Conception, Embryonic Development and Corpus Luteum Function in Beef Cattle Open for Two Consecutive Breeding Seasons

High-fertility (control cows) and low-fertility (cows and heifers not pregnant after two consecutive breeding seasons -twice-open) cyclic bovine females were treated with a single injection of 1000 IU of human chorionic qonadotropin (HCG)orlOO ng of gonadotropin releasing hormone (GnRH) to enhance and/or hasten corpus luteum formation and progesterone secretion, and improve conception rate in the low-fertility females. Hormone treatments were administered to 38 parous control cows, 34 twice-open parous cows, and 27 twice-open nonparous heifers immediately after natural mating by a fertile bull. Blood samples were collected on Days 3, 6, 9, 12, and 18 after mating for determination of systemic progesterone concentrations. Pregnancy rate at necropsy approximately 33 days after mating (range 31-37) was higher in control cows (73.0%) than in twice-open cows (48.4%; P<O.O5) or twice-open heifers (34.6%; P<O.Ol). Pregnancy rate was not affected by the HCG or GnRH treatment. The HCG treatment increased plasma progesterone concentrations in twice-open heifers but not in control or twice-open cows. Progesterone was unaffected by the GnRH treatment. Systemic progesterone concentrations were higher in control than in twice-open females but did not differ between pregnant and nonpregnant females of Days 3, 6, 9 and 12 after mating. Enhanced gonadotropin stimulation at estrus by injection of either HCG or GnRH did not increase pregnancy rate or systemic progesterone concentrations (except in HCG-treated twice-open heifers) in lowor high-fertility females. Lower pregnancy rates in twice-open females were not associated directly with the lower systemic progesterone concentrations.