We all want to ride in the summer but the weather poses major challenges to our horses. The horses control their body temperature primarily by sweating. Horses doing endurance training, eventing, taking trail rides, giving lessons, even standing in the field in hot weather are at risk of dehydration and heat related illness, even death in extreme cases.
Sweating causes prodigious losses of water and salt. Water loss can be as high as about 4 gallons/hour with heavy sweating and salt losses the equivalent of as much as 4 ounces of salt. The sodium in salt, which is sodium chloride, is absolutely essential for the horse’s body to be able to hold on to normal levels of water.
Without adequate sodium/salt regardless of how much water the horse drinks they will not be able to retain a normal amount of water in the body. Hydration requires normal body sodium levels to hold the correct amount of water.
The brain reads sodium levels in the blood to control when the horse is stimulated to drink. When water levels drop and sodium concentration rises, the horse is signaled to drink. However, before any changes in water or sodium levels in the blood can occur, the body will pull what it needs out of the tissues to maintain blood levels. This can, and does, result in severe dehydration at the tissue level before it shows up in the blood.
The consequences of this include poor exercise tolerance, cramping, apparent weight loss (actually water loss) and increased risk of colic and heat stroke. Sweat production may decrease. Exercising horses and lactating mares are at particularly high risk.
To monitor for dehydration, do the skin pinch test. A fold of skin on the neck lifted up away from the body should snap back into place quickly when you let go. Somewhat more reliable is the capillary refill time. Press a finger firmly against the gum above the upper teeth. The white indentation this forms should return to a pink color again in 1.5 seconds or less. As dehydration becomes advanced, the interior of the mouth will begin to feel tacky. Urine production drops and urine becomes darkly colored.
Clean water must be available at all times and in unlimited amounts. Let the horse drink freely and as much as they want immediately after exercise. Have a salt block or loose coarse salt available at all times. Also add 2 to 3 ounces per day to grain feeds or dissolved and sprayed on hay. If the horse is on less than 1% of body weight in hay per day, or if working an hour a day or more, also use an electrolyte replacement product containing twice as much chloride as sodium and about half as much potassium as sodium.
Close attention to the horse’s hydration is the most important factor in maintaining health and performance in the summer.
Eleanor M Kellon, VMD
Dr. K's Horse Sense 