Estrus synchronization and pregnancy rates in cyclic and noncyclic beef cows and heifers treated with syncro-mate B or Norgestomet and Alfaprostol

Postpartum lactating cows (N=118) and virgin heifers (N=60) were treated with subcutaneous Norgestomet implants for nine days and received either an intramuscular injection (im) of 5 mg estradiol valerate and 3 mg Norgestomet at the time of implant insertion or an im injection of 5 mg Alfaprostol 24 hr before implant removal. Animals were artificially inseminated 12 hr after detection of estrus. Of the cows and heifers, 78% and 88%, respectively, were in estrus within five days after implant removal (P<0.09). There was no difference between treatments in the proportion of animals in estrus or in the timing of estrus (P<0.85). Estrus was detected in a greater (P<0.05) proportion of animals that were cyclic prior to treatment (88%) than among those that were anestrous prior to treatment (77%). Pregnancy rates after five days were similar between heifers that were cyclic (42%) or anestrous (47%) prior to treatment; however, the five-day pregnancy rate in cows that were anestrous prior to treatment was 38% lower than that in cows that were cyclic prior to treatment (17 vs 55%, P<0.01). Although the treatments synchronized or induced estrus in both cyclic and anestrous animals, marked variability in estrous response and fertility among previously cyclic or anestrous postpartum cows limited the effectiveness of the treatments.