Don’t panic. That’s probably the most helpful thing I can say about calculating the balance of nutrients in your horse’s diet. Most people are perfectly comfortable speaking about   calcium or vitamin E levels in the abstract sense, but when it comes to sitting down with a calculator and actually figuring out how much of each nutrient your horse’s current ration delivers, you freeze in terror. Books full of hideously complicated mathematical equations, using Pearson’s squares and the like, don’t help the widely held impression that doing this sort of math is impossibly convoluted.

Ration balancing

Fear not, gentle reader. It’s true that you can make an infinite number of intricate calculations for the optimum concentrations of each and every micro-nutrient required or supplied by your horse’s diet. But, you also can zero in on what’s really important, eliminate from worry those details that are not, and simplify the whole process, all without a graduate degree.

Much of the time, your horse will tell you if he’s receiving good nutrition–by his shiny coat, good appetite and weight maintenance, pleasant attitude, and appropriate energy level. If you are feeding average-to-excellent quality forage and grain, you can be reasonably assured that your horse’s diet adequately meets his daily requirements. This is nearly always the case when you feed a commercial ration without supplements; and only during growth (especially in the first year), lactation, and the last three months of pregnancy are horses likely to need extra nutritional support. Supplementing might be necessary if the forage you’re feeding is of poor quality (assuming, of course, that you’re unable to replace it with something better).

It’s worth noting, however, that contrary to popular belief, horses do not exercise “nutritional wisdom” when it comes to their diets. Some feed and supplement manufacturer’s advertising departments would have you believe that horses instinctively choose the plants and nutrients they need for good health. Alas, that’s not the case–if it were true, we would never have a problem with horses gorging themselves with grain until they colic. With the exception of salt and water, horses do not develop cravings for the nutrients they require; they simply function according to appetite and taste preference, just as humans do.

A horse’s daily requirements include about 40 different nutrients–the proper amounts of energy and protein, 15 minerals, plus chlorine, 14 vitamins, plus beta carotene, fat, water, and at least four amino acids (lysine, methionine, tryptophane, and threonine). Fortunately, most of these are known to be provided in adequate amounts by any common equine diet. Or, at least, there are no documented cases of clinical deficiencies or toxicities. (We know that horses have a need for trace amounts of sulfur in their diets, for example, but there has never been a documented case of sulfur deficiency, so we safely assume that all equine diets deliver enough sulfur for their needs.)

As a result, we can zero in on a few key nutritional elements, for which deficiencies or excesses might occur–protein, digestible energy (DE), calcium, phosphorus, and selenium.

For growing horses, we’ll add zinc and copper to that list (for adult horses, the levels of these two minerals generally are adequate in any diet). Levels of vitamins A and E also might be a concern for growing horses and those in high-performance situations, especially in the winter months when they have no access to growing forage. It’s only these nutrients that really need to be looked at carefully when you are formulating a ration.

Step By Step

Before you prepare to number-crunch, you need to answer a few basic questions about the horse you’re feeding. These will have an important bearing on the concentrations of nutrients they require. For example:

1. What stage of life is your horse in? Are you feeding a growing foal, a lactating broodmare, an adult pleasure horse, or a 100-mile endurance champion?

2. Is your horse idle, or doing work of light, medium, or serious intensity? (Light work generally is defined as a few hours a week at a slow pace–a weekend pleasure horse or children’s hunter, for example. Medium work is perhaps up to an hour a day of trotting and cantering, such as might be required of a dressage horse or working cow horse. Intense work usually is considered such strenuous tasks as playing polo, racing, or upper-level three day eventing, where the horse is exercising at a high-performance level for more than an hour a day.) Is his workload due to be increased, decreased, or remain constant?

3. Is your horse in the condition you’d like him to be in? Is he too fat or too thin?

4. What feeds are available to you? Which are reasonably priced? (There’s no point in formulating a ration based on ingredients you can’t buy, or which are outrageously expensive.)

Next, you need to know your horse’s body weight. The most accurate way to do this is to stand him on a scale specifically designed for horses, but you’ll likely only have access to this equipment if you are lucky enough to live near a veterinary school.

A rough approximation of your horse’s weight can be obtained by using the “heart-girth” measuring tapes sold at many feed stores for a couple of dollars, but bear in mind these devices are not terribly accurate (they’re sometimes off by more than a hundred pounds). Incidentally, heart-girth tapes are of no use for pregnant mares; their heartgirths alone will not tell the story.

