Ovarian follicular growth in sows.

The resumption of ovarian follicular development during lactation and after weaning in sows is a complex process that ultimately determines rebreeding efficiency of sows. Ovarian follicular development before weaning is heterogeneous because multiple patterns of development are observed when individual sows are compared. Sows can have relatively inactive ovaries before weaning with follicles of < 2 mm in diameter. Other sows have non-ovulatory follicular waves in which follicles grow to approximately 5 mm and subsequently regress before weaning. Sows may also have preovulatory follicular development and ovulation, or may develop cystic ovaries before weaning. Weaning is a random event relative to follicular development on the ovary. Therefore, variation in the weaning to oestrus interval in sows is caused by weaning at random stages of follicular development. Most sows experience a rapid period of follicular growth after weaning and return to oestrus within 3-7 days. Delayed intervals to oestrus after weaning are associated with inactive ovaries before weaning (follicles < 2 mm in diameter) or weaning during the regression phase of a follicular wave. An integrated model for follicular growth and oestrus in weaned sows should include endocrine mechanisms (that is, individual differences in insulin, insulin-like growth factor I (IGF-I), LH and FSH), behavioural mechanisms (relationship between follicular growth and the initiation of oestrus) and morphological mechanisms (that is, timing of weaning relative to ovarian follicular development).