Fillies mature sexually as early as the yearling year, but it can depend on the season, writes Dr. Atwood C. Asbury in the January issue of The Horse. 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 their yearling year.