Effect of GnRH administration on different days following embryo transfer on conception rates in beef heifers

This study investigated the effect of GnRH, administered on days 4 and 5 after the transfer of fresh embryos to beef heifers, on the conception rate. For this purpo-se, thirty Simmental cows were used as donors in the study. Progesterone-based protocol (eleven-day) was applied to the donors for estrus synchronization. In the synchronization of the recipients, the classical ovsynch protocol was applied. Uterine flushing was performed on the 7th day following artificial insemination of the donors. Code I and II quality fresh embryos were transferred to 117 recipient heifers suitable for transfer (≥15 mm CL). Recipient heifers were divided into three groups; GnRH-4, GnRH-5, and control. After the embryo transfer intramuscular GnRH (10 μg, Buserelin Acetate) was administered to the recipients on the 11th and 12th days of the cycle (4 and 5 days following the transfer). No administration was performed on heifers in the control group. On day 23 following the transfer, pregnancy examinations were performed with real-time ultrasound. Consequently, the conception rates of GnRH-4, GnRH-5, and control groups were 38.7%, 43.3%, and 33.9%, respectively (p > 0.05). As a result, GnRH administration on the 4th and 5th days following the transfer did not increase significantly the pregnancy rate. Since the pregnancy rates obtained in a high-cost biotechnological method such as embryo transfer are very important, the post-transfer GnRH administration can be recommended because there is a statistically insignificant numerical increase (approximately 10%) in the presented study. However, this strategy should be applied to a larger number of animals and be molecularly and endocrinologically evaluated.