Endocrine control of estrous cycle in mithun (Bos frontalis)

The objective of the present study was to establish the profiles of luteinising hormone (LH), follicle stimulating hormone (FSH), estradiol 17β (E2) and progesterone (P4) secretion and their interrelationships during the natural estrous cycle of mithun (Bos frontalis). Daily blood samples were collected from second or third postpartum estrous cycles for determination of plasma concentrations of LH, FSH, E2 and P4. Concentration of P4 was found to be lowest on the day of estrus. It increased following estrus, attained the highest concentration on day 11 and decreased thereafter. Concentrations of LH and FSH varied significantly (p < 0.01) during the first and last 6 days of the cycle and their variations were found to be synchronised. Both LH and FSH attained a biphasic peak during the estrous cycle. This biphasic peak lasted on from day −5 to day 3 of the cycle. The variations in maximum LH and FSH concentrations of both the phases did not differ significantly. During the entire estrous cycle, the E2 concentrations attained either one peak or two peaks. The first peak, approximately on day 4 before estrus was common in all animals. One additional peak was found on the day of estrus in 45% animals. A significant (p < 0.01) negative relationship was found between P4 and, LH and FSH during the first and last 6 days of cycle. But a significant (p ≤ 0.01) negative relationship between E2 and, LH and FSH was found only during the last 6 days of cycle. The results suggest a negative feedback mechanism on LH and FSH release by E2 and P4 during the respective phases of cycle. A sustained increase in LH and FSH levels during the period before estrus was probably necessary for the final maturation of ovulatory follicle and subsequent ovulation.