Reproductive performance of Thoroughbred mares on six commercial stud farms

The records of 1630 mare years from 6 Thoroughbred stud farms in south eastern Australia were analysed for the years 1981 to 1986. Overall pregnancy and foaling rates were 83.9% and 69.3%, respectively. When calculated per served oestrous cycle, pregnancy and foaling rates were 54.7% and 43.1%, respectively. Pregnancy and foaling rates were higher (P < 0.001) for mares 3 to 10 years of age than for older mares. There was no difference in the pregnancy rates of maiden, barren and foaling mares. The foaling rate was significantly higher (P < 0.001) in mares that became pregnant during the first served oestrous cycle (77.8%) than in mares that needed two served oestrous cycles to become pregnant (65.4%). Of all diagnosed pregnancies, 19.5% were not completed. Pregnancy loss was lower (P < 0.05) in maiden (12.4%) than in barren (19.7%) or foaling (20.9%) mares. Twins were diagnosed in 7.8% of all pregnancies. If one conceptus was lost without external interference, 84.1% of pregnancies went to term. If one conceptus was manually crushed, 55.9% of pregnancies were maintained. If prostaglandin was used to terminate twin pregnancies, 60% of mares so treated produced foals the following year.