Comparison of ovarian follicles (stage, number and estradiol concentration) in high fecundity Olkuska sheep, low fecundity Polish mountain sheep and their crossbreeds.

The objective of this study was to identify whether high ovulation rate in high fecundity Polish Olkuska Sheep results from within ovary differences by comparing the number, size and estradiol concentration in ovarian follicles obtained from high fecundity ewes of Olkuska breed, non prolific Polish Mountain Sheep and the crossbreed between them. The ovaries were obtained after laparotomy from the anesthetized animals on the 17th day after the onset of estrus. The diameter of individual follicles was estimated with the use of operating microscope with a calibrated scale in the ocular. Dissected follicles were stored at -20 °C for the estradiol estimations in the homogenates. The mean number of highly estrogenic and medium follicles in crossbreed was significantly lower than that in Olkuska and significantly higher than that in Polish Mountain. However, mean estradiol level in high estrogenic and medium follicles in three investigated groups was not significantly different. Mean ovulation rates were 3.1 for Olkuska, 1.5 for Polish Mountain and 2.2 for crossbreed. It is concluded that, like in Booroola sheep, the increased number of follicles that mature and ovulate results from the differences within the ovary. The recruitment of ovulatory follicles in high fecundity Olkuska sheep, lasts longer than in non prolific Polish Mountain sheep and similar tendency has been observed in crossbreed.