MAINTENANCE OF PREGNANCY IN POSTPARTUM BEEF-COWS THAT HAVE SHORT-LIVED CORPORA-LUTEA

The first two experiments examined the role of the uterus in low pregnancy rates of beef cows induced to ovulate by early weaning. At 20 to 25 d postpartum, one-half of the cows in Exp. 1 and 2 received a s.c. implant containing 6 mg of norgestomet (NOR) for 9 d (NOR-pretreated) and the remaining cows were untreated controls (CON). Lengths of first postpartum luteal phase after weaning of calves at d 7 after implant insertion were expected to be normal in NOR-pretreated and short in CON cows. In Exp. 1, cows of both groups received an implant containing 3 mg of NOR at d 4 after first estrus and a silastic implant with 15 or 25 mg of NOR at d 7 after first estrus. At 7 d after first estrus, two embryos were transferred into the uterus of each cow and pregnancy was diagnosed by ultrasonography at d 35. Blood samples were collected daily from onset of treatment to d 8 after estrus and then every other day to d 24. Only 4 of 22 cows were pregnant at d 35, concentrations of estradiol (E2) were elevated after luteolysis, and large follicles were present at d 35. In Exp. 2, all cows were injected with 100 mg of progesterone (P4) twice daily from d 4 to 35 after first estrus. Embryos were transferred, pregnancy was diagnosed, and blood samples were collected as in Exp. 1, except blood sampling was continued to d 34. A subset of seven cows was examined by ultrasonography every other day from d 6 to 24 for change in size of each large follicle. Pregnancy rates in cows injected twice daily with 100 Mg Of P4 were 5 of 18 in CON cows and 9 of 16 in NOR-pretreated cows (P < .10). In Exp. 2, waves of follicular growth occurred and concentrations of E2 were low during d 6 to 24 after first estrus. A third experiment using nonlactating cows examined the ability of NOR implants, as used in Exp. 1, to suppress development of large follicles and to decrease secretion of E2 after luteolysis induced at d 6 of the estrous cycle. Large (21.6 +/- .8 mm), persistent follicles, elevated concentrations (5.9 +/- .4 pg/mL) of E2, and intermittent estrous activity without standing estrus or ovulation were observed after luteolysis. It is concluded that uteri of some postpartum cows with short-lived CL are capable of maintaining gestation, if provided good-quality embryos and twice-daily injections Of P4. However, NOR implants, as used in Exp. 1, were ineffective in maintaining pregnancy, suppressing secretion of E2, and preventing persistence of large follicles.