After synchronization of oestrus, normally cyclic heifers (n = 31) received 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. and had a progesterone-releasing intravaginal device (PRID) without the oestradiol capsule inserted on day 10 of the oestrous cycle and received 15 mg prostaglandin (PG) i.m. 48 h later. PRIDs were removed 96 h after insertion and 16 heifers received 1.0 mg GnRH i.m. while the controls (n = 15) received 10 ml saline i.m. All heifers were injected with anti-PMSG i.v. 10 h later. Peripheral blood concentrations of PMSG, progesterone, oestradiol and LH were compared. Ovaries were collected on death 7 days after the GnRH or saline injection and the number of corpora lutea counted. Heifers were considered to have responded well (> 60 pmol l⁻¹) or poorly (< 60 pmol l⁻¹) to superovulation on the basis of the oestradiol concentration 24 h after PG administration. During PRID treatment, LH concentrations remained at basal values. In the heifers treated with GnRH, a single LH surge occurred 2.3 ± 0.1 h (sd) after the GnRH injection with a maximum concentration of 14.6 ± 2.3 (sem) μg l⁻¹ and a duration of 6–8 h. In 12 of the 15 control heifers, LH concentrations remained low (range 0.10–1.94 μg l⁻¹) during the 72 h following the saline injection; three controls showed a spontaneous LH surge at 18, 23 and 23 h after the saline injection, respectively, with a maximum concentration of 6.0–12.5 μg l⁻¹ and a duration of 10–12 h. The oestradiol concentration increased continuously during PRID treatment until the injection of GnRH or saline, when it was four times higher in the heifers that responded well than in the heifers that responded poorly. It decreased sharply 6 h after GnRH indicating that the follicles still responded normally to a preovulatory LH signal, whereas in control heifers a similar decrease took place 4 h later following anti-PMSG treatment. In the GnRH-treated heifers, the heifers responding well showed a significantly higher number of corpora lutea than did the animals showing a poor response, 16.4 ± 2.2 (n = 9) and 5.4 ± 1.4 (n = 7), respectively. Five of the 12 control heifers without an immediate LH surge showed a single corpus luteum, and seven heifers did not have a corpus luteum. For the three controls with an LH surge, 33.0 ± 8.5 corpora lutea were observed. In conclusion, the preovulatory LH signal can be effectively postponed in PMSG/PG-superovulated heifers using a PRID. However, the PRID treatment has to be followed by GnRH to obtain the LH surge at a defined time of preovulatory follicular development. Follicular function with regard to oestradiol secretion and the potential to ovulate remains unchanged.

Normally cyclic heifers (n = 34) received 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. at day 10 of oestrus, and 15 mg prostaglandin (PG) i.m. at day 12. Thereafter, a monoclonal antibody against PMSG was administered i.v. before (n = 24), at (n = 6) or shortly after (n = 4) the preovulatory LH surge. Peripheral blood concentrations of LH and oestradiol were compared; follicular development was monitored by daily ultrasound scanning; and the numbers of preovulatory-sized follicles and ovulations were counted 96 h after injection of PG following death. Anti-PMSG treatment before the LH surge inhibited the LH surge in 16 heifers (67%). In these heifers, the initial increase in oestradiol concentration upon PMSG stimulation to 167.5 ± 35.0 pmol l⁻¹ was terminated immediately after anti-PMSG treatment and decreased rapidly to basal values, while the number of preovulatory-sized follicles remained constant until 68 h after PG injection; on average 0.4 ± 0.1 ovulations were counted. In the remaining eight heifers, five animals showed an immediate, but temporary, 20–60% drop in oestradiol concentration after anti-PMSG treatment. In all eight heifers 25% of the preovulatory-sized follicles ovulated. Treatment with anti-PMSG at or shortly after the LH surge did not affect the pattern of oestradiol concentration, but a significantly higher ovulation rate was observed in the animals treated shortly after the LH surge: 20.3 ± 2.6 versus 6.3 ± 2.3 in animals treated at the LH surge, which corresponded to 76% and 24% of the preovulatory-sized follicles monitored shortly before the period of multiple ovulations. Thus, neutralization of PMSG at any time before the maximum concentration of the preovulatory LH surge severely suppresses the functionality of the majority or all of the stimulated follicles, which is dependent on the time of injection of anti-PMSG. Although the follicles retain their size, they lose the potential to ovulate. It is concluded that interfollicular asynchrony of development is present at the time the LH surge occurs.

Dutch Friesian heifers (n = 13) and cows (n = 13) were used to obtain information about the number, size and micromorphology of antral follicles (≥ 3 mm in diameter) in cattle after induction of luteolysis with the PGF_2α analogue luprostiol. Special attention was paid to the presence of atypical granulosa cells in these follicles to obtain additional data to help evaluate the hypothesis that these cells are markers of follicular atresia. Animals were injected i.m. with 15 mg of the synthetic prostaglandin on day 10 or day 11 of the oestrous cycle. The ovaries were collected on day 12, that is 48 and 24 h after injection of luprostiol, respectively. After prostaglandin-induced luteolysis, the mean number of medium-sized and large nonatretic follicles and of medium-sized atretic follicles had not changed in heifers and in cows, compared with those of untreated animals. However, in heifers, contrary to cows, the development of a preovulatory-sized follicle was initially accompanied by an increase in the number of large definitely atretic follicles. Atypical granulosa cells can be considered as markers for a lower quality follicle, on the basis of their absence in preovulatory-sized follicles and their presence in large numbers in a high proportion of definitely atretic follicles. If it is assumed that only a nonatretic follicle without atypical granulosa cells will grow to preovulatory size, growth of this follicle within 2 days after prostaglandin treatment was almost 9 mm and over 10 mm in heifers and cows, respectively. In cows, most growth of follicles was observed at the first day after prostaglandin treatment; in heifers, this occurred one day later.