Reproductive performance of thoroughbred mares on six commercial stud farms.

This study investigates the breeding performance of thoroughbred mares in southeastern Australia from 1981 to 1986. It shows that both age and number of estrus cycles required for pregnancy significantly influence successful foaling rates.

Research Context and Methodology

• This study analyzed breeding records of 1630 mare years from six Thoroughbred stud farms located in Southeastern Australia, over a five-year period (1981 to 1986).
• The key reproductive performance indicators considered in the study were pregnancy rates, foaling rates, and the occurrence of twin pregnancies.

Pregnancy and Foaling Rates

• The cumulative pregnancy rate recorded was 83.9% and the foaling rate was 69.3%.
• When evaluated per served estrus cycle, these rates were lowered to 54.7% and 43.1% respectively. This suggests that getting mares pregnant on the first estrus cycle they are served enhances the likelihood of a successful foal.
• Mares aged 3 to 10 years had significantly higher pregnancy and foaling rates than older mares, indicating a decrease in reproductive efficiency with age.

Effect of Previous Breeding Status

• The study reported no difference in pregnancy rates between maiden (first-time breeders), barren (not currently or recently pregnant), and foaling mares.
• However, miscarriage rates significantly varied based on the mare’s previous reproductive status, being lower in maiden mares (12.4%) compared to barren (19.7%) or foaling mares (20.9%).

Twin Pregnancies

• Twin pregnancies were detected in 7.8% of all conceptions.
• In situations where one fetus was naturally lost (without human intervention), the chances of the remaining pregnancy progressing to a full term were 84.1%.
• If one fetus was manually obliterated, the success rate of the remaining conception carried to term dropped to 55.9%.
• When prostaglandins, hormones that induce labor, were used to terminate twin pregnancies, 60% of such mares gave birth to a single foal in the following year.