It is fire ant season. The mounds are popping up everywhere. Initially, mounds may be only an inch tall, but they will grow rapidly to easily a foot in diameter. If you or your horse steps in them, the ants quickly attack. They first bite to hold on and then sting and inject venom. They will keep stinging until they are removed.
Treating the mounds on your horse farm is not difficult if you keep on the project. Many stores and tack shops sell mound treatment. Follow the directions on the bag to sprinkle poisonous bait around the mound. I am sure it is better to treat the mound when your horses do not have access to the area, however I have found my horses avoid the mounds and the bait, so I treat all mounds when I find them whether it is in my yard or my pasture. Usually the ants are inactive within a day.