When are Fillies Fertile?

At what age does a filly go into heat? Our 18-month-old Haflinger/Arab mix shows signs of coming into heat.
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Q:What I am interested in knowing is at what age does a filly go into heat? We purchased a Haflinger/Arab mix four days ago and were told she was 18 months old. Today we put her in with our other horses and she showed signs of coming into heat. She was not submissive at all. She showed none of the normal submissive actions that a yearling would show. Your help would be greatly appreciated.

—Tracy

A: Fillies mature sexually as early as the yearling year, but it can depend on the season. Those born early in the year might cycle and be fertile as early as 14-15 months of age (May-June of the yearling year). Foals born in late fall might miss cycling on the yearling year because the length of daylight affects the cycle. Submissive behavior probably is not related to the behavior of true “heat.”

The signs of estrus (“heat”) usually are distinct from the typical submissive attitude of foals. Fillies in estrus raise the tail, urinate, “wink” the vulvar lips in the presence of other horses (either sex), and might stand if mounted. The best management for prevention of early, unwanted pregnancy is to separate colts and fillies by the yearling year

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A. C. (Woody) Asbury received his DVM from Michigan State University in 1956, then spent 21 years in California in breeding farm practice and at UC Davis. He joined the faculty at the University of Florida in 1977 and was involved in teaching, research, and administration until 1996. Asbury was a long-time member of The Horse’s advisory board. He died in 2011 after a lengthy illness.

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