The Influence of Supplemental Zinc and Ractopamine Hydrochloride on Mineral and Nitrogen Retention of Beef Steers

The study objective was to determine if N retention was improved with supplemental Zn above NRC concentrations with or without ractopamine hydrochloride (RH) inclusion. Angus crossbred steers (n = 32, 485 ± 57 kg) with Genemax gain scores of 4 or 5 were utilized in a 2x2 factorial arrangement (8 steers/treatment). Steers were blocked by BW to one of two finishing diets with no supplemental Zn (analyzed 32 mg Zn/kg DM; CON) or supranutritional Zn (analyzed 145 mg Zn/kg DM; SUPZN), fed for 56 days and then assigned to RH (Actogain, Zoetis) supplementation strategies of 0 (NORAC) or 300 mg·^-1steer·^-1d (RAC) fed the last 30 d before harvest. On d 57 (d 1 of RH 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 2x2 factorial in Proc Mixed of SAS with group as random and steer as the experimental unit. Initial 56 d ADG was not affected by treatment (P = 0.67) but DMI was lesser in SUPZN vs. CON (P < 0.01). No interactions between dietary Zn and RH strategies were noted during collection (P ≥ 0.19). Collection DMI did not differ by treatment (P ≥ 0.23); however, SUPZN intake of Mn and Zn was greatest (P < 0.01). Fecal and urinary excretion of Zn and retention of Mn and Zn were increased in SUPZN vs. CON (P < 0.01); however, Zn retention was not different between RAC and NORAC (P = 0.11). Retention of Cu and Fe were unaffected by treatment (P ≥ 0.85). Urine output and urine N excretion were decreased by RAC (P ≤ 0.05). Nitrogen retention as a percent of total ingested N tended (P = 0.10) to be greater in SUPZN (44.4%) vs. CON (40.0%) and tended (P = 0.06) to be greater in RAC (44.8%) vs. NORAC (39.7%). Using Proc Corr, Zn and N retention were found to be positively correlated (r = 0.48, P < 0.01). Average daily gain and G:F across the 86 d trial were increased in RAC vs. NORAC (P < 0.01). Overall, SUPZN appears to improve N retention, suggesting that increasing dietary Zn may be important for cattle growth beyond that induced by RH.