norgestomet and Weight-Gain

Crossbred heifers (n = 75) fed for rapid (R; .82 kg/d) or slow-then-rapid (SR; .41 kg/d for 90 d then .82 kg/d) postweaning gain were used to examine the effects of age or pattern of gain on induction of puberty by a progestin. At 9.5, 11.0, and 12.5 mo of age, 12 prepuberal heifers from each growth treatment received progestin (a 6-mg Norgestomet implant for 10 d) or control treatments. Induction of puberty, LH secretory profiles, and ovarian follicular characteristics were assessed in Norgestomet-treated and control heifers. Body weights of R heifers were greater (P < .01) than those of SR heifers at all ages. At 12.5 mo, more Norgestomet-treated heifers exhibited a puberal estrus within 5 d after implant removal compared with controls (82% vs 9%, respectively), but Norgestomet did not induce puberty at 9.5 or 11 mo of age (progestin x age, P < .05) in heifers of either gain pattern. Norgestomet increased (P < .01) LH pulse frequency at all ages, whereas Norgestomet increased only mean LH concentrations at 12.5 mo of age (progestin x age, P < .03). Norgestomet treatment altered (P < .01) ovarian follicular characteristics at all ages. Gain pattern did not affect (P > .1) LH secretory profiles, ovarian characteristics, or induction of puberty by Norgestomet. We conclude that progestins induce puberty by hastening the normal cascade of endocrine and ovarian events associated with spontaneous puberty. Furthermore, age, but not pattern of gain, seems to be the critical factor influencing the efficacy of progestins to induce puberty in heifers.

Topics: Aging; Animals; Body Weight; Cattle; Drug Implants; Female; Luteinizing Hormone; Ovarian Follicle; Pregnenediones; Progesterone Congeners; Random Allocation; Sexual Maturation; Time Factors; Ultrasonography; Weight Gain

The efficacy of erodible norgestomet implants for preventing pregnancy in postpubertal heifers was evaluated in two experiments at five locations each. Heifers (n = 896) within each study location were stratified by weight and allotted randomly to receive an ear implant containing either 0, 24, 36, or 48 mg of norgestomet (d 0). Heifers were exposed 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). Weights were recorded on d 0 and 74 (Exp. 1 and 2) and d 154 (Exp. 2). Each heifer was palpated rectally for pregnancy at the end of each experiment. Pregnancy rates were higher (P < .01) for control heifers (0 mg implant) than for heifers that received 24, 36, or 48 mg of norgestomet. 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 received 0, 24, 36, and 48 mg implants of norgestomet, respectively. Estrous activity during the first 3 wk of bull exposure was reduced (P < .05) among heifers that received norgestomet implants compared to control heifers but was not completely abolished at any dosage in Exp. 1. During 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 heifers. Combined across both experiments, ADG during the first 74 d were .53, .56, .59, and .60 kg/d for heifers treated with 0, 24, 36, and 48 mg implants of norgestomet, respectively. These data indicate that norgestomet implants increased rate of weight gain, reduced estrous activity, and reduced the occurrence of pregnancy in heifers on pasture.

Topics: Animal Feed; Animals; Cattle; Dose-Response Relationship, Drug; Drug Implants; Estrus; Female; Fertility; Male; Pregnancy; Pregnancy Rate; Pregnancy, Animal; Pregnenediones; Progesterone Congeners; Random Allocation; Weight Gain