Artificial Control of Estrus and Ovulation in Female Yaks

Introduction Yaks are seasonal breeders and the breeding season lasts from July to October. If the females do not show obvious estrus in a particular breeding season, they will express estrus in the next breeding season or in some cases not until several breeding season later. The observed estrus rate of yaks is rather low, and the postpartum period is long. Most females deliver calves once every two years, some of them only once every three years. Yak heifers attain puberty much later than cattle and usually first get pregnant at 3 to 4 years old [1]. Silent estrus constitutes the single largest factor responsible for poor reproductive efficiency in yaks. Aside from seasonal factors affecting the display of estrus, the most important contributing factor is an inability to detect estrus. Application of reproductive biotechnologies in the yak requires a reliable method for estrus control of female and such a method is also essential in artificial insemination or embryo transfer programmers. The possibility of inducing estrus and ovulation in acyclic females and of synchronizing estrus and ovulation in groups of females offers an opportunity to increase the efficiency of yak production and to makes artificial insemination more convenient. A reliable method for estrus control would benefit not only the use of artificial insemination but would also be important for establishing embryo transfer programmers. As in other livestock animals, exogenous gonadotropins in various treatment regimens have been used for estrus induction in the yak [2-4]. The effects of these treatment are, however, considerably variable. There is only limited information from field studies on such protocols. There remains a considerable need for more studies on methods to induce estrus and ovulation during acyclic periods in females yaks, either by the administering of exogenous hormones and/or by manipulating environmental factors that suppress ovarian activity.