|Hall, John - UNIV MINNESOTA|
|Staigmiller, Robert - RETIRED USDA-ARS|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 1997
Publication Date: N/A
Interpretive Summary: Beef heifers that are younger at puberty (first heat) conceive earlier and produce more kilograms of calf (beef) in their lifetime. Induction of puberty may be a method to decrease age at puberty. This experiment examined how age and growth rate affected the ability of norgestomet, a synthetic progestin, to induce puberty in beef heifers. Seventy-five crossbred heifers were grown at two different rates rapid or slowly- then-rapid. At 9.5, 11, and 12.5 mo of age 10-12 heifers that were not pubertal were given a progestin implant norgestomet. Reproductive hormones and ovarian follicular characteristics were examined. The ability of norgestomet to induce puberty in heifers was dependent upon age at treatment. The best response was in 12.5 mo old heifers. Growth rate did not affect response to norgestomet in heifers fed to grow normally or slowly-then-rapidly. Norgestomet seems to induce puberty by triggering the cascade of hormonal events and ovarian follicular changes similar to spontaneous puberty.
Technical Abstract: 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 growth rate on induction of puberty by a progestin. At 9.5, 11.0 and 12.5 mo of age, twelve non-cycling heifers from each growth treatment received progestin (6 mg norgestomet for 10 d) or control treatments. Induction of puberty, LH secretory profiles, and ovarian follicular characteristics were assessed in progestin and control heifers. Body weights of R heifers were greater (P <.01) than SR heifers at all ages. At 12.5 mo, more progestin treated heifers exhibited a puberal estrus within 5 d of implant removal compared to controls (82% vs 9% respectively), but progestin did not induce puberty at 9.5 or 11 mo of age (progestin x age, P <.05) in heifers of either growth rate. Progestin increased (P <.01) LH pulse frequency at all ages, whereas progestin only increased mean LH concentrations at 12.5 mo of age (progestin x age, P <.03). Progestin treatment altered (P <.01) ovarian follicular characteristics at all ages. Growth rate did not affect (P >.1) LH secretory profiles, ovarian characteristics, or induction of puberty by progestin. We conclude that progestins induce puberty by hastening the normal cascade of endocrine and ovarian events associated with spontaneous puberty. Furthermore, age (but not growth rate) appears to be the critical factor influencing the efficacy of progestins to induce puberty in heifers.