Follicular persistence, oestradiol level and conception on the first and second day of oestrus and influence of thermal stress in cross-bred dairy cows manifesting prolonged oestrus

Increasing incidence of prolonged oestrus reported in cross-bred dairy cows bred through artificial insemination (AI), is attributed to various causative factors and underlying mechanisms. The present study was to assess the pattern of follicular growth, oestrogen secretion, and conception on the first and second day of oestrus and to understand the influence of thermal stress as a causative factor for oestrus prolongation. The year-round study involved 52 crossbred cows intensively managed under a hot humid climate, during their second and third months postpartum. Animals were examined for the status of ovarian follicles, oestrogen level, and conception to AI on Day 0 and Day 1 of oestrus confirmation and the findings were compared across seasons. While oestrus detection rate (ODR) and conception of AI were lowest during the summer season attributable to maximum TS, better ODR (P<0.05) and more prolonged oestrus (P<0.01) occurred during the seasons of favourable climate having least TS and better green fodder availability. Besides, the occurrence of larger follicles and almost similar oestradiol levels on Day 0 and Day 1 of oestrus, better conception rate for double AI (63.16 %) than total AI (48.48 %) indicated prolonged oestrus and involvement of delayed ovulation among the causation. Comparison of the seasonal pattern of follicular growth, oestrogen level, and conception to AI between Day 0 and Day 1 further clarifies the influence of TS and increased milk production as the potential contributors to the occurrence of prolonged oestrus in cross-bred cows under the hot humid tropical climate. Keywords: Crossbred cow, Oestrogen, Prolonged oestrus, Thermal stress