Pattern of Ovarian Follicular Development and Steroid Hormone Concentrations during Estrous Cycle of Lohi Sheep

ABSTRACT The objective of the study was to determine ovarian follicular development and steroid hormones concentrations during estrous cycle of Lohi sheep. Ovaries of nine Lohi ewes were monitored for two consecutive estrous cycles using B-mode trans-rectal ultrasonography during the breeding season (Sep-Nov 2018). Plasma progesterone and estradiol-17β concentrations were determined throughout the cycle using radioimmunoassay. The average length of the estrous cycle in Lohi sheep was 17.0±0.1 days and follicular and luteal phases were 4.6±0.2 and 11.3±0.2 days long, respectively. Estrous cycles had either three or four follicular waves; 3-wave cycles were more frequent than 4-waves (87vs.13%; p≤0.05, respectively). In 3-wave cycles (n=14), follicles (≤ 3mm) emerged on Days 0.7, 5.2 and 10.5. In 4-wave cycles (n=2), follicles emerged on Days 0.1, 4, 8.5, and 11.5. The maximum diameter of preovulatory follicle and corpus luteum was 5.4±0.3 and 10.4±0.3mm, respectively. Regardless of the wave pattern, single ovulation occurred in each cycle. CL was first detectable on Day 4.0±0.1, it reached maximum diameter on Day 9.0±0.1 and luteolysis began on Day 12.2±0.2 of the cycle. Peak estradiol-17β concentration was observed 48h before ovulation and correlated with the diameter of the preovulatory follicle (r=0.84; p