while in 2200VA steers increased by day 56 and remained greater than 0VA until harvest (VA × Day; P = 0.01). Liver Cu was lowest in 120ZN-2200VA compared to all other treatments (ZN × VA; P = 0.04). In finishing beef steers, dietary vitamin A supplementation presents a trade-off between optimal growth performance and carcass merit. Vitamin A supplementation increased plasma Zn concentration, and in this study supplementing both vitamin A and Zn promoted a shift in carcass composition.