Development of fertility programs to achieve high 21-day pregnancy rates in high-producing dairy cows

Our objective is to overview the research that lead to the development of fertility programs for high-producing lactating dairy cows using only GnRH and Prostaglandin F_2α (PGF_2α), such as Ovsynch with a focus on the role of progesterone in fertility. A key factor affecting fertility to timed-AI is the response to each hormonal treatment of the Ovsynch protocol. Although not required for fertility, cows ovulating to the first GnRH treatment of the Ovsynch protocol (G1) have greater P/AI than cows failing to ovulate. The association between progesterone concentrations at each treatment during the Ovsynch protocol and fertility is reviewed, and data from 7792 cows from 14 studies are presented. Overall, medium P4 (between 0.5 and 6 ng/mL) at G1, high P4 concentrations (>1.0 ng/mL) at the PGF_2α, and low P4 concentrations (<0.4 ng/mL) at final GnRH (G2) are associated with greater P/AI. The use of presynchronization treatments that increase the percentage of cows initiating the Ovsynch protocol during early diestrous increases the percentage of cows with medium P4 concentrations at G1 and with high P4 concentration at PGF_2α. Some cows, however, fail to completely undergo luteal regression after a single PGF_2α treatment, particularly cows that initiate the Ovsynch protocol in a low P4 environment and cows with a young (d 6) CL at the time of treatment with PGF_2α. Addition of a second PGF_2α treatment increased the percentage of cows with complete luteal regression and P/AI. The use of fertility programs that include the concepts described in this review have resulted in more P/AI than inseminating cows after an induced estrus at first insemination. In addition, extending these concepts to strategies for resynchronization of ovulation and the implementation of an aggressive reproductive management program for first and subsequent inseminations results in reproductive performance that is unprecedented for high-producing Holstein dairy cows.