PREGNANCY RATES FOR EMBRYOS TRANSFERRED FROM EARLY POSTPARTUM BEEF-COWS INTO RECIPIENTS WITH NORMAL ESTROUS CYCLES

Survival rate of embryos from first ovulations of postpartum cows with SHORT (6.9 +/- 0.3 days; n = 35) or NORMAL (17.1 +/- 0.3 days; n = 42) luteal phases and quality of the embryos on Day 6 were compared. At 19 to 23 days postpartum, cows were allotted to receive a norgestomet implant for 9 days (normal luteal phase) or to serve as untreated controls (short luteal phase). Calves were weaned 7 days after initiation of treatment to induce behavioral estrus in cows for mating. In 25 cows, growth of the ovulatory follicle was monitored by ultrasonography. On Day 6 after estrus, embryos were recovered nonsurgically, and live embryos were transferred into recipient cows exhibiting normal estrous cycles. The medium used to flush the embryos from the uterus of each donor cow was assayed for prostaglandin F2alpha (PGF2alpha). Days from calf removal to estrus and size of ovulatory follicles at ovulation (4.1 +/- 0.3 days and 16.7 +/- 0.7 mm, respectively) did not differ between NORMAL and SHORT cows. Interval from detection of the ovulatory follicle to ovulation was longer in NORMAL (10 +/- 0.7 days) than in SHORT cows (8 +/- 0.6 days; p < 0.05). Rates of recovery of an embryo or ovum (64%), rates of fertilization (65%), and quality or stage of development of Day 6 embryos did not differ between SHORT and NORMAL cows. Overall pregnancy rate from recovered oocytes was 13% for SHORT and 32% for NORMAL cows (p = 0.06); survival of fertilized oocytes was 23% for SHORT and 47% for NORMAL cows (p = 0.08). Survival rate of transferred embryos (65%) did not differ with respect to length of the luteal phase. Concentrations of PGF2alpha in flushing medium were higher in SHORT (577 +/- 68 pg/ml) than in NORMAL cows (337 +/- 65 pg/ml; p < 0.05). Quality of Day 6 embryos tended to be correlated to concentrations of PGF2alpha in uterine flushings (r = -0.42; p = 0.07). In conclusion, an improper uterine environment appears to play a role in the lower survival of embryos in cows with short as compared to normal luteal phases.