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.(ABSTRACT TRUNCATED AT 250 WORDS)