In earlier studies, prolongation of follicle lifespan was consistently observed in ewes given PGF2a early in the es-trous cycle (Days 5 or 6 after ovulation), with a subsequent treatment to create low serum concentrations of progesterone or with MAP for 10 or 14 days, respectively. The proposed mechanism when the treatments began early in the luteal phase was an increase in LH pulse frequency, similar to the explanation given for cattle; however, LH pulse frequency was measured only once, after 13 days of MAP treatment in sheep.
In the present study, a shorter treatment with MAP was applied at midcycle, and no increase in LH pulse frequency was noted as determined 3 days after sponging. In a previous study in which intra-vaginal MAP sponges were inserted for 12 days on Day 12 after ovulation (i.e., just before natural luteolysis), prolonged follicle lifespan was also observed, but LH pulse frequency did not change over the first 6 days of treatment, and only a numerical but not significant increase was seen after 12 days. Therefore, the extended follicular lifespan and increased ovulation rate after PGF2a/MAP treatment commenced late in the sheep estrous cycle, and applied for a short period of time, may not be dependent on changes in LH pulse frequency.