Effect of long-term and short-term progestagen treatment on follicular development and pregnancy rate in cyclic ewes.

The aim of this study was to evaluate the effect of the length of a progestagen treatment (12 d vs. 6 d) on follicular dynamics, estrus synchronization and pregnancy rate using medroxyprogesterone acetate (MAP) with or without an eCG dose at the end of MAP treatment. One hundred sixty Polwarth ewes were divided into four equal groups: long-term treated (LT, n=40); short-term treated (ST, n=40); long-term treated plus eCG (LTeCG, n=40); and short-term treated plus eCG (STeCG, n=40). Five ewes of each group were separated to undergo daily transrectal ultrasonography, and blood samples were taken for hormone determination. Until 96 h after sponge withdrawal the number of ewes in estrus was higher in both long-term-treated groups than in both short-term-treated groups but at the end of the observational period (144 h) no significant differences were found among groups. The pregnancy rate was higher in the ST group (87%) than in the other groups (LT, 63%; LTeCG, 67%; and STeCG, 58%; P< or =0.03). The ovulatory follicle emerged before sponge withdrawal in long-term-treated ewes (-3.8+/-0.4 d and -2.2+/-0.8 d for LT and LTeCG, respectively), whereas in short-term-treated ewes it emerges around sponge removal (0.4+/-1.1 d and 0.5+/-0.5 d for ST and STeCG, respectively; P< or =0.01). The ovulatory follicle in the LT group had a longer lifespan and attained a larger (P< or =0.05) maximum diameter than in the ST group. We conclude: a) that the lower pregnancy rate observed after long-term progestagen treatment was related to a slower follicular turnover that promoted the ovulation of persistent dominant follicles; (b) that short-term treatment resulted in a higher pregnancy rate probably due to the ovulation of newly recruited growing follicles; and (c) treatment with eCG had no advantage in association with long-term treatment and had a deleterious effect in combination with short-term treatment with MAP.