Increasing the dose of GnRH at a synchronized timed AI increases pregnancy rates in Bos indicus influenced cattle

Compared to Bos taurus cattle, Bos indicus influenced cattle have a history of decreased pregnancy rates following artificial insemination (AI). These decreased pregnancy rates among Bos indicus influenced cattle may be attributed to their higher excitability response to stressful situations, which can result in increased circulating cortisol that can delay or suppress ovulation. This experiment was designed to evaluate the effect of an increased GnRH dose at a synchronized timed artificial insemination (TAI) on pregnancy rates in Bos indicus influenced cattle. Over two years, Bos indicus influenced heifers (n = 50) from four locations, Bos taurus heifers (n = 123) and lactating Bos indicus influenced cows (n = 83) were inseminated with conventional semen using the CO-Synch+CIDR protocol. Heifers were inseminated between 48 to 56 h and mature cows between 56 to 66 h of last PGF2 alpha administration. Non-lactating Brahman cows (n = 32) were also synchronized in the above manner and inseminated between 56 to 66 h with sexed Bos indicus influenced semen. All cows were randomly selected to receive either 100 mu g (n = 144) or 200 mu g (n = 144) of GnRH at insemination and examined via ultrasonography for pregnancy similar to 30 days post-TAI. The administration of 200 mu g of GnRH at the time of AI to Bos indicus influenced cattle significantly (P < 0.004) increased pregnancy rates (0.43 +/- 0.05) compared with 100 mu g of GnRH (0.21 +/- 0.04). This pattern of increased pregnancy rates in the 200 mu g GnRH group occurred at all locations and in all cow types. Among Bos taurus heifers, the increased dose of GnRH at the time of AI did not affect pregnancy rates; 200 mu g (0.49 +/- 0.06) compared with a 100 mu g dose (0.55 +/- 0.06). These results indicate that increasing the dose of GnRH at the time of AI can increase synchronized pregnancy rates in Bos indicus influenced cattle, but not among Bos taurus heifers.