The influence of supplemental zinc and ractopamine hydrochloride on trace mineral and nitrogen retention of beef steers.

The study objective was to determine whether N retention was improved with supplemental Zn above NRC concentrations with or without ractopamine hydrochloride inclusion. Angus crossbred steers (n = 32, 485 ± 26 kg BW) with Genemax gain scores of 4 or 5 were utilized in a 2 × 2 factorial arrangement (8 steers/treatment). Steers were blocked by BW to a finishing diet with 1 of 2 mineral supplementation strategies (ZNTRT), no supplemental Zn (analyzed 32 mg Zn/kg DM; CON) or supranutritional Zn (CON + 60 ppm ZnSO4 + 60 ppm Zn-amino acid complex; analyzed 145 mg Zn/kg DM; SUPZN), fed 56 days in pens equipped with GrowSafe bunks and assigned to β-agonist (BA) supplementation strategies of 0 (NON) or 300 mg steer-1 d-1 ractopamine hydrochloride (RAC) fed the last 30 d before harvest. Initial 56-d ADG was not affected by ZNTRT (P = 0.66), but DMI was greater in CON vs. SUPZN (P < 0.01). On day 56 (day 1 of BA supplementation), steers (4 groups; 8 steers/group; 2 steers/treatment) were moved to metabolism crates and adapted for 10 d, followed by 5 d of total fecal and urine collection. Total retention of Zn, Mn, Fe, Cu, and N were calculated. Data were analyzed as a 2 × 2 factorial arrangement, with group as a fixed effect and the 3-way interaction of ZNTRT × BA × group as random. No interactions between ZNTRT and BA were noted for any data (P ≥ 0.19). Collection DMI did not differ among treatments (P ≥ 0.23); however, Zn intake was lesser in CON vs. SUPZN (P < 0.01). Fecal and urinary Zn excretion and Zn and Mn retention were lesser in CON vs. SUPZN (P ≤ 0.03); however, Zn retention was not different between NON and RAC (P = 0.43). Retention of Cu and Fe was unaffected by strategies (P ≥ 0.49). Urine output and urine N excretion were greater in NON vs. RAC (P ≤ 0.05). Nitrogen retention (as percent of N intake) was lesser (P = 0.05) in CON (40.0%) vs. SUPZN (44.3%) and lesser (P = 0.02) in NON (39.5%) vs. RAC (44.8%). Zinc and N retention were found to be positively correlated (r = 0.46, P < 0.01). Average daily gain and G:F across the 86-d trial were lesser in NON vs. RAC (P < 0.03). Overall, SUPZN appears to improve N retention, suggesting that increasing dietary Zn may be important for cattle growth beyond that induced by ractopamine hydrochloride.