Follicle deviation and diurnal variation in circulating hormone concentrations in mares

The temporal relationships between follicle deviation and systemic hormone concentrations were studied in mares. Blood samples were obtained at 01:00, 07:00, 13:00, and 19:00 h from nine mares throughout an interovulatory interval. Diurnal variation in progesterone occurred on Days 4–12 and in LH on Days 4 and 5; the lowest concentration for both hormones was at 13:00 h. Ultrasonically observed deviation in the ovulatory follicular wave began on Day 15.7 ± 0.5 (ovulation = Day 0). An increase (P < 0.002) in LH began on Day 14 before the beginning of deviation, and an increase (P < 0.05) in estradiol began at the beginning of deviation. Testosterone concentrations began to increase (P < 0.05) 2 days after the beginning of deviation and reached maximum 1 day before the next ovulation. The beginning of deviation was encompassed by a decline (P < 0.003) in cortisol concentrations, and the concentrations remained low during the preovulatory period.