Different types of postpartum luteal activity affected by the exposure of heat stress and subsequent reproductive performance in Holstein lactating cows.

The aim of the present study was to determine the effect of heat stress on postpartum (PP) luteal activity and subsequent reproductive performance in lactating cows. Thirty Holstein-Friesian (HF, >or=75%) cows (hot season, n = 15 and cool season, n = 15) were studied over 60 days after calving. The changes of temperature-humidity index (THI) were monitored within both seasons. Body condition scores (BCS), uterine involution and the ovarian structures were recorded. Plasma samples were obtained three times a week for the analysis of progesterone (P4) and prostaglandin F(2 alpha) metabolite (PG metabolite). Subsequent reproductive performance of the cows, calved within the same period was also evaluated. The means of THI for hot and cool seasons varied between 84-87 and 78-83 respectively. A drop of the mean BCS was recorded at 5 weeks PP in the cows during hot season (p < 0.01), however a delay of uterine involution during early PP period in hot season was not clearly seen. The cows with normal PP ovarian cyclicity during hot and cool seasons were 4/15 (26.7%) and 9/15 (60.0%) respectively. A higher percentage of abnormal luteal activity was found in the cows during hot season (p = 0.07) and delayed luteal cyclicity/anovulation was the most pronounced atypical P4 profile. The levels of PG metabolite were not different between groups and the relationship between the levels of PG metabolite and the time of uterine involution was not evident (p > 0.05). The heat detection rates and the pregnancy rates were higher in the cows during cool season (p < 0.05). Additionally, the higher first AI conception rates during cool season were recorded (p = 0.06). In conclusion, heat stress conditions had negative effects on BCS and altered a normal process of ovarian resumption PP, consequently resulted in lower reproductive performance in a tropical dairy herd.