摘要
This paper reviews seasonal breeding, puberty, postpartum anestrus, embryonic loss and calf survival and their constraints in female yaks. Methods for improving fertility in postpartum yak cows are also considered. Yaks are seasonal breeders with mating and conception restricted in the warm season. Puberty generally occurs in the 2nd to the 4th warm season following birth, i.e. between 13 and 36 months of age. The cows usually have a long postpartum anestrus period; only a small proportion of the cows return to estrus in the 1st breeding season after calving, most come into estrus in the 2nd and 3rd years. Nutritional status is the most important determinant of reproduction in female yaks. Reproductive success is a direct result of the availability of pasture determined by climate, season, and management practices. Milking delays puberty by reducing milk intake (restricted suckling) and growth rate for the calf. Milking interferes with grazing and prolongs the duration of postpartum acyclicity in cows. Calves born early in the season have a longer suckling season than those born later in the season before the onset of winter. Thus, they can have their first cycle in the breeding season of the following year, while those born late in the season may not have their first estrus until 25 or 26 months of age. Cows calving early in the season are more likely to return to estrus in the year of calving because they have a longer period to recover from the demand on body reserves before the onset of winter.
Inbreeding in smallholder yak farms is also discussed and minimizing inbreeding by exchanging bulls among different herds is suggested. Reproductive efficiency can be improved by nutritional supplementation during the winter, however, the most cost-effective and practical strategy for this needs to be determined. Early weaning or restricted suckling may shorten the duration of postpartum acyclicity, however, it is impractical due to reduced growth rates and increased winter mortality of early weaned calves. A single treatment with either GnRH, or PGF2α+GnRH can successfully induce estrus in yak cows that calved in previous years (with or without calf) but did not calve in the current year, however, it has little effect in cows nursing a calf born in the current year. The effects of administration of exogenous progestogens plus GnRH on the fertility of yak cows are worthy of further study.
摘要译文
本文回顾了季节性繁殖，青春期，产后不育，胚胎损失和小牛存活及其对雌性牦牛的制约。也考虑提高产后牦牛生育力的方法。牦牛是季节性的育种者，在温暖的季节交配受孕。青春期一般发生在出生后的第二至第四个暖季，即在13至36个月之间。奶牛通常有一个很长的产后期;产犊后第一个繁殖季节只有一小部分奶牛恢复发情，第二和第三年大部分进入发情期。营养状况是雌性牦牛繁殖的最重要决定因素。繁殖成功是由气候，季节决定的牧场的直接结果，和管理实践。挤奶通过减少牛奶摄入（限制哺乳）和小牛生长速度来延缓青春期。挤奶会干扰放牧，延长奶牛产后非周期性的持续时间。在本赛季初出生的犊牛比在冬季来临之后出生的犊牛有更长的吮吸季节。从而，他们可以在第二年的繁殖季节开始他们的第一轮，而在赛季末出生的那些人可能在25或26个月之前没有第一次发情。在产犊年份早期产犊的母牛更容易在产犊年份恢复发情，因为在冬季开始之前，他们有更长的时间来恢复对体内储备的需求。还讨论了小型牦牛农场的近亲交配，并建议通过交换不同牛群之间的公牛来减少近交。冬季的营养补充可以提高生殖效率，但是要确定最具成本效益和最实用的策略。早期断乳或限哺乳可能缩短产后非周期性的持续时间，但由于早期断奶犊牛生长速度降低和冬季死亡率增加，这是不切实际的。用GnRH或PGF2α+ GnRH单独处理，可以成功诱导前几年（有或无小牛）犊牛发情但当年未产犊，对今年出生的小牛的奶牛影响不大。外源孕激素加GnRH对牦母牛育性的影响值得进一步研究。
Xiang-DongZi;. Reproduction in female yaks (Bos grunniens) and opportunities for improvement[J]. Theriogenology, 2003,59(5-6): 1303-1312