ENDOCRINE AND FOLLICULAR STUDIES IN MEISHAN PIGS

The enhanced early embryonic survival in Chinese Meishan compared with Large-White gilts may be due, in part, to differences in ovarian and endocrine function, particularly during the periovulatory period. The overall patterns of oestradiol, LH and FSH secretion were not differen t between Meishan and Large-White hybrid controls during this period, although circulating inhibin concentrations were higher in Meishan gil ts. Thus, there appeared to be a decreased sensitivity to inhibin feed back on FSH secretion in Meishan gilts. Behavioural oestrus was exhibi ted earlier relative to the LH surge in Meishan gilts than in Large-Wh ite hybrid gilts, but the time interval from the oestradiol peak until the LH surge was similar in both breeds. This finding suggests that M eishan gilts are more sensitive in terms of initiating a behavioural r esponse, but not in terms of positive feedback. Although preovulatory follicular characteristics were as variable in Meishan as in Large-Whi te hybrid gilts, follicles from Meishan gilts were smaller, but contai ned a higher concentration of oestradiol in the follicular fluid. This was probably due to increased aromatase activity in both granulosa an d theca cells of Meishan follicles. The enhanced maturation of the int rafollicular environment in Meishan gilts was reflected in the oocyte population which was at a more advanced stage of development in the pe riod preceding ovulation. In addition to decreasing the time between o nset of oestrus and ovulation, advancing the LH surge to coincide with onset of oestrus (via hCG administration) decreased embryo survival a t day 30 of gestation. It is concluded that both endocrine and follicu lar mechanisms have a role in ensuring the prolificacy in the Meishan breed.