Extreme heat can be hard on your horse, especially during riding sessions. When temperatures soar, slow down the work. Don’t think that because your horse has been working every day in 80-degree weather that he can necessarily take the heat when the mercury tops 90. If you must work him in that kind of heat, lighten the work or break it up into short sessions. This is especially important if humidity is high, making it harder for him to cool himself through sweating—plus contributing to poorer air quality.
Source: University of Pennsylvania’s School of Veterinary Medicine