Managing the Summer Heat
Tweak Your Schedule
If your schedule allows it, move your riding and training times to the morning or evening to avoid the hottest part of the day (typically from 11:00 AM - 3:00 PM, but check your local forecast). You and your horse will both appreciate this accommodation to avoid exertion during excessive temperatures, and reduce the risk of heat stress and potential heat stroke. If intense heat is unavoidable with your schedule or with a lack of cooling in the evening hours, consider reducing the duration or intensity of the workout, or skipping it entirely.
Modifying your turnout schedule during the summer can be beneficial as well, particularly if your paddocks and pastures lack shade. Keep horses in during the day and turned out at night when the sun goes down and the air temperature drops.
Provide shade, airflow, and free access to clean water during hot weather. This might mean keeping your horse stabled during the day, or adding a run-in shed or sun sail to your paddock to give your horse relief from the summer sun rays.
If stalled, open doors and windows where possible to increase ventilation. Fans can also make a huge difference in airflow. Fans with mist attachments can be used, though if you live in a high humidity area, they may not provide much additional benefit versus a standard fan.
Water intake - on average, a 1,000-pound adult horse will need 8-10 gallons of fresh water per day, but this can reach 20-25 gallons for horses in work in temperatures above 70°F. Ample, fresh water should be provided at all times. You can encourage your horse to drink by adding loose salt to feed or providing salt blocks. You might also consider adding an additional bucket of water with electrolytes - but this should only be in additional to fresh, plain water. Electrolytes can be beneficial particularly when your horse is losing them through sweating.
Your fly protection strategy might include topical treatments, horse 'clothing', feed-through supplements, and facility management strategies. Most owners are familiar with the commercially available fly sprays, but in addition to the commonly used chemical insecticide-and-deterrent-based sprays, there are also more natural essential-oil based solutions can be applied to keep flies at bay. Use caution and read the label, as essential oils can be quite strong - your formula may instruct you to dilute it in water. Be sure to apply as a fine mist vs saturating your horse's coat. There are also spot-on solutions (similar to flea products for cats and dogs).
Absorbine has some great fly spray education here.
Your horse may benefit from fly masks, boots, and sheets, depending upon your specific needs. Flfly predators, traps, and tapes can be utilized from a facility-level to minimize the population of pesky insects, and there are even feed-through supplements that can deter flies from your horses or interrupt their life cycles.
How Hot is Too Hot?
Avoid riding your horse when the Heat Index (the combined air temperature (F) and relative humidity) is over 150, especially if the horse is not acclimated to the heat.
Normal, resting core body temperature for horses ranges from 99-101°F. Per the AAEP,
"Additional heat generated during exercise results in an elevation of core body temperature from resting values (range, 99-100 F) to an excess of 102-106°F. The horse’s body attempts to cool itself by dissipating excess heat via sweating.
The subsequent evaporation of sweat carries heat from the body and helps to lower body temperature to a safe range. An important point to remember is that high humidity decreases the sweat evaporation rate, therefore slowing the cooling process. This is especially important in the heat and humidity of the summer. High temperatures and humidity decrease the horse’s ability to cool. If the rate of body temperature cooling is not satisfactory to match the ambient conditions, heat stress can develop. "
Horses who live in areas where the are acclimated to hotter weather may have greater resiliency than others to tolerate heat, but caution should still be exercised when the Heat Index reaches 150 or greater - especially if high humidity is a component of the score.
Here is some guidance directly from AAEP, the American Association for Equine Practitioners, for managing the summer heat.
If you have a horse with a long coat (such as from a disorder such as Cushing's Disease, or from unseasonably warm weather), consider clipping so they can cool more effectively.
Horses with anhidrosis (little or no ability to sweat) should be watched closely as they are prime candidates for overheating. Without the ability to sweat, horses cannot effectively cool themselves in warm weather. Speak to your vet, as there may be helpful medication to manage this disorder.