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.
Accurately determining a feed’s digestible energy content requires a feeding trial. Digestible energy content of feeds can vary widely with chemical composition. While two feeds might have similar carbohydrate contents, and therefore similar gross energy, they could vary widely in digestible energy if one is mostly starch (which is easily digested) while the other is structural carbohydrate such as cellulose (which isn’t completely digestible).
Within the world of nutrition and feeding trials the above digestible energy is what we call “apparent digestible energy.” This is because we make the assumption that all the calories recovered in feces came from feed, when in reality some will be the result of digestive secretions, intestinal cell wall lining, etc. True digestible energy takes these into account but adds an even greater layer of complexity. Therefore, in the real world, we work with apparent digestible energy and call it good.
It’s also worth noting that there are systems that go to even greater levels of accuracy. This is because energy within the feed isn’t only lost through feces; some will be lost as gaseous energy leaving you with an estimate of metabolizable energy, with still more lost in urine, leaving a net energy. If you truly want to know how much energy in a feed is available for production purposes, this is the level of detail you would need to reach. It’s fairly easy to see that to get to this level of detail requires a substantial amount of research work, and while this work is done in other livestock species, such as pigs and chickens, there is very little work in horses. So, instead North America relies on digestible energy when considering horses’ nutritional needs and outlines requirements as such.
How do feed manufacturers determine the digestible energy content of their feed? There are a couple of options. They can rely on data from feeding trials, or they can do chemical analysis of the feed and calculate the DE based on the chemical composition. Most will be doing the latter.
Feeding trials: In rare cases, companies might perform feeding trials to establish DE; however, this is more likely to be done when trying to determine the DE of a new feed ingredient to assess how it might work within a feed or ration and whether it’s a suitable horse-feed ingredient. It’s the most accurate way of determining the DE of a feed.
However, since the nutritional composition of each batch of ingredients going in to a feed will vary slightly in its chemical composition, the data collected during a trial will never be exactly repeatable. Plus, digestibility is impacted by a number of factors, such as others items in the diet and individual animal variability.
Many individual feed ingredients, such as oats, beet pulp, and alfalfa, have had their digestible energy contents determined, and these values are available in various tables, such as those in the back of the National Research Council’s (NRC) Nutrient Requirements of Horses (2007). The problem with these tables, though, is that researchers might not have collected DE data during equine feeding trials, and the data might instead be from feeding trials using species such as beef cattle or pigs. This presents problems, because each animal species has different digestive anatomy and areas of strength within its digestive tract. For example, cattle have a greater ability to digest alfalfa hay than pigs, so the DE for pigs is lower than that for cattle.
When the work is not done in horses we are left using data collected from other species and making assumptions that they are similar. In reality they’ll be different, maybe only slightly different, but different none the less.
Chemical analysis: Because of the issues discussed above, the NRC adopted a way of calculating the DE in feeds mathematically based on their chemical composition. There’s a calculation for forages and another for energy and high-protein feeds. The calculations are based on a summary of data collected from 108 digestion trials performed in horses and require knowledge of the feeds crude protein and acid detergent fiber (a carbohydrate fraction). These are then multiplied by a coefficient.
Since these equations’ development in the early 1980s, a number of researchers have contested their accuracy especially when using them to determine the DE of feeds with high fat, such as rice bran, or fermentable fiber, such as beet pulp. Indeed, when testing rice bran’s DE in a feeding trials and comparing it to that calculated using different equations the feeding trial gave a DE of 3.17 Mcal/kg and the calculations 2.62 and 2.71 Mcal/kg, a fairly large underestimate.
Where Does This Leave the Horse Owner?
So here lies the major reason why energy content of feeds is not guaranteed on feed labels: There’s no industry standard for determining DE. For values such as crude protein or crude fat and fiber, which are also calculated, there are consensus standards of chemical analysis and calculation that the Association of Analytical Communities (AOAC) as approved. The AOAC is a globally recognized independent third-party organization that develops internationally accepted standards. For a feed company to place a value on a feeds guaranteed analysis, it must be repeatable otherwise it is not a guarantee. If a regulatory agency were to test a feed to determine whether a guarantee for DE is true they could get a value different than the feed manufacturer, depending on which calculation they used.
Where does this leave you the consumer? Your feed company should be able to provide you with a value for a feed’s DE if you ask for it. As a general guideline you can make some informed guesses based on what is on the feed label. Look at the feed ingredients. Most feed ingredients, even hay, have a DE of at least 2.2 Mcal per kilogram on a dry matter basis—grass hays are closer to 2.2 and legumes such as alfalfa are a little higher. Grains are higher with oats at around 3.3 Mcal/kg; barley and corn are higher still. Beet pulp is between forages and grains at about 2.8 Mcal/lb, and wheat middling are closer to grains at around 3.4 Mcal/kg. Fats are much higher: Vegetable oil has a DE of 9.19 Mcal/kg but rice bran has an energy level closer to grain at about 3.35 Mcal/kg. Based on what you see on the ingredient list you can make a rough guess.
Another area to check is the level of crude fat and fiber. It used to be that as crude fiber decreased, a feed’s energy content increased; this is because lower crude fiber concentrations meant the feed contained more high starch grain. However with the shift towards fermentable fiber as an energy source, this isn’t always a safe assumption. Compare the ingredient list against the fiber content to see where that fiber is coming from.
If fermentable fibers such as beet pulp or soy bean hulls are early on the ingredient list the feed will likely be higher calorie than if the fiber sources are hays. Also look at the crude fat. Most feed ingredients have a relatively low natural fat content of about 3-4%. Feeds with a crude fat greater than this likely have an added fat source. Once you get to 8% fat or higher, you have a feed that has a considerably higher calorie content.
While this method is certainly less than perfect, you can use it to make informed choices about feeds, especially if you back it up with a call to the feed company. One day we might have an approved method and see more energy values placed on feed labels the way they are now on cat and dog food. However do not expect this to happen any time soon. Getting them on pet food labels took years of discussion and negotiation between regulatory officials and the feed companies.