PHYSIOLOGICAL FACTORS THAT AFFECT PREGNANCY RATE TO ARTIFICIAL INSEMINATION IN BEEF CATTLE

The development of effective methods of synchronizing estrus and ovulation has been based on our understanding of the physiological and hormonal mechanisms controlling the estrous cycle and the initiation of estrous cyclicity in prepubertal heifers and postpartum cows. Although estrus synchronization products (e.g. CIDR and GnRH) and protocols have changed over time, the basic physiological principles underlying how these products work have not. An understanding of the importance of estrus expression and the basic physiological factors affecting pregnancy rate following estrus synchronization and artificial insemination can facilitate the application of these technologies in cycling and anestrous females. The purpose of this paper is as follows: 1) review some management practices that increase the probability of a successful estrus synchronization and AI program, 2) emphasize the importance of estrus expression for the establishment of pregnancy, 3) discuss some basic physiological mechanisms that increase pregnancy rate following synchronization of estrus and(or) ovulation in cattle, and 4) provide guidelines for determining why the pregnancy rate to AI may have been lower than expected. Important Considerations Before Implementing an Estrus Synchronization Program Factors Affecting Pregnancy Rate. When it comes to reproductive management the things you do well will not compensate for the mistakes you make. Instead, the mistakes you make cancel out all the things you do well. This is best illustrated by examining the primary factors that affect pregnancy rate. In an AI program, pregnancy rate is the product of estrous detection rate and conception rate (Pregnancy rate = estrous detection rate x conception rate; see definitions below). The following definitions can be applied to an entire breeding season or to the synchronized period (period of time during which estrus is expressed after treatment with an estrus synchronization protocol [normally 5 to 7 days]). Pregnancy rate – total number pregnant during the breeding season/ number of females exposed to breeding (expressed as a percent).