A common concern this time of year is horses that lack energy for exercise. There are no apparent health or lameness issues but the horse is either “flat” or starts out well but hits a wall that is at a level of exercise below where they normally perform. What’s wrong?
Is it just too hot?
Could be! The horse is a very large animal and this makes it difficult to dissipate heat. Horses expected to work in the heat should be gradually accustomed to it by step wise increases in work duration and intensity. An important adaptation that occurs is development of an extensive system of blood vessels in the skin. These supply the sweat glands and also radiate body heat from the hot blood on the surface to the surrounding air.
Conditions can be dangerously hot even for accustomed horses. A good rule of thumb uses a heat index calculated by adding the temperature (in Fahrenheit) and the relative humidity. If 120 or less, no barrier to heat dissipation. If over 150, especially with high humidity, the horse will have some difficulty cooling. Over 180, cooling mechanisms are severely compromised and the horse should not be worked. Flagging energy and slowing down will be the first sign your horse is overheating. Heed it.
Is fluid topped off?
Exercise research has documented that as little as 2% dehydration will compromise performance. Work at low and moderate speeds will be affected more than high intensity efforts. This level of dehydration occurs even before you can detect a problem with the skin pinch test so it’s easy to see how dehydration can easily be a problem with work in the heat.
Providing plenty of fresh, clean water whenever the horse wants it is obviously important to avoiding dehydration but it’s not the whole story. To retain that water in the body the horse needs adequate sodium. The major electrolyte lost in sweat is sodium and it is also highly deficient in both hay and concentrate feeds.
I use a 2-2-2 rule to help guard against dehydration – 2 oz of salt the night before a competition or heavy work, 2 oz the morning of and replenish with an electrolyte supplement correctly balanced for sweat if work is longer than 2 hours.
Is the gas tank full?
Many people today are feeding diets designed to limit starch and sugar intake. There is a lot to be said for this but it can sometimes backfire if the horse is in regular work.
The major fuels for muscle work are fat and glucose, with branched chain amino acids also contributing. The fat for muscle work is liberated from fat deposits throughout the body and there is never a shortage. Glucose is taken from the blood but primarily from glucose stored in the muscle as glycogen. Glycogen in the liver is also used to keep blood glucose normal. Glycogen stores are limited so this is the fuel with the potential to limit work. Fat cannot be used to replace glucose. There is always a baseline requirement.
Glycogen stores are lowered by work and need to be replenished. The very low sugar and starch diet may not be able to keep up with losses. Timed feedings can get maximum benefit from a higher carbohydrate meal and also avoid aggravating insulin resistance. Feed 1 to 1.5 lbs of beet pulp (dry weight) with 1 to 1.5 pounds of plain oats within the first hour after work is finished.
There is a window after exercise where muscle takes up glucose very readily. Even insulin resistant horses can receive extra carbohydrate in that time frame. The oats are easily digested to glucose to begin replacing glycogen. The beet pulp is fermented to acetate which is more slowly released and can be used instead of glucose for energy functions, freeing up glucose for glycogen. Studies have confirmed acetate supports the glycogen replacement process. As a plus, you can get water and electrolytes into the horse at the same time – which research has shown is also important for replenishing glycogen.
The above three issues account for the vast majority of horses with energy issues in the summer. If you are having problems, consider them first.
Eleanor Kellon, VMD