Simple SummaryIn recent decades, embryo transfer in dairy cows has been widely applied, and improving the conception rate after embryo transfer has become a major challenge in dairy cow reproduction. Progesterone is a critical hormone during early pregnancy in the cow. One strategy to increase progesterone levels and improve the fertility of dairy cows is inducing accessory corpus luteum through the administration of GnRH or human chorionic gonadotropin after embryo transfer. Many studies tried to improve conception rate of ET recipients, but failed to deliver a clear consensus on the therapeutic benefits of such treatments. Thus, a meta-analysis to evaluate the efficacy of hCG and GnRH in improving pregnancy rates after ET was carried out in this review. Our results indicated that the use of GnRH or hCG can improve pregnancy rates in dairy recipient cows with very poor fertility (<40%), but does not have a significant effect on cows with good fertility. Compared with GnRH or GnRH analogue, hCG treatment acquired better improvement in conception rates after ET. Treatment between days 5 and 7 after synchronized ovulation was beneficial, while later than this was not. Furthermore, the treatment was more effective in parous lactating dairy cows compared with heifers. This review may have great significance in the management of reproductive performance on modern dairy farms.AbstractThe circulation of progesterone (P4) concentrations of recipients has positive correlations with embryo survival and pregnancy success of embryo transfer (ET) in dairy cows. One strategy to improve P4 concentration is the administration of gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin (hCG), thereby inducing the formation of accessory corpus luteum (CL). This study aimed at determining the efficacy of GnRH or hCG treatment regarding embryo transfer (ET) and providing a better clinical veterinary practice guidance. A meta-analysis was conducted on the data from 2048 treated recipient cows and 1546 untreated cows. By inducing the formation of accessory CL with GnRH (100 µg), GnRH analogue Buserelin (8–10 µg), or hCG (≥1500 IU) 5–11 days after synchronized ovulation, hCG alone achieved an improvement (RR = 1.39,p< 0.05), while GnRH and GnRH analogue did not result in significant changes (RR = 1.04,p= 0.26). Treatment with GnRH or hCG 5–7 days after synchronized ovulation was associated with increased chances of pregnancy compared with later treatment (11–14 days). Owing to the treatment, the pregnancy rate of cows with very poor fertility (<40%) was improved, while that of cows with good fertility (≥40%) was not affected. Treatment with GnRH or hCG greatly improved pregnancy rates of parous lactating cows (RR = 1.32,p< 0.05) compared with heifers (RR = 1.02,p> 0.05). Additionally, as indicated by pregnancy loss analysis, the treatment had no benefit on late embryo/early fetus survival at days 28–81. In conclusion, the induction of accessory CL with GnRH or hCG may benefit fertility and have important implications for the management of reproductive performance in the dairy industry.