RESUMEN
Heat stress causes reduced fertility and significant economic loss in dairy cattle. To override the suppressive effects of heat stress, various hormonal manipulations have been utilized. The aim of this study was to compare the effect of progesterone [in the form of CIDR] and administration of GnRH after insemination on the conception rate of heat stressed dairy cattle. All cows were inseminated at estrus and were then alternately assigned into three groups on day 5 after artificial insemination [AI]: i] GnRH group [n=44] received an IM injection of 500 micro g GnRH [GONAbreed, PARNEL, Australia,]; ii] CIDR group [n = 44] received a CIDR [EAZI-BREED, Hamilton, NZ, containing 1/9 g progesterone] which was removed after a week; and iii] control group [n = 36], which did not receive any treatment. Conception was diagnosed on day 32-39 after AI by ultrasonography. Conception rate in GnRH, CIDR and control groups were 54.5%, 54.5% and 58.3%, respectively. The results demonstrated that there was no significant difference among the three groups [p >0.05]. These treatments had had no statistically different effects on lactation, milk yield, days in milk and number of AI [p>0.05]. Conception rates within GnRH and CIDR groups in <150 and >150 days in milk subgroups were 74.4%, 40.7%, 84.6% and 41.9%, respectively and differed statistically significantly [p>0.05]. Conception rate within control and CIDR groups among <3 and >3 numbers of AI were 80%, 31.2%, 84.2% and 32%, respectively, which was statistically significant [p>0.05]. According to the results of this study, the use of GnRH and CIDR after AI did not improve conception rates of mildly heat stressed dairy cattle