Photo: Anne M. Eberhardt/The Horse

A: In the United States we express the energy content of both human foods and animal feeds in calories. Specifically, we measure the energy content of equine feeds in kilocalories (kcal) or megacalories (Mcal). A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius. The energy within feeds varies with their chemical composition. The energy contained within fats is about 2.25 times greater than in an equal weight of protein or carbohydrate. So the amount of each of these in the feed impacts the energy content.

This energy within a feed is that feed’s gross energy or intake energy. It’s the feed’s total potential energy and is obtained through the use of a piece of equipment called a bomb calorimeter, which is basically a chamber the item of interest is placed into and combusted, and the heat released measured.

This is all pretty straight forward, and so you might be wondering why it is then that calorie contents on horse feeds is so hard to come by. The problem is that not all energy contained within a feed is available for use by the animal. Digestion is not 100% efficient, and so some material passes out of the digestive tract undigested in feces. That leaves us with digestible energy (DE), the energy remaining after the energy in feces is subtracted from the gross energy or intake energy of the ration