NORGESTOMET IMPLANTS PREVENT PREGNANCY IN BEEF HEIFERS ON PASTURE

The efficacy of erodible norgestomet implants for preventing pregnancy in postpubertal heifers was evaluated in two experiments at five loca tions each. Heifers (n = 896) within each study location were stratifi ed by weight and allotted randomly to receive an ear implant containin g either 0, 24, 36, or 48 mg of norgestomet (d 0). Heifers were expose d to fertile bulls immediately after implantation for 75 d (d 0 to 74) in Exp. 1 (n = 476) or for 80 d (d 75 to 154) in Exp. 2 (n = 420). We ights were recorded on d 0 and 74 (Exp. 1 and 2) and d 154 (Exp. 2). E ach heifer was palpated rectally for pregnancy at the end of each expe riment. Pregnancy rates were higher (P <.01) for control heifers (0 mg implant) than for heifers that received 24, 36, or 48 mg of norgestom et. In Exp. 1, pregnancy rates were 96, 29, 6, and 4% for heifers that received 0, 24, 36, and 48 mg implants of norgestomet, respectively. In Exp. 2, pregnancy rates were 85, 36, 19, and 9% for heifers that re ceived 0, 24, 36, and 48 mg implants of norgestomet, respectively. Est rous activity during the first 3 wk of bull exposure was reduced (P <. 05) among heifers that received norgestomet implants compared to contr ol heifers but was not completely abolished at any dosage in Exp. 1. D uring the first 75 d of Exp. 1 and 2, heifers treated with 36 or 48 mg norgestomet implants gained weight faster(P <.05) than control heifer s. Combined across both experiments, ADG during the first 74 d were .5 3, .56, .59, and .60 kg/d for heifers treated with 0, 24, 36, and 48 m g implants of norgestomet, respectively. These data indicate that norg estomet implants increased rate of weight gain, reduced estrous activi ty, and reduced the occurrence of pregnancy in heifers on pasture.