The aim of the present study was to investigate the ovarian activity in high and average producing Holstein cows under heat stress conditions (temperature敮摡獨humidity index [THI]=81.5±3.9). Cows were kept in free-stall barns and fed a total mixed ration consisting mainly of alfalfa, corn silage, beet pulp, cotton seed, soybean, corn and barley. Forty high producing (HP, mean 305-day fat-corrected milk [FCM]= 8707.3 kg) and 30 average producing (AP, FCM=6115.1 kg) multiparous healthy (free of detectable reproductive disorders) Holstein cows were monitored from14 days before to 70 days after calving. Transrectal ultrasound scanning was performed twice weekly from 7 to 65 days postpartum. Blood samples were also collected twice weekly to measure serum proges-terone and biweekly to detect serum β -hydroxybutyrate (摯瑬敳獩 HB) and non-esterified fatty acid (NEFA) concentrations. Body condition score (BCS) was taken biweekly after calving. Cows were observed visually four times a day to determine the duration of standing estruses. Classification of the serum progesterone profiles revealed a non-significant difference between HP and AP cows with abnormal ovarian activity (63.6% [21/33] and 46.2% [12/26], respectively, p>0.05). The highest type of abnormal ovarian activity was delayed first postpartum ovulation 36.4% (12/33) in HP and 38.5% (10/26) in AP cows (p>0.05). The lowest type of abnormal-ity was the cessation of ovarian activity in HP and AP cows, 3.0% (1/33) vs 3.8% (1/26), respectively. The mean concentrations of both metabolites in HP cows were greater than AP cows at any time after calving (NEFA and β HB [p>0.05]). There was significant difference in estrous duration between groups ([mean±SD] 6.1±2.7 h in HP and 10.5±2.9 h in AP cows, P<0.05). The loss of body condition score (BCS loss >0.5) significantly affected calving to first detected estrus interval in HP cows (P< 0.05), whereas calving to first detected estrus interval was not significantly affected in AP cows. The most important abnormality of ovarian activity was delayed first ovulation in high and average producing dairy cows under heat stress conditions. Keywords High producing dairy cows . Ovarian activity . Heat stress conditions . Progesterone

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