Origin and fate of preovulatory follicles after induced luteolysis at different stages of the luteal phase of the oestrous cycle in goats

The effect of day of induced luteolysis on follicle dynamics, oestrus behaviour and ovulatory response in goats was studied by administering cloprostenol on Day 5 (n = 10), Day 11 (n = 10), or Day 16 (n = 10) after detection of oestrus. Stage of the luteal phase affected the interval from cloprostenol injection to onset of oestrus, with behavioural oestrus being observed earlier in goats treated early in the luteal phase (43.4 ± 3.2 h on Day 5 versus 57.0 ± 2.6 h on Day 11 and 56.7 ± 2.7 h on Day 16, P < 0.01). The group treated on Day 5 also tended to have a higher proportion of does which exhibited oestrus behaviour (P = 0.07) and ovulation (P = 0.06). In all the cycles, at least one of the ovulatory follicles arose from antral follicles present in the ovary at cloprostenol injection. In 66.7% of monovular cycles, the ovulatory follicle was the largest follicle on the day of luteolysis. In 33.3% of polyovulatory cycles, one of the ovulatory follicles was the largest one present when cloprostenol was administered. In 80% of polyovulatory cycles, the second ovulatory follicle was present on the day of luteolysis; but in the three remaining cycles, the second ovulatory follicle emerged later. This shows that the largest follicle may not exert dominance over other follicles in the goat. Evaluation of follicular dynamics in different phases of luteal activity in current experiment showed an attenuation of dominance in the mid-luteal period. In does treated early or late in the luteal phase, the number of new growing follicles decreased with time (P < 0.01 and 0.05, respectively), the mean number of follicles reaching 4–5 mm in size also decreased (P < 0.001 and 0.01, respectively) and the number of regressing follicles increased (P < 0.05). These effects did not reach statistical significance in does treated in the mid-luteal phase.