In the present study, mean serum concentrations of estradiol were significantly higher in ewes that produced as many CL as ovulations compared with animals that had inadequate CL, on Days 1 and 2 after ovulation, but normal CL were observed in both groups of ewes in question. This suggests that luteal insufficiency may be a product of inadequate developmental competence of preovulatory follicles; this would explain the coexistence of apparently normal and inadequate CL in a ewe, or even in an ovary of a ewe.
The first experiment was replicated to confirm its findings. There was close agreement between the first two experiments in terms of the incidence of breakthrough ovulations (i.e., ovulations detected during MAP treatment), and the number of ovulations and time at which they occurred after sponge withdrawal in MAP-treated ewes.
However, the ewes studied late in the breeding season (experiment 2) appeared to have a delayed onset of estrus, by approximately 1.5 days, and a somewhat greater number of inadequate CL compared with the ewes in the middle portion of the breeding season (experiment 1). These differences could be due to decreased estrogenicity of antral follicles, or to diminished ovarian responsiveness to gonadotropic stimuli, or both, in ewes approaching seasonal anestrus (experiment 2). The important point, however, is that nonprolific ewes at different stages of the breeding season exhibited similar patterns of change in follicular development and ovulation rate in response to a PGF2a /MAP treatment applied at midcycle.