[Follicular dynamics after treatment with hCG for ovulation induction in mares]

In this study the use of hCG for induction of ovulation is described. Factors such as follicle diameter at the time of administration of hCG (3000 IE hCG i.v.), follicular growth after hCG and the rate of double ovulations were evaluated. A total of 168 mares presented for artificial insemination were used. In 249 estrous periods hCG was given to mares exhibiting standing estrous when a minimum follicle diameter of 30 mm and a well developed edema of the endometrium could be detected by ultrasonography. In nine estrous periods ovulation occurred within 24 hours after hCG. The majority of mares (216; 86.7%) ovulated 24 to 48 hours after hCG and in 24 cases ovulation was delayed beyond 48 hours. Follicle size at the time of hCG administration (30-34 mm, 35-39 mm, > or = 40 mm) had no influence on the percentage of mares ovulating 24 to 48 hours after hCG (89.2%, 87.9%, and 83.7%, respectively). Double ovulations could be observed in 17.7% of estrous periods. The one cycle pregnancy rate was not influenced by follicle size (small 45.9%; medium 41.6%; large 47.5%). Repeated treatments with hCG during successive estrous cycles within one year did not influence the rate of responding to hCG. Mares in standing estrous respond well to hCG if a minimum follicle size of 30 mm and a well developed endometrial folding is present.