ORIGINAL ARTICLE Ovarian activity in high and average producing Holstein cows under heat stress conditions Abdolah Mirzaei & Mojtaba Kafi & Mohsen Ghavami & Mehrdad Mohri & Hamid-Reza Gheisari Received: 2 October 2006 / Accepted: 30 January 2007 / Published online: 6 March 2007 # Springer-Verlag London Limited 2007 Abstract The aim of the present study was to investigate the ovarian activity in high and average producing Holstein cows under heat stress conditions (temperature–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 from 14 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 (βHB) 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 βHB [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 Introduction The onset of normal ovarian activity is one of the most important events for the modern dairy cow to regain her maximum breeding potential after calving (Opsomer et al. 2000). It was reported that high producing (HP) dairy cows show a higher incidence of reproductive disorders than average or low producing cows (Butler and Smith 1989; Nakao et al. 1992; Taylor et al. 2000). Opsomer et al. (1998) and Lamming and Darwash (1998) reported that HP dairy cows have a greater incidence of delayed first ovulation and prolonged luteal phases. Shrestha et al. (2004) reported that 37% of cows showed normal ovarian Comp Clin Pathol (2007) 16:235–241 DOI 10.1007/s00580-007-0672-4 A. Mirzaei : M. Kafi (*) : M. Ghavami : H.-R. Gheisari Department of Clinical Science, School of Veterinary Medicine, Shiraz University, Shiraz 71345, Iran e-mail: kafi@shirazu.ac.ir M. Mohri Department of Clinical Science, School of Veterinary Medicine, Ferdowsi University, Mashad, Iran H.-R. Gheisari Department of Food Hygiene and Epidemiology, School of Veterinary Medicine, Shiraz University, Shiraz 71345, Iran