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My utmost appreciation goes first to my family, who always supported me even though they did not understand what I was doing, despite being perplexed when I left my interest in sheep behind. They raised me to have a strong work ethic and to be independent, which were both essential in completing this project. Second, I would like to thank the faculty on my committee. Thanks go to Dr. Inskeep for his guidance, sharp proofreading eye and for enduring one cold winter day of cow work in my absence. I am just now starting to get the hang of things. Dr. Lewis accompanied me many times on trips to Eglon, but perhaps most importantly taught me that there is more to life than work. Finally, thanks go to Dr. Dailey for his keen point of view and his attempts to show me that I really do know more than I think. I would like to thank those at the Bachtel Dairy; Carl, Mark, Alan and Ronnie, for their cooperation, interest and assistance with this project, even though much of that assistance was related to vehicles. I would like to thank the farm crew at the WVU dairy, Will, Jimmy, Greg and Gary, for their assistance in retrieving unruly cows and for being patient with me because I did things out of routine. My gratitude is extended to those who took time from their schedules to be my traveling assistant and extension cord overseer,

Four experiments were conducted to evaluate two measures of reproductive efficiency in cattle, conception rate and retention of pregnancy. In experiment I, the relationship between concentrations of progesterone and retention of pregnancy was examined. The corpus luteum (CL) was removed on day 29 of pregnancy in 40 suckled, beef cows, and pregnancy was supported with exogenous progesterone to day 53. Concentrations of progesterone (1.8 ± 0.1 ng/mL between days 30 and 38), were below those normally recorded for pregnant cows. Retention of pregnancy (77%) was monitored frequently using ultrasonography until day 53 and was negatively associated with age of cow, but not directly related to concentrations of progesterone or estradiol, or body condition score. On day 53, a new CL was induced using hCG and exogenous progesterone support was withdrawn (experiment II). Pregnancy was maintained to term in all 20 cows that formed a new CL, including four in which the new CL was contralateral to the uterine horn of pregnancy. Effects of a single treatment with rbST at artificial insemination on conception rates and retention of pregnancy in dairy cows and heifers and beef cows were examined in experiment III. Conception rates were increased only in dairy cows, specifically those > 100 days in milk. Retention of pregnancy was not affected by treatment with rbST. In experiment IV, the relationship between the number of antral follicles that developed within a follicular wave and fertility was examined. Development of follicles ≥ 4 mm in diameter was monitored by ultrasonography on alternate days from day 6 of the cycle until ovulation or diagnosis of pregnancy in beef cows and heifers before and after insemination during two (Trial 1) or three (Trail 2) estrous cycles. Cows were artificially inseminated 12 h after second estrus (Trial I) or third estrus (Trial 2). Numbers of follicles ≥ 4 mm in diameter differed with estrous cycle, follicular wave, and cow, but the average number of antral follicles per wave did not affect conception rates. Additional research is needed to determine the specific roles of age of cow and concentration of progesterone on reproductive efficiency. iii