Amir Hossein Nasiri - pardis College of Agriculture and Natural Resources, University of Tehran, Animal Science Department
Armin Tohidi - Tehran university animal science department
Malek Shakeri - Tehran university animal science department
Mahdi Jhendi - Tehran university animal science department

Two series of experiments were conducted to investigate the reproductive performance of transition dairy cows in response to supplementation with a live yeast culture (Probio- Sacc®, Biochem, Germany) under high ambient temperature. In Exp. 1, two groups of 6 periparturient Holstein cows were fed a diet without or with 4 g yeast/d/head starting 21 d prepartum through 8 wk postpartum to investigate estradiol and progesterone and ovarian follicular dynamics. In Exp. 2, from d 1 through d 70 postpartum, a total of 150 Holstein cows (75 cows per group) were assigned at random into 1 of 2 groups and received either 0 or 4 g live yeast/d/head to investigate their reproductive performance at 120 and 150 d postpartum. In 1st exp, circulating progesterone and estradiol-17 beta concentrations were found to be greater in yeast-supplemented cows than those receiving no yeast. The mean diameter of the ovulatory follicles was greater (17.2 vs. 18.4 mm; P < 0.01) in yeast-fed cows than those in control treatment. Duration of estrus was also shortened by an average of 2.6 d (P = 0.05), which is indicative a faster resumption of ovulatory activity. In 2nd exp., days open, days to first service, conception rate at d 120 and 150, and the proportion of pregnant cows at 120 and 150 postpartum were observed to be higher for cows receiving yeast supplement compared with no yeast.Overall, it appears that live yeast supplementation would beneficially improve the reproductive performance and ovarian dynamic of dairy cows during the heat stress.

Heat stress, Live yeast, Reproductiveperformance, Transition dairy cow