RESUMEN
It has been demonstrated that heat stress has a detrimental effect on conception rate of dairy cows, via early embryonic death. To reduce heat stress, providing shade, fan, appropriate diet and application of ovulation induction agents [GnRH and hCG] is recommended. Endogen PGF[2]alpha has positive effect on ovulation. Repeat Breader. The aim of the present study was to evaluate the role of PGF[2] alpha injection on reproductive performance of Repeat Breader [RB] dairy cows in heat stress condition at artificial insemination [AI]. This study was conducted in two large dairy farms in Tehran province during the summer 1386-87. All cows were inseminated [A M -PM] based on heat detection at 12 hours after observed standing heat. Mean temperature and humidity were recorded throughout the study. In this study 150 RB cows were randomly divided into three groups. Group A [n=50] received PGF[2] alpha simultaneously with AI, group B [n=50] received GnRH at the time of AI and group C [n=50] did not receive any treatment and served as control. Based on rectal palpation on 40-50 days of AI, 5 cows of group A, 9 of group B and 10 group C were pregnant. There were not any significant differences among groups [p>0.05]. The results revealed that simultaneous using of PGF[2]alpha or GnRH with AI has no beneficial effects on conception rate in RB cows