Control of follicular development during and after lactation in sows.

Follicular development during early lactation in the sow is characterized by a large population of small-sized follicles and a small population of medium-sized follicles. As lactation progresses there is a gradual shift in number of follicles into medium- or large-sized categories and the percentage of follicles classified as atretic declines. Weaning at birth often leads to aberrant follicular development, apparently because the positive feedback response of LH to oestradiol does not occur during the first week post partum. Secretion of LH during lactation is primarily controlled by suckling intensity of the litter while FSH is controlled by a nonsteroidal ovarian factor, presumably inhibin. Suckling apparently limits secretion of GnRH and weaning leads to an increase of GnRH within the hypothalamus coincident with an increase of LH in the anterior pituitary and plasma and increased follicular growth. Follicular development during both lactation and post-weaning anoestrus can be stimulated by exogenous gonadotrophins and pulsatile administration of GnRH. Factors such as nutrition, season, boar exposure, litter size and split weaning affect follicular development during lactation and after weaning, probably because they affect secretion of LH.