The last major analysis of reproductive performance in South African Thoroughbreds was published in 1999, reporting a live foaling rate of 62 percent. To update these figures, researchers in South Africa recently reviewed the age and foaling rates of barren and maiden mares from 2018 and 2019. These mares were part of Southern Africa’s Stud Health Scheme (SHS), which conducts annual testing for bacterial venereal diseases in mares prior to breeding.

In total, 990 mares were tested in 2018, and another 1,130 mares were tested in 2019. The majority of maiden mares were young, aged five years or younger, while most barren mares were older, with 79 percent falling between the ages of six and 15. “Failure to conceive” was the most common reason for mares being classified as barren.

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The foaling rates for both maiden and barren mares were 68.1 percent in 2018 and 63 percent in 2019, aligning closely with the 1999 figures.

The study’s key findings emphasized mare age over the number of previous live foals as the primary factor in foaling success. Mares aged 10 years or younger had the highest foaling rates, ranging from 68.9 percent to 71.9 percent. However, as mares aged, their foaling rates dropped significantly, with mares aged 16 years or older showing a 54.7 percent rate in 2018 and just 45.7 percent in 2019.

Mares aged 10 years or younger were nearly three times more likely to produce a live foal than mares aged 16 years or older, explained Kathleen Crandell, Ph.D., of Kentucky Equine Research.  “Advancing age is a well-known predictor of reproductive performance, and studies show that advancing age negatively influences the condition of the endometrium, the lining of the uterus.” 

Although age cannot be reversed, there are ways to help support the reproductive health of older mares. “Supplementing mares with long-chain polyunsaturated omega-3 fatty acids DHA and EPA has reproductive benefits such as improved fertility, particularly in older mares,” Crandell added.

Scholtz, M., A.J. Guthrie, G.T. Fosgate, and M.L. Schulman. 2022.

Predictors of foaling outcomes in barren and maiden Thoroughbred mares in South Africa. Journal of the South African Veterinary Association 93(2).

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Information provided by Kentucky Equine Research.

Photo: Clix Photography