Response of fall-born calves to progesterone-estradiol benzoate implants and reimplants

Two trials, involving 469 steer and heifer calves, were conducted to evaluate the practices of implanting or reimplanting suckling fall-born calves with progesterone-estradiol benzoate (PE) implants (100 mg of progesterone, 10 mg of estradiol benzoate). Calves having an average initial weight of 67 kg were randomly allocated within sex and location to remain either nonimplanted or to receive a PE implant in the late fall, in the spring just before green grass, or in both fall and spring. All animals grazed good- to excellent-quality native Bluestem range. Nonimplanted calves in Trial 1 gained .57 kg/d during the winter months and PE implants improved growth rate by 7% to .61 kg/d (P less than .05). Growth rate of calves on dry winter range in Trial 2 was only .20 kg/d, and was not affected by PE implants. Average daily gain of nonimplanted calves during the spring and summer months was .96 and .87 kg/d for trials 1 and 2, respectively. Performance of implanted calves during that period was improved an average of 4.3 to 10% (P less than .05) by PE implants in both trials regardless of when the calves were implanted. Growth rate of calves over the entire 8-mo trials was .66 kg/d for nonimplanted calves and was improved (P less than .01) to .70, .70 and .71 kg/d for calves on the two single-implant and reimplant schedules, respectively. Implanted calves gained an average of 10.4 kg more than nonimplanted calves during the study.