52 The influence of supplemental zinc and dietary energy level on mineral retention of beef steers.

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Abstract
The objective was to determine if Zn retention improved with supplemental Zn above recommended concentrations with increasing growth rate. Angus steers (n = 32; 309 ± 4.2 kg BW) with Genemax gain scores of 3, 4, or 5 were utilized in a 2 × 2 factorial arrangement (8 steers/treatment). Steers were blocked by BW within growing diets to one of two Zn strategies (ZNTRT) 1) no supplemental Zn (analyzed 36 mg Zn/kg DM; CON) or 2) supranutritional Zn (analyzed 153 mg Zn/kg DM; SUPZN), fed for 60-d and then assigned to a dietary energy strategy (ENERGY) to emulate two different growth rates representing ~1.5 kg ADG (LOW) or ~2 kg ADG (HI). Beginning on d-60, HI steers were transitioned to HI for 14-d. On d 74, steers (4 groups; 8 steers/group; 2 steers/treatment) moved to metabolism crates and adapted for 10-d, followed by 5-d of total fecal and urine collection. Data were analyzed as a 2 × 2 factorial in Proc Mixed of SAS with ZNTRT×ENERGY×group as random; steer was experimental unit. No ZNTRT and ENERGY interactions were detected (P ≥ 0.12). Dry matter digestibility tended to increase due to SUPZN (P = 0.10). Water intake was lesser (P ≤ 0.03) in SUPZN vs. CON and LOW vs. HI; urine output tended (P = 0.08) to decrease in LOW vs. HI. Copper apparent absorption was unaffected by ZNTRT (P = 0.28), but Cu apparent absorption increased in LOW (22.1%) vs. HI (15.3%; P = 0.04). SUPZN increased (P < 0.01) intake and retention of Zn (377 vs. 100 mg/d) and decreased Zn apparent absorption vs. CON (30.9 vs. 25.5%; P = 0.04). Zinc apparent absorption (32.3 vs. 24.1%) increased in LOW vs. HI (P < 0.01). Zinc absorption and retention appear to be influenced by diet energy concentration and more positively impacted by LOW energy diet.