Ovarian follicular responsiveness and oocyte quality after gonadotrophic stimulation of mature pigs

The influence on the oestrous cycle and ovulatory response of injecting pregnant mare serum and/or chorionic gonadotrophin has been examined in both the luteal and follicular phases. After a single injection of 1500 i.u. PMSG on Day 13 of the cycle, oestrus was expressed within five to seven days. By contrast, oestrus was delayed until Days 27 to 29 in six gilts injected similarly with PMSG on Day 14. Superovulation was not a characteristic of the response to PMSG injected in the late luteal phase. A single treatment with PMSG on Days 17, 18 or 19 slightly shortened the oestrous cycle and provoked superovulation in some of the animals, but treatment in the early follicular phase (Days 15 or 16) was considered the optimum time at which to initiate a superovulatory response. These observations correlate well with the incidence of ovarian compensatory hypertrophy after unilateral ovariectomy performed at different times during the follicular phase.