LAUSR

The initial impact of the original Ovsynch protocol on 21-d pregnancy rates in U.S. dairy herds was to increase AI service rates; however, a deeper understanding of the physiology underlying the Ovsynch protocol has allowed for modifications to the original protocol that dramatically increase in pregnancies per artificial insemination (P/AI). A key factor affecting P/AI to timed AI is the response to each hormonal treatment of the Ovsynch protocol. Cows that ovulate to the first GnRH treatment of the Ovsynch protocol (G1) have more P/AI than cows that fail to ovulate. The association between progesterone (P4) concentrations at each treatment during the Ovsynch protocol and P/AI was analyzed based on data from 7,792 cows across 14 experiments. Cows with P4 concentrations from 0.5 to 6.0 ng/mL at G1, >1.0 ng/mL at PGF2α, and <0.4 ng/mL at the last GnRH treatment had the most P/AI. Presynchronization strategies that combine GnRH and PGF2α induce ovulation in anovular cows and optimize P4 concentrations at G1 and the PGF2α treatments of the Ovsynch protocol to increase P/AI. A subgroup of cows, however, fail to undergo complete luteal regression after the PGF2α treatment of the Ovsynch protocol, particularly multiparous cows that initiate G1 in a low P4 environment and cows with a CL that has not yet acquired luteolytic capacity. Addition of a second PGF2α treatment 24 h after the first in the Ovsynch protocol increases the percentage of cows with complete luteal regression thereby increasing P/AI. The key to increasing P/AI to Resynch protocols is to segregate nonpregnant cows based on the presence or absence of a CL and to treat cows without a CL with exogenous P4 during the Resynch protocol. Adoption of fertility programs developed using these concepts have yielded annualized 21-d pregnancy rates that exceed 30% in healthy, high-producing dairy herds.