Effect of age and pattern of gain on induction of puberty with a progestin in beef heifers.

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.