RESUMO
The pregnancy rate following embryo transfer (ET) is a very important factor in the success of embryo production programs. Different strategies were therefore developed to increase pregnancy rates. The aim of this meta-analysis was to investigate the effects of hormone treatments used to increase the success of embryo transfer programs on pregnancy rates. A meta-analysis was performed of 46 trials from 39 publications involving treated (n = 7856) and control (n = 6663) cattle. The meta-analysis explained the effect size with its 95 % confidence interval (CI) for pregnancy per embryo transfer (P/ET) after hormonal treatment under different moderators. Hormonal support was found to increase P/ET compared to the control group (P < 0.05). However, GnRH treatment was found to increase P/ET by approximately 4.3 % and hCG treatment by 8.0 %. Progesterone supplementation was not found to have a statistically significant effect on P/ET. In addition, GnRH treatment significantly increased P/ET when used to transfer in vitro or frozen-thawed embryos or in studies using cows as recipients. It was observed that hCG treatment had a positive effect on P/ET according to all moderators. Progesterone supplementation significantly increased P/ET when frozen embryos were transferred and reduced P/ET, especially in publications where fresh or in vitro produced embryos were transferred or cows were used as recipients. The results of this meta-analysis showed that the use of GnRH, and hCG, in bovine embryo transfer programs increased P/ET, whereas the use of progesterone had no effect on P/ET. However, it was found that P/ET could increase/decrease depending on the moderator.