Canadian Journal of Animal Science 56 3 467 473

Twenty-five lactating dairy cows were treated with either saline (control), 500 mu g gonadotropin-releasing hormone (GnRH), GnRH + 10 mg oestradiol (E), oestradiol alone, or GnRH + E + 25 mg prostaglandin F2 alpha at approximately 11 days postpartum. Blood samples were taken at the time of injection and then twice each week for about 30 days. Treatment with GnRH alone gave the largest LH peak; LH production appeared to be inhibited when E was given with GnRH. Plasma oestrogen concentrations were above 1 ng/ml for 12 h in all animals receiving oestrogen alone or in combination. None of the treatments hastened initiation of cyclic activity which began at an average of 19 days postpartum. It seems unlikely that postpartum anoestrus is an important constraint to early rebreeding in dairy cows.