Submaximal concentrations of progesterone prolonged the lifespan of large antral follicles in cyclic ewes. A similar effect was observed when luteolysis was induced with PGF2a and progesterone was replaced by MAP being released from intravaginal sponges, or when MAP-impregnated sponges were inserted on Day 12 after ovulation and ovaries were exposed to MAP in the absence of functional CL. This suggests that MAP treatment mimics a low progesterone regimen in sheep. During the midbreeding season, mean serum concentrations of progesterone are higher in nonprolific Western white-faced than in prolific Finn sheep. We speculate that treatment of nonprolific Western white-faced ewes with MAP after PGF2a-induced luteolysis created the equivalent of a low progesterone environment, and hence promoted an increase in ovulation rate to resemble that of the Finn sheep.
There is marked variation in prolificacy among breeds of sheep. Earlier studies failed to produce convincing evidence that differences in circulating concentrations of gonadotropic hormones were solely responsible for differences in ovulation rate in ewes. It has been suggested that the high ovulation rate in prolific sheep is due to intraovarian rather than pituitary factors. We propose that the difference in the pattern of follicle development and ovulation rate between breeds of sheep differing in prolificacy may be partly due to a difference in circulating concentrations of progesterone.