# Determining Dietary Energy

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Given the opportunity and good health, horses will choose to consume enough feed to meet their energy needs as a rule.

Photo: Thinkstock

Not all the energy contained in a feed is accessible to the horse. A significant portion of it is lost in the conversion process of digestion. The digestible energy (DE) is the value most often used to describe the usable portion of the total energy, or gross energy. It consists of the portion of energy not lost in the feces. However, like many things in the nutritional world, it isn’t perfect: The DE value doesn’t take into account energy lost in urine (and to a lesser extent, in gastrointestinal gases such as methane), nor energy lost as heat in the actual digestion and absorption of the food. Nonetheless, DE values for foods are far easier to come by than values that do take these minor factors into account (which are far more difficult to calculate), so DE is the unit in common usage. Just keep in mind that when you see a DE value, it’s likely to be a little generous.

Another way of calculating the energy content of feeds is by the familiar Calorie, the amount of heat generated by oxidation (burning) to raise the temperature of a kilogram of water by one degree Celsius. (The capital "C" calorie is actually shorthand for a kilocalorie; the original small "c" calorie unit is the amount of heat required to raise the temperature of one gram of water one degree. It’s too small a unit to be of practical use when discussing nutrition.) When dealing with horses and other large animals, nutritionists usually switch to the megacalorie (Mcal), which is 1,000 kilocalories. It saves writing a lot of zeros.

A third unit in common use is TDN, or total digestible nutrients, a measure of digestible energy expressed in either weight or percentages. This measure is the sum of a feed’s digestible carbohydrates, its digestible protein, and its digestible fats multiplied by 2.25 (because fats provide about 2.25 times more energy than carbohydrates or proteins).

One kilogram (kg) TDN is approximately equal to 4.4 Mcal. (If you use TDN as the basis of your ration formulating, make sure you have a TDN value for horses, not ruminants, such as cattle. Ruminants are much more efficient digestion-wise than horses, so calculations for energy available from forages are generally 5% to 15% higher. If you formulate a ration for a horse based on ruminant TDN, you will likely be providing too little feed in the long run.)

A couple of formulas can help you calculate how much digestible energy your horse requires for his daily maintenance needs (without weight change).

For the average horse weighing less than 600 kg (1,320 lbs), use this formula:

Mcal DE/day = 1.4 + 0.03 x (kg body weight)

So for example, if your Standardbred mare weighs 450 kg (that’s 990 pounds), she would require 1.4 + (0.03 x 450) Mcal, which equals 14.9 Mcal of digestible energy per day for her maintenance metabolism.

If your horses weigh more than 600 kg, they will have lower energy needs per kilogram than smaller animals. So they have a slightly altered formula:

Mcal DE/day = 1.82 + (0.0383 x kg body weight) – [0.000015 x (kg body weight)²]

Using this formula, a 750 kg Belgian gelding, for example, would require 1.82 + 28.73 – 8.44 = 22.11 Mcal/day.

You also can do rough calculations for how much additional energy your horse will need for various kinds of work. For ponies and light horses, the Mcal DE/day for light, medium, and intense work has been estimated at (respectively) 1.25, 1.5, and 2.0 times the amount needed for maintenance.

What constitutes light, medium, or intense work? It depends, of course, on a number of factors, but generally speaking, light work includes such activities as Western or English pleasure, trail riding, quiet pleasure driving, and acting as a beginner level lesson horse. Medium work encompasses functions like ranch work, roping, cutting, jumping, barrel racing, and dressage. Intense work includes race training, polo, endurance riding, and upper-level three-day eventing.

Given the opportunity and good health, horses will choose to consume enough feed to meet their energy needs as a rule. Four things can contribute to a horse’s not getting enough energy:

1. A sufficient quantity of food is not available;
2. His gastrointestinal tract will not hold enough of the available feed because the DE density of the feed is too low (as with poor-quality hay, for example);
3. He can’t consume enough because of a physical problem (such as an injury or dental problem); or
4. He doesn’t want to consume the feed because illness, stress, unpalatable feed, or inadequate water intake has left him with no appetite.

Regardless of the reason, the first sign of inadequate energy intake is a depressed attitude. Eventually, hormonal changes will decrease the body’s energy utilization, shutting down growth in youngsters or milk production in broodmares, and reducing physical activity.

These changes also will call on the system to draw on stored fats and carbohydrates, resulting in weight loss. The horse’s stores of carbohydrates are depleted within the first few days of total food deprivation, and within a week the body adapts, drawing on body fat and conserving the body protein.

But if starvation continues, the horse will have no choice but to turn to his structural protein for energy once the fat stores are depleted. First, proteins in the blood, intestines, and muscle are drawn on, followed by those lending structural support to bones, ligaments, tendons, and cartilage. By the time muscle wasting or weakness is evident, feed-deprivation-induced changes in other body functions are already well under way. The good news is that carefully providing adequate calories usually can reverse the damage over time.

Far more common with domestic horses at least, is an energy excess. Horses that routinely receive too much feed will develop increased fat stores for a start.

Some of the excess energy also will be given off as heat (a mechanism used by many animals, including humans); but the horse is unique in that he also compensates for excess energy intake by increasing his physical activity. The result is familiar to many of us: a snorting, shying, bucking explosion looking for a place to happen!

In the young horse, excess energy also contributes to rapid growth, which can sometimes increase the risk of developmental orthopedic (bone and joint) problems. Reducing the amount of feed, especially grains, in the diet and providing more outlets for exercise usually will take care of this problem.