Hot Weather:The hot weather during summer months is the obvious way your horse can quickly become overheated. Especially in humid weather, your horse will have a harder time shedding heat from his body.
Hot Weather Fixes:
Go out early in the morning and rest during the heat of the day. Use tack that allows airflow to their skin to help keep them cooler. Be sure to offer your horse plenty of water throughout the day, keeping them hydrated will help them reduce their inner body temperatures.
Overexertion:Another way your horse can overheat is from working too hard. Heat is a byproduct of muscle exertion, over half the energy used is converted into heat. Overexertion and high body temperatures can lead to fatigue and weak muscles, potentially causing accidents. In addition, if your horse is deficient in any vitamins and minerals, they are more prone to their muscles "tying up" from overexertion.
Avoid working your horse too hard in the heat. Go at a mild pace, don't push your horse too hard or too fast. Throughout the ride take frequent breaks, allowing your horse to breathe and rest, preferably in the shade and near water. Cool your horse off with water on their neck, head, chest, and legs. Loosening the tack is another way to help reduce heat and keep them cool. In preparation for the trail season make sure your horse has been receiving a vitamin-mineral supplement to prevent muscle fatigue and tying up.