Factors affecting reproductive efficiency of cattle

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 On day 53, a new CL was induced using hCG and exogenous progesterone support was withdrawn (experiment II).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 \u3e 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 (Trial 2) estrous cycles. Cows were artificially inseminated 12 h after second estrus (Trial 1) 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