This study reviewed the protocols for ovulation induction in sows by human chorionic gonadotropin (hCG) and gonadotropin releasing hormone (GnRH). In pigs, it was documented that ovulation usually occurred about 70% of the way through oestrus. However, the duration of oestrus in sows highly varies, producing difficulty in performing artificial insemination (AI) in sows at optimal time. In practice, sows are usually inseminated for 2 to 4 times at 12 to 24 hours interval during oestrus, with an attempt to deposit sperm at the optimal time of ovulation. Many protocols have been developed to control time of ovulation, allowing fixed time insemination in gilts and sows. Hormones used for ovulation induction included hCG, GnRH and porcine luteinizing hormone (pLH). The purpose of treatment for weaned sows with hCG, GnRH or pLH performed after gonadotrophic treatment at weaning is to stimulate follicular growth. Such treatments at weaning period are associated with an increase in embryo mortality due to poorer oocyte quality. GnRH and hCG mechanisms of action on ovulation induction are different: GnRH induces ovulation by providing a pre-ovulatory peak of LH, while hCG acts directly on LH receptors on ovarian follicle cells.
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