Effects of supplemental zinc sulfate concentrations on growth performance and carcass characteristics of feedlot heifers, and in vitro ruminal fermentative activity

Abstract Text:

Effects of supplemental Zn as Zn sulfate on feedlot performance and carcass characteristics were evaluated using 480 crossbred heifers (BW = 385 kg ± 13.08) in a randomized complete block design. Heifers were blocked by BW and randomly assigned within block to receive 0, 30, 60, or 90 mg supplemental Zn/kg diet DM. Heifers were housed in dirt-surfaced pens (20 animals/pen; 6 pens/treatment) equipped with fence-line feed bunks and automatic water fountains. Heifers were fed once daily ad libitum. Plasma was collected days 0, 63, and 115 from 4 or 5 heifers/pen to determine changes in plasma Zn concentrations. Heifers were transported on d 144 to a commercial abattoir where HCW and incidence of liver abscesses were recorded at harvest and carcass data were recorded after 36 h of refrigeration. Plasma Zn concentration increased linearly in response to increasing concentrations of dietary Zn (P = 0.02). Final BW and ADG were not affected by supplementation (P ≥ 0.29). Increasing supplemental Zn concentrations tended to decrease DMI (linear effect, P = 0.07), resulting in a linear improvement in feed efficiency with increasing Zn concentration (P = 0.03). No differences were detected for HCW, dressing percentage, LM area, 12^th rib fat, percentages of carcasses grading Select or Choice, or yield grade (P > 0.53). There was a tendency for a quadratic effect of Zn concentration on percentage of carcasses graded as USDA Prime, with percent Prime peaking at 60 mg/kg added Zn. Carcasses from heifers supplemented 60 mg Zn/kg diet DM yielded the greatest numerical increase ($25/carcass) in carcass value compared to other treatments (P = 0.32). In vitro fermentations were performed using ruminal fluid cultures containing 0, 30, 60, 90, 120, 150 mg Zn/kg substrate DM to determine impact of Zn on gas production, VFA concentrations, and IVDMD. There was no Zn × time interaction or effect of Zn on in vitro gas production (P ≥ 0.59). Zinc supplementation tended to reduce acetate (quadratic effect; P = 0.07), and decreased isovalerate (linear effect; P = 0.05), but did not affect other VFA (P ≥ 0.17) or IVDMD (P ≥ 0.20).  Overall, Zn supplementation up to 150 mg/kg substrate weight minimally affected in vitro fermentation. Supplementing up to 60 mg Zn/kg diet DM improves feed efficiency of feedlot cattle.

Keywords: feedlot cattle, feed efficiency, gas production, zinc