A more accurate result can be obtained by using the following simple formula:

Bodyweight in pounds = [(heartgirth in inches) squared times length in inches] divided by 330 where the length of your horse is measured from the front point of his shoulder blade to the point of his rump.

If you prefer to do metric calculations, use the following:

Bodyweight in kg = [(heartgirth in cm) squared times length in cm] divided by 11,880.

For light-horse foals one to six weeks of age, use the following:

Bodyweight in pounds = [heartgirth in inches – 25.1] divided by 0.07


Bodyweight in kg = [heartgirth in cm – 63.7] divided by 0.38.

It’s worth doing these calculations, because once you have your horse’s weight, there’s a simple rule of thumb you can use to determine how much feed your horse should be getting each day. The total weight of feed per day should be between 1.5% and 3.0% of your horse’s body weight.

Simple Principles, Applied

Let’s use an example here. If, for instance, you owned a Standardbred mare who weighs 950 pounds, our rule would suggest that she would need between 14.25 and 28.5 pounds of total feed every day. That’s a good bit of leeway, of course, which allows you some adjustability. For example, if your mare is overweight and only in light work, you would lean toward the lower end of the scale, at 14.25 pounds a day. If, on the other hand, she was in top physical condition and was competing in a high-intensity sport like harness racing or upper-level combined driving, she might require closer to 28 pounds of feed a day to provide her with the energy she needs.

The 1.5% to 3.0% rule works for almost all types of horses, except for nursing foals (which only will eat between 0.5% to 0.75% of their body weight in solid food while they are nursing) and weanlings, which might consume up to 3.5% of their body weight per day. Intense work, lactation (nursing), and growth all need to be fueled by larger amounts of nutrients. To some extent, the intake also will be affected by temperament (laid-back, easy keepers will be at the lower end of the scale, while nervous or high-strung horses which are hard keepers will need more), and climate (because it takes more energy to maintain internal body temperature in below-freezing weather).

In order to use the rule effectively, you will have to weigh your horse’s feed. Most horse people, of course, are used to feeding by volume (this horse gets half a scoop, this one a whole scoop, that pony a handful). Weighing the ration need not be a daily routine–once you’ve done it a few times, you should be able to estimate by eye fairly effectively. But it’s important to weigh both the hay and the grain your horse receives at least a few times; otherwise your “guesstimates” might not be very accurate.

Here’s why: hay bales can vary in weight from under 40 pounds to more than 100 pounds, depending on their size, balance of legumes and grasses, and moisture content. They also vary in regard to the number of flakes they contain. And some grains are far heavier and more energy-dense than others. For example, a one-quart scoop of corn will have considerably more energy than a one-quart scoop of oats. (So if you feed that quart to a horse which is used to oats, you might have one wired-for-sound horse on your hands!) Any time you substitute one type of grain for another, you should repeat the weighing process to make sure that you’re delivering an equivalent weight, not an equivalent volume.

The simplest way to determine the weight of a hay bale is to bring your bathroom scale out to the barn. Stand on it empty-handed to find your own weight, then repeat the process while hoisting a bale of hay. Subtract your weight from the weight of you plus the bale to get the hay’s weight. Then crack open a couple of bales and weigh yourself holding some representative flakes. Find an average of the weights, and you’ll have a much better idea how much hay you’re really feeding your horse when you toss him three flakes in the evening.

Weighing grain is most easily done with an ordinary kitchen scale. (Be sure to subtract the weight of the empty container you use to scoop your grain.)

Under almost all circumstances, a horse’s total diet should be at least 50% forage by weight. (The only exceptions are weanlings and yearlings, for whom grain might make up 70% and 60%, respectively, of their total diet as a maximum; and two-year-olds in intense race training, which might receive up to 65% grain, if necessary.) So of the 1.5% to 3.0% total feed an adult horse consumes daily, he should receive a minimum of 1% of his body weight in pasture, hay, or other fiber sources. He might, of course, receive up to 3.0% pasture or hay, if he is an easy keeper and/or idle or in light work.

