Zinc supplementation strategies in feedlot heifers receiving an extended-release implant or an aggressive re-implant program
Abstract Two-hundred and eight Angus-crossbred heifers (291 ± 23 kg) from four sources were used in a randomized complete block design. The objective of the study was to determine the effects of implant strategy and Zn supplementation on performance, carcass characteristics, muscle fiber diameter, and mineral status of heifers. Heifers were assigned to a 2 × 2 factorial study for 168 d, factors included Zn and implant (IMP). Heifers were supplemented Zn (mg/kg dry matter [DM]; ZnSO4) at national (30; NRC) or industry (100; IND) recommendations. Implant strategies (Merck Animal Health, Madison, NJ) included extended-release Revalor-XH on d 0 (REV-XH; 20 mg estradiol + 200 mg trenbolone acetate) containing 4 uncoated pellets and 6 coated pellets or the uncoated implant Revalor-200 on d 0 and again on d 91 (REV-200; 20 mg estradiol + 200 mg trenbolone acetate). Heifers were blocked by weight within source to pens of 5 or 6 heifers per pen (9 pens per treatment). A corn silage-based diet was fed during the growing period (d 0-55) followed by transition to a corn-based finishing diet. Weights were taken consecutively on d -1/0, 55/56, and 167/168. Liver and muscle from the longissimus thoracis were collected from one heifer per pen on d -5, 14, 105, and 164. Data were analyzed via Mixed Procedure of SAS. Average daily gain (ADG) and liver mineral used Period as the repeated effect. Corresponding to periods of high hormone payout from each implant, d 0-28 and 91-120 ADG were greatest for REV-200, while REV-XH numerically peaked during d 56-91 (IMP × Period; P = 0.02). Day 91 IND body weight tended to be heavier (P = 0.06) and d 120 body weight was heavier (P = 0.05) than NRC heifers. No effect of Zn or IMP on final body weight was observed (P ≥ 0.21). Muscle fiber cross-sectional diameter on d 164 was greater (P = 0.05) in IND than NRC. Liver Mn concentrations decreased by d 14 regardless of implant, though d 105 and 164 concentrations were lesser for REV-200 than REV-XH (IMP × Period; P = 0.02). No effects of Zn, IMP, or the interaction were observed for carcass-adjusted gain to feed, d 0-168 DM intake, hot carcass weight, or ribeye area (P ≥ 0.11). The nominal differences in performance between implant strategies suggests extended-release implants may be an effective implant strategy to replace re-implant programs in heifers, while the improved performance of heifers fed IND vs. NRC during times of peak hormone payout suggests a role for Zn in periods of rapid growth.