Identification at the onset of luteolysis of follicles capable of ovulation in the ewe

Follicles of various sizes at the surface of the ovary were ablated by electrocautery at the time of cloprostenol-induced luteolysis in ewes and the interval from cloprostenol treatment to the onset of the LH surge determined as an index of the time from luteolysis to ovulation. When follicles 2-4 mm or greater than 4 mm diameter remained in the ovaries, the interval from cloprostenol treatment to the onset of the LH surge was similar to that in sham-operated (control) ewes (55-60 h), whereas when the only follicles remaining were less than 2 mm, the interval was extended by 24 h (P less than 0.05). This study demonstrates that follicles capable of ovulating can be selected from those greater than or equal to 2 mm diameter at luteolysis, emphasizing the flexibility of the sheep ovary in its final selection of the ovulatory follicle.