The UC Davis College of Veterinary Medicine indicated in their newsletter that horses are susceptible to problems in the heat and these problems are even more likely to occur in Texas than in California. Many equine events are scheduled in the summer and the triple digit heat can lead to problems such as dehydration, exhaustion, and heat stroke. During these events, all horses must be allowed access to water at all times including on the trailer or after a class at a show. You can't make a horse drink but you can offer hay, which will sometimes encourage the horse to drink; you can even soak the hay in water to increase water consumption. You can also add warm water to grain and make a gruel or mash. Adding electrolytes or even table salt to the feed can increase water consumption.
The most important fact is to know how much water your horse is drinking and if it's not enough, contact a veterinarian before dehydration develops. Also, always provide as much shade as possible for your horse and don't tie it to the trailer out in the sun. Another good idea is to make sure to leave vents and windows open in trailers so as to allow cross ventilation, but a screen over the window is recommended. We have all seen horses traveling down the road with their heads out of the trailer windows, but horses can develop eye irritation and even corneal damage resulting from flying debris.
Know the signs of overheating and fatigue in your horse. One symptom is a high respiratory rate that is persistent 10 to 20 minutes after resting. A change in attitude and alertness (mentation) and a decrease in energy indicate fatigue, and your horse should be allowed to rest.