Q: I’ve read articles that talk about the horse’s energy requirement in terms of Mcals of digestible energy. What is digestible energy and what is Mcal?
A: In the United States, we measure daily energy requirements for both humans and animals in terms of calories. One calorie is actually a heat measure and is the energy it takes to heat 1 gram of water 1 degree Celsius. Many areas of the world use joules as their measure of energy and 1 calorie is equal to 4.1868 joules.
It’s important to note whether we’re talking calorie with a lowercase c or Calorie with an upper case C—the the latter is actually the measure of energy needed to raise the temperature of 1 kilogram of water 1 degree Celcius. Therefore 1 Calorie is actually 1,000 calories, often referred to as one kcal where k stands for 1,000.
The daily calorie we’re used to seeing on United States nutrition panels is the Calorie even though it’s often written incorrectly with a lowercase c. Horse’s requirements are greater still and are measured in Megacalories (or Mcals), which would be the same as 1,000 kcals.
Energy in the Equine Diet
The food your horse eats contains a gross amount of energy. This is the energy that would be released if you were to combust that food in a special chamber (called bomb calorimeter) designed to measure the heat released during combustion. However, the processes of digestion and metabolism are not 100% efficient, and some of that potential energy is lost.
Some energy is never captured and, rather, is lost in feces. Subtracting the energy lost in feces from the gross energy that originally existed in the feed being fed gives the digestible energy (DE). The daily energy requirements recommended by the National Research Council (NRC) is digestible energy, which is an estimate of what energy your horse is going to absorb.
In reality, not all digestible energy is available to your horse either. Energy will be lost as urinary energy and gaseous energy leaving what’s known as metabolizable energy … and even then some metabolizable energy will be lost in heat. What’s truly available to your horse is net energy, which is what’s left after all these subtractions have been made from the gross energy consumed.
Why, then, are the daily requirements given as digestible energy if this isn’t a true measure of what the horse can use? A big part of that is the amount of research that would need to be conducted to determine the net energy content of all the feed ingredients fed to horses. It’s not so difficult to perform a feeding trial—feeding a set amount of a feed of known gross energy content and then collecting feces and determine how much energy horses lost in feces to estimate digestible energy. However, it’s much more involved to determine the energy lost with urine, gas, and heat production.
Other countries have worked to create different systems in an attempt to be more accurate, such as the French horse feed unit (UFC) system, which is a net energy system where the net energy content of a feed is compared to that of barley and given a UFC rating. This system has not been adopted in the United States in part because the data available is limited compared to what we have for the digestible energy system. It would be a massive undertaking to get all U.S. stakeholders to switch; also, because we don’t feed a great deal of barley to horses in the United States, using barley as a reference is not particularly logical.
The NRC calculates the horse’s daily digestible energy requirement in Megacalories as a function of body weight (0.0333 x body weight in kilograms for average maintenance), with multipliers added to account for workload. For example, the above calculation is multiplied by 1.4 to estimate calories needed for a horse in moderate work.
The Mcal’s provided by common feeds are ideally determined through feeding trials; however, in reality, it is more commonly determined through calculation. There are several calculations that can be used depending on whether the feed is a roughage or energy feed. Most use the acid detergent fiber (a measure of plant digestibility) content of the feed in part of the calculation, but this difference in potential calculation methods is one reason you’ll never find feed energy contents on feed bags—it’s very hard to guarantee the energy content of feed due to the variability in the calculations used and the potential sources of error that can enter the equation. Even though Mcal content of feeds is not freely available on feed bags, feed companies should be able to provide this information if you contact them.
While in many cases recommended energy-requirements are our best estimate of what a horse needs—and energy contents of feeds are estimates—both are important parts of a good nutrition program. This is especially true when dealing with horses that are either under- or over-weight. Determining requirements and comparing these to the energy content of the diet could shed light on where the problem lies.