Changes in Fertility Associated with Estrus Expression and the Influence of Varying Time of Insemination with Liquid and Frozen Semen on Pregnancy Success

In the beef industry, maintaining a 365 d calving interval relies on the reproductive management of the females and is essential in order to maximize productivity and profitability of any operation. Expression of estrus has a strong influence on pregnancy success, and relies on elevated estradiol concentrations to prepare the reproductive tract for sperm transport (Hawk, 1983) and early embryo development (Zollers et al., 1993). The use of frozen semen for artificial insemination allows for the long-term storage and international dispersal of gametes from genetically superior animals (Vishwanath, 2003), but requires precise timing of insemination due to the reduction in viable lifespan and motility characteristic of cryopreserved semen (Watson, 1995). Development of fixed-time AI (FTAI) protocols that eliminate the need for detection of estrus greatly benefit operations where labor is limited. Fixed-time AI protocols are more time efficient but induction of ovulation without expression of estrus may lead to decreased conception rates due to inadequate estrogen concentrations to properly regulate the uterine environment for sperm transport and survival (Perry and Perry, 2008). Initiation of normal estrous cycles is an important criteria for expression of estrus, and is largely dependent on nutritional status (Hurnik, 1987). A meta-analysis conducted on 10,116 animals from 26 different studies reported a 27% improvement in pregnancy rates among cows that expressed estrus prior to FTAI (Chapter 3). Management factors influencing expression of estrus were investigated and days postpartum did not impact expression of estrus, whereas BCS and estrous cycling status did influenced estrus expression. Additionally, no effect of repeatability of estrus expression within an animal over multiple years was detected (Chapter 3). Liquid semen is not exposed to stress associated with cryopreservation and may have a longer viable lifespan in the female reproductive tract compared to frozen semen. Insemination at the correct time is essential for pregnancy success, and depends on the viable lifespan of gametes, the time it takes for viable sperm to reach the site of fertilization, and the timing of ovulation relative to insemination (Nebel et al., 2000). We examined the effect of varying the interval from CIDR removal to AI with fresh or frozen semen (Chapter 4). There was no difference in pregnancy rates between liquid or frozen semen, but insemination at 60 h and expression of estrus resulted in greater pregnancy rates compared to 36 h and no expression of estrus, respectively. Among cows that failed to exhibit estrus, insemination with liquid semen resulted in greater pregnancy rates compared to insemination with frozen semen and those that became pregnant had elevated concentrations of estradiol (Chapter 4). In summary, incorporation of estrus detection into FTAI breeding programs should be considered as a strategy for improvements in pregnancy success. Management of the herd can have a tremendous impact on the variability of estrus expression. Furthermore, there was no difference in pregnancy success between liquid and frozen semen, but cows that exhibited estrus and were inseminated at 60 h after CIDR removal had greater pregnancy success compared to cows that did not exhibit estrus