Cryobiology of Gametes Department, Institute of Biology and Immunology of Reproduction, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria
HRISTOV, P., 2005. Comparison of follicular dynamics and plasma steroid concentrations in natural estrous cycles of normally - and high-producing dairy cows. Bulg. J. Agric. Sci., 11: 373-380
Follicular dynamics from the last follicular waves and plasma steroid concentrations were compared between two groups lactating dairy cows: Group A (normally-producing, n=6, 20.3 ± 1.5 kg milk/day) and Group B (high-producing, n=7.45 ± 2.4 kg milk/day). All cows had typical spontaneous estrous cycles after cycles were initiated. Follicular dynamics and plasma åstradiol-17b (E2) and progesterone (P4) concentrations were monitored through ultrasonography and blood sampling between 6-10 h, 18-22 h (late follicular phase) and 30-34 h (early metestrous phase) after the onset of estrus (OE). The results showed that normally-producing cows had larger dominant follicles between 6-10 h after OE (19.7 vs. 15.6 mm), larger preovulatory follicles around 7 h before ovulation (20.0 vs. 16.8 mm) and larger ovulated follicles (17.2 vs. 16 mm) compared with high-producing cows. However, high-producing cows had lower plasma E2 concentrations between 6-10 h after OE (18.7 pg/ml vs. 26.8) and similar plasma Ð4 concentrations compared with normally-producing cows. Therefore, high-producing cows had lower or similar plasma steroid concentrations than high-producing cows, despite having smaller dominant folliles.