Patterson D J, Kiracofe G H, Stevenson J S, Corah L R
Dept. of Anim. Sci., Univ. of KY, Lexington 40546.
J Anim Sci. 1989 Aug;67(8):1895-906. doi: 10.2527/jas1989.6781895x.
Expanded use of artificial insemination in the beef cattle industry depends on successful application of treatments designed to synchronize estrus. Regulation of estrous cycles is associated with control of the corpus luteum (CL), whose life span and secretory activity are subject to trophic and lytic mechanisms. The advantages of melengestrol acetate (MGA) in estrous synchronization incorporate ease of administration, lower cost relative to other estrous synchronization products, and potential for use to induce estrus in prepubertal heifers. Treatments first designed to synchronize estrous cycles of normally cycling heifers by feeding MGA were imposed daily for 14 to 18 d at levels of .5 to 1 mg. The minimal daily effective dose required to inhibit ovulation was .42 mg. Longer feeding periods of MGA were associated with low fertility at the first synchronized estrus, but at the second estrus, conception was normal. Low fertility at the synchronized estrus resulted in development of alternative treatment practices, which combined feeding of MGA with injections or implants of estradiol-17 beta, estradiol cypionate, luteinizing hormone, human chorionic gonadotropin, pregnant mare serum gonadotropin, or oxytocin. Estrus was synchronized after MGA and estradiol-17 beta or estradiol cypionate treatments, but fertility was low. Short-term feeding of MGA (5 to 7 d) combined with prostaglandin F2 alpha or its analogs (PGF) on the last day of MGA reduced fertility at the synchronized estrus. The reduced conception at first service occurred in animals that began treatment after d 12 of the estrous cycle. However, feeding MGA for 14 d and then injecting PGF 17 d later avoided problems with reduced conception. Fertility of animals after this treatment was similar to that of contemporaries synchronized with Syncro-Mate-B. However, the length of the treatment period creates a need for increased management and may extend management beyond practical limits. Further research is warranted to address problems associated with reduced fertility after short-term treatment with MGA.
肉牛行业人工授精的广泛应用取决于旨在同步发情的处理方法的成功应用。发情周期的调节与黄体（CL）的控制有关，黄体的寿命和分泌活动受营养和溶解机制的影响。醋酸甲地孕酮（MGA）在发情同步方面的优势包括易于给药、相对于其他发情同步产品成本较低，以及可用于诱导青春期前小母牛发情。最初设计通过每天饲喂0.5至1毫克的MGA来同步正常发情的小母牛的发情周期，持续14至18天。抑制排卵所需的最小每日有效剂量为0.42毫克。较长时间饲喂MGA与首次同步发情时的低生育率相关，但在第二次发情时，受孕情况正常。同步发情时的低生育率导致了替代处理方法的发展，这些方法将MGA的饲喂与注射或植入17β-雌二醇、环丙孕酮、促黄体生成素、人绒毛膜促性腺激素、孕马血清促性腺激素或催产素相结合。MGA与17β-雌二醇或环丙孕酮处理后发情得以同步，但生育率较低。在MGA的最后一天短期饲喂MGA（5至7天）并联合前列腺素F2α或其类似物（PGF）会降低同步发情时的生育率。首次输精时受孕率降低发生在发情周期第12天后开始处理的动物身上。然而，饲喂MGA 14天，然后在17天后注射PGF可避免受孕率降低的问题。这种处理后动物的生育率与使用Syncro-Mate-B同步发情的同期动物相似。然而，处理期的长度需要增加管理，并且可能会使管理超出实际限制。有必要进一步研究解决短期使用MGA处理后与生育率降低相关的问题。