Going back to our 950-pound Standardbred mare, if we know she is used for pleasure riding only, she could survive quite nicely on 14.25 pounds of forage per day. If, on the other hand, she is very fit and is actively racing, she might need a diet consisting of 14.25 pounds of hay, and another 14.25 pounds of grain daily to fuel her performance (for a total of 28.5 lbs. of feed, or 3% of her body weight). When you do those figures, remember there’s some room for adjustment–and that you also can factor in a 10% to 15% wastage factor for hay.

If you have good-quality pasture and/or hay and are feeding a well-formulated commercial grain mix, you might never need to do more ration balancing than the above. (To make sure your horse is getting enough protein, calcium, and phosphorus, use the chart in The Horse Interactive at and compare the requirements to the total amount in the diet you’re providing. The chart also contains the amount of digestible energy found in select feeds.)

For example, if you are feeding your breeding stallion a legume hay with 15% crude protein, and it makes up 50% of his ration, choose a low-protein grain (under 10%) to balance that high-protein hay. If, on the other hand, you are feeding a grass hay with only 8% crude protein, you can afford to offer a higher-protein grain ration.

How do you know what nutrients are contained in your hay? Getting a hay analysis done is the best way. Your local agricultural extension specialist or feed store likely offers this service, for a cost of approximately $20 to $40. If, however, you’re getting hay from a number of sources, a hay analysis for each batch might be prohibitive. In that case, rely on the standard values listed in such publications as the National Research Council’s 1989 “Bible” of equine nutrition. (Be sure to note whether the hay is early-, mid-, or late-bloom, determined by the percentage of mature seedheads in a representative sample. Early-cut hay is generally more nutritious than late-cut.)

One snag I should mention at this point–if your horse is on good pasture, and is receiving no other fiber source in his diet, you might have to do a lot of estimating, since it will be virtually impossible to determine his actual daily intake of grass. But an analysis of your growing pasture grasses still will allow you to analyze the percentage of protein and macro-minerals (primarily calcium and phosphorus) he’s receiving, so you can at least get a ballpark idea of his intake.

What about your grain? If you’re feeding a commercial sweetfeed, pellet, or extruded ration, the feed tag on the bag will supply most, or all, of the information you need. You can contact the manufacturer for a full nutritional rundown if what’s printed on the tag is an abbreviated version. If you’re feeding individual grains, such as oats, you’ll need to rely on the NRC (National Research Council) charts for standard nutrient values. (Fortunately, grains don’t vary tremendously from year to year or location to location, so these values are relatively accurate.) In a pinch, you can ask your local lab to do a nutritional analysis on the grain you’re feeding.

A Little Number Crunching

Some simple calculations will help you determine whether your horse’s diet is sufficient on the basic scores of crude protein, calcium, and phosphorus. For example, if your horse’s ration currently is 70% grass hay, with a calcium level of 0.4%, and 30% sweetfeed, with a calcium level of 0.75%, you would multiply 0.4 by 0.7, and 0.75 by 0.3, and add the two results together, for a total calcium concentration of 0.505%. (If your horse’s diet is 50% forage and 50% grain, your job is easier–you can just split the difference.)

Remember that with calcium and phosphorus (both instrumental in correct growth and repair of bones, muscles, cartilage, and tendons, among other functions) the ratio between the two is as important as the levels of each individually. There always should be at least as much calcium in the diet as phosphorus (in other words, a 1:1 ratio), and ideally, the calcium percentage should be a little higher than the phosphorus (1.2:1 to 1.6:1 for adult horses). An inverted ratio, with more phosphorus than calcium, usually is an indication of a diet that is too heavy on grain and too light on forage, and might put your horse at risk for brittle bones and nutrition-related disease. The minimum recommended percentage of phosphorus in the diet of adult equines is 0.20%, and the minimum recommended amount of calcium is 0.25%.

As for protein, the amount your horse needs will depend on his stage of life. Adult horses need very little–only about 8% to 10% crude protein overall, an amount which is supplied easily by almost any half-decent diet (as a result, protein deficiencies in mature horses are almost unheard of). Young, growing stock and horses used for breeding will need considerably more crude protein because greater demands are being placed on their systems.

One other ingredient we mentioned earlier was selenium, a micro-mineral that horses need only in very small quantities. It wouldn’t be significant at all except that selenium pairs up with vitamin E to play an important role in immune function, and selenium content in the soil (and thus in plants) varies widely from place to place in North America.

