Low pregnancy rate is an important problem in livestock. Therefore it is possible to control the process of reproduction in animals by using synthetic hormones and increase the pregnancy rate through estrus synchronization. The aim of this study was to investigate the effect of GnRH on days 10, 11, and 12 after artificial insemination on pregnancy rate of Makui sheep breed. For this purpose, estrus was synchronized in 226 ewes using CIDR. After removing CIDR, all ewes received 400 IU PMSG. After two days, ewes in estrus were artificially inseminated with fresh sperms and divided into three groups. The control group did not receive any hormone. The second group was divided into subgroups D, E and Fwhich received 20 µg GnRH on days 10, 11, and 12 after artificial insemination respectively. Finally, the third group or group G received 40 µg GnRH on day 12 after artificial insemination. Blood samples (5 ml) were collected two days after GnRH injection by using vacuum tubes. The serumic concentrations of glucose, urea, total protein, phosphorus and progesterone were measured. To analyze the dependent and discrete variables, GLM and FREQ procedure of SAS software were used respectively. The results showed that the effect of GnRH on plasma concentration of progesterone was significant (p<0.01). Maximum pregnancy rate was achieved in the third group with a significant difference (p<0.01) observed between this group and the other groups regarding pregnancy rate. It can be concluded that injection of 40 µg of GnRH hormone on day 12 after insemination improves pregnancy rate in ewes.
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