Ovarian function in Holstein cows immunized against pregnant mare serum gonadotrophin

Six Holstein-Friesian cows were immunized against pregnant mare serum gonadotrophin (PMSG) using Freunds' adjuvant during the mid-luteal phase of the estrous cycle. Antibody response was maintained by five booster immunizations at 2- to 3-wk intervals. Four cows were treated with a single intramuscular injection of PMSG (2350 I U) 107 d after primary immunization. Cloprostenol (500 ug) was administered at 56 h and 72 h after the treatment with PMSG; the cows were inseminated three times at 12-h intervals starting 56 h after cloprostenol treatment. Five days after insemination, the animals were slaughtered and their reproductive organs were recovered to quantify the population of corpora lutea and unovulated follicles (>10 mm dia). Antibody titres and progesterone concentrations were determined from blood samples collected either on alternate days or twice a week. Initially, progesterone concentrations were measured in milk samples. All cows produced antibodies, and titres were elevated within 6 to 9 d following each booster immunization. After each boost, however, the antibody titres declined rapidly. Progesterone concentrations declined to below 1 ng/ml after two weeks of initial immunization and remained low throughout the study, except in one cow that ovulated on Day 75. All animals were observed to have large follicular cysts during this period. Treatment with PMSG induced a single ovulation in one cow. Ovulations were neither induced by PMSG nor observed in any of the other animals. In PMSG-treated animals, the mean number of large follicles (5.0) was greater than in those which were not treated (2.0). The results of this study suggest that low titres of antibodies against PMSG are sufficient to disturb ovarian activity, result in follicular cysts and block multiple ovulations in response to exogenous PMSG.