Because selenium has a very low toxicity level in horses compared to other minerals, it’s important to keep tabs on how much is supplied by your horse’s diet. That might require a pasture or soil analysis if your local agricultural extension office doesn’t know what the selenium concentration is in your area. If you live in a selenium-deficient area, you’ll likely have to supplement this mineral (don’t overdo it!), whereas if there’s sufficient selenium in the soils in your area, you should avoid supplementing.

What About Energy?

In addition to crude protein, calcium, phosphorus, and selenium, you’ll probably want to determine whether your horse’s diet supplies enough energy for the work you ask him to do. For horses weighing less than 600 kg (1,320 lbs), you can calculate their digestible energy (DE) requirement at a maintenance level (doing no work) using this formula:

DE/day (in Megacalories) = 1.4 + 0.03 times (kg body weight)


DE/day (Mcal) = 1.4 + 0.03 (body weight in pounds divided by 2.2)

Horses weighing more than 600 kg (generally draft horses or draft-crosses) have lower energy needs, kilo for kilo, than their lighter brethren, so the formula gets slightly modified to the following:

DE/day (Mcal) = 1.82 + (0.0383 times kg body weight) – [0.000015 times kg bodyweight squared ]

The amount of energy your horse needs is directly related to the amount of work he does. For ponies and light horses, the DE requirement per day for light, medium, and intense work has been estimated at 1.25, 1.5, and 2.0 times the amount needed for maintenance.

For example, a 500 kg horse which is being used in a riding school six days a week could be considered to be in medium work, and would have the following energy needs:

1.4 + (0.03 times 500) = 16.4 Mcal/day if he were idle

16.4 times 1.50 (factor for his workload) = 24.6 Mcal/day.

Keep in mind that the DE content is one value usually not printed on a commercial grain ration’s feed tag, so you might have to ask the manufacturer, or try to generate an estimate yourself based on the ingredient list. As a rule, of course, carbohydrate-rich grains always will be higher in digestible energy than forages, although early-cut alfalfa can come close.

That being said, in many cases it’s not really necessary to calculate the amount of megacalories your horse’s diet supplies. Your horse will tell you if his energy supply is sufficient, and given the opportunity, he’ll always consume enough feed to supply his needs (unless he’s prevented from doing so by poor teeth, illness, or poor quality hay or grain). He’ll also tell you if he’s getting far too much energy–by bouncing off the walls! If that’s the case, try cutting back on the amount of grain he’s receiving, and compensating with a larger percentage of low-energy forage.

A Few Other Tips

That’s really all most people need to know in order to balance their horse’s diets. Focus on the major concerns, don’t sweat the small stuff, and most importantly, let your horse tell you whether he’s nutritionally well-covered by his appearance and attitude. Here are a few other parting tips to help you in your mathematical quest.

  • If you’re planning to calculate the concentrations of other nutrients in your horse’s diet, such as vitamin A or E, pay close attention to the units in which the ingredient is expressed; they might not be the same on your grain feed tag and your hay analysis. Most vitamins are described in terms of IU, which stands for International Units, while many minerals are expressed as ppm, short for parts per million. Mg/kg is another unit you might see; it works out to exactly the same thing as ppm. In the case of any unit that isn’t a percentage, you’ll have to take into account how many kilograms (or pounds) of that feed your horse is eating every day, and multiply appropriately for a total-per-day.

  • When using NRC charts, be careful to note whether the nutrient values are expressed in terms of “dry matter” (what the concentrations of those nutrients would be if there were no moisture in the feed). Dry matter numbers are more concentrated than the “real world” versions, and you’ll have to multiply by the appropriate percentage to get a picture of what’s actually going on in your horse’s feed bowl. For example, most hays are between 85% and 95% dry matter.

  • Don’t forget the most important nutrient of all–water. Without free access to clean water, your horse can’t absorb any nutrients or have any of his bodily functions operate normally.

  • If your horse’s diet comes up short on a nutrient, there are a number of routes you can take to compensate, from changing the hay or grain you feed, to juggling the amount of each you offer, to choosing an appropriate vitamin or mineral supplement. But don’t supplement gratuitously, on the assumption that “if some is good, more is better.” That’s not always the case–nutrient toxicities can be just as damaging as deficiencies.

  • If you’re really going cross-eyed over all this, consider investing in some computer software that can do all the calculations for you, and even generate impressive three-dimensional charts or pie graphs. The Kentucky Equine Research feed company has one on its web site at