Growth, reproductive development, and estrous behavior of beef heifers treated with growth promotants

Charolais × Balancer heifers (n = 65; BW = 179 ± 30 kg; 255 ± 12 d of age) were used to determine the influence of growth-promoting implants on growth, reproductive development, estrous behavior, and pregnancy rate. Heifers were assigned to 1 of 4 implant treatment groups: (1) control, no implant (n = 16); (2) trenbolone acetate (TBA; 200 mg of TBA; n = 15); (3) trenbolone acetate plus estradiol (TBA+E2; 40 mg of TBA and 8 mg of E2; n = 17); or (4) zeranol (ZER; 36 mg of ZER; n = 17). Reproductive-tract scores (RTS) were determined via ultrasonography on d 106 and 195 (d 0 = implant treatment). Estrous behavior was monitored by radiotelemetry. Average daily gain was greater (P < 0.03) for TBA+E2 heifers compared with other treatment groups. A lower percentage (P < 0.03; 18%) of heifers treated with ZER were classified with cyclic reproductive-tract scores on d 106 compared with control (53%) and TBA heifers (67%); heifers treated with TBA+E2 (35%) were similar to all treatments. Heifers treated with TBA had increased mounts received (P < 0.05; 60.1 ± 10.4 mounts) during estrus compared with all other treatments (27.0 ± 8.2 mounts). Overall pregnancy rate did not differ (P > 0.10) among treatments (72%). Implanting with TBA+E2 after weaning resulted in heavier heifers at breeding, and reproductive development was delayed in ZER heifers. Implant strategies after weaning may alter heifer growth and development on forage systems, but overall fertility was not affected in this experiment.