Eighty Hereford heifers were injected to induce superovulation with 3000 or 5000 i.u. of purified pregnant mares' serum (pms) in one, two or three doses, administered on Day 12, 14, 16 or 18 of the oestrous cycle, with or without enucleation of corpus luteum; 2000 i.u. of human chorionic gonadotrophin (hcg) were given 3, 4, 5, 6 or 7 days after the first pms injection; clay models of the ovaries were constructed daily following palpation per rectum. The animals were bred and slaughtered 2 to 5 days after ovulation. The reproductive tracts were flushed and the ova recovered and examined microscopically; fertilized ova were cultured in vitro in saline solution and autologous serum.
Mean number of mature follicles and percentage that ovulated were almost identical for the right and left ovaries. The development of palpable follicles started 1 to 2 days after first pms injection; ovulation occurred when they reached an estimated average diameter of 14 mm (15 mm for the controls). The number of ovulated follicles was not related to increase in volume of the ovary.
Superovulatory response in the ovary where enucleation was performed was similar to that in the other gonad, but where the corpus luteum was not wholly expressed the total number of follicles developed was appreciably lower. Response was greater in animals that underwent corpus luteum enucleation than in those that remained intact. No advantage was obtained by initiating pms treatment earlier than the 16th day after oestrus nor by dividing 3000 i.u. of the gonadotrophin in two or three daily doses. The total number of follicles developed was higher at 5000 i.u. than at 3000 i.u., but there was a substantial increase in percentage of luteinized and haemorrhagic follicles.
Enucleation of the corpus luteum appeared to facilitate superovulation and decrease fertilization rate. The higher dose of pms (5000 i.u.) did not adversely affect the proportion of ova that were either recovered or fertilized. The time from first to last ovulation did not appreciably affect the recovery or fertilization of ova. The proportion of ova recovered tended to fall with increasing ovulations, but the factors associated with poor recovery were not necessarily related to high ovulation rate. Fimbriae appear capable of picking up ova released from ten follicles, on an average. There was no consistent relationship between the fertilization rate and the number of ovulations. No conclusion could be made relative to the rate of transport of ova due to the scatter in time of ovulations in superovulated animals. Of the ova, 15% were considered morphologically abnormal. Degenerating ova showed different morphological characteristics, indicating that different mechanisms may be responsible for degeneration. In-vitro culture of fertilized ova was unsuccessful.