Age-Dependent Effect of Foal Heat Breeding on Pregnancy and Embryo Mortality Rates in Thoroughbred Mares.

This study analyzed the effect of age and heat cycle timing on reproduction efficiency in Thoroughbred mares, finding that older mares should be bred during their second heat cycle to reduce embryo mortality and increase reproductive success.

Objective and Methodology of the Study

• The main aim of this research was to determine the potential correlation between a mare’s age and heat cycle in relation to their reproductive efficiency. Two points of the mare’s heat cycle – foal heat (immediately after she has had a foal) and second heat (the next cycle after foal heat) were considered.
• Thoroughbred mares of various ages were observed and their rates of successful pregnancy and embryo mortality were recorded. Embryo Mortality (EmbM) was considered whenever a mare did not remain pregnant after an initial positive pregnancy diagnosis.
• The research focused on the interval from birth of a foal to next breeding, the rate of successful pregnancies, and the rate of EmbM as primary variables. The approach to breeding – during foal heat or the second heat – were the different intervention methods applied.

Outcomes

• The study found that Pregnancy Rate (PregR) was lower for all mares bred on foal heat compared to those bred on their second heat, irrespective of the age of the mare.
• Interestingly, the age of the mare did not have a significant effect on her ability to conceive after foal heat, indicating that younger mares were not necessarily more fertile during this time.
• Regarding foal heat and second heat, there were significant differences in pregnancy rates in younger mares. In contrast, the age of the mare didn’t produce a significant difference in pregnancy rate when bred on the second heat.
• Furthermore, EmbM rates were not found to be significantly different based on whether the breeding happened during the foal heat or second heat. However, the age of the mare did affect EmbM rates, with older mares having higher rates of EmbM in general.
• Significantly, older mares bred during foal heat had a much higher rate of EmbM compared to younger mares bred at the same time.

Conclusion and Recommendations

• The results indicated that the reproductive efficiency of Thoroughbred mares bred during foal heat was affected by the age of the mare.
• The study concluded that for older mares (10 years and above), it is recommended to breed during their second heat after the birth of a foal to improve chances of successful gestation and reduce the EmbM rates.