Fertility of pasture bred mares in synchronized oestrus

Oestrus was synchronized in 220, 300 and 272 mares in 1983, 1984 and 1985 respectively. Mares were given two injections of 250 micrograms fenprostalene 15 days apart except in 1983 and 1984 when 56 and 53 of the synchronized mares were given 1-10 daily injections of 150 mg progesterone and 10 mg oestradiol-17 beta to delay and synchronize post-partum oestrus. At 2 days after the second PG injection or 7 days after the last progesterone + oestradiol treatment, mares were divided into groups of 15-21, and each group was placed in a separate pasture with a stallion for 7 weeks. Pregnancy rates were 87.7, 93.7 and 97.1%, and foaling rates were 72.3, 89.7 and 94.1% in 1983, 1984 and 1985 respectively. The number of abortions occurring mainly between 3 and 6 months of gestation varied from 34 (17.8%) in 1983 to 12 (4.3%) in 1984 and 8 (3.0%) in 1985.