Bulking Up: Does Your Horse Need to Gain Weight, Muscle, or Both?

If you can see your horse’s ribs or his topline is lacking, he might need to gain weight or muscle. But how can you tell which he needs?

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Bulking Up: Does Your Horse Need to Gain Weight, Muscle, or Both?
You can't put weight or muscle on your horse without adequate calories. | Photo: iStock

Some of the biggest indicators of a horse’s health are his body condition, weight, and muscle development. So if you’re starting to see your horse’s ribs, or if he has a less-than-­desirable topline—the muscles that support the spine, neck, and ­hindquarters—then he might need to build weight or muscle. But what is the difference between adding weight and muscle? How do you know which your horse needs, or is he deficient in both? And, then, how can dietary changes help your horse bulk up?

Dietary Energy 101

It’s important to understand dietary energy and how it relates to a horse’s food sources. Basically, energy equals calories, which are measured in kilocalories. In horses, which need thousands of kilocalories per day, energy requirements are expressed as megacalories (Mcal).

There are various types of energy—gross energy (GE), digestible energy (DE), metabolizable energy (ME), and net energy (NE)—but, for the purposes of this article, we’ll focus on DE. This is the energy listed on feed labels and that nutritionists reference most. It’s the energy that can be digested from a feed after adjusting for the energy lost in fecal output. Because calculations within the equine nutrition industry vary, DE values of feedstuffs are considered estimates.

For energy, horses consume fat, carbohydrates, and protein. Fats are the most calorically dense feed, at 9.4 kcal/gram of GE (the heat produced when a feed is completely oxidized, or burned). Carbs offer 4.15 kcal/gram, and proteins 5.65 kcals/gram.

Carbohydrates (fiber, starches, and sugars) are the main components of forages. Horses require them for digestive health, to help buffer stomach acid, and as a good energy source.

Protein is the least-efficient energy source, says Russell Mueller, MS, PAS, a member of the Equine Research and Innovation Team at Cargill Animal Nutrition, in Minneapolis, Minnesota. Horses can expend more energy digesting protein than they gain from it, he says.

Now let’s think about how a veterinarian or nutritionist might recommend you add weight, muscle, or both to a horse. Note that if a horse is calorie-deficient, he will also be muscle-depleted. You cannot build muscle without adequate calories, says Clair Thunes, PhD, an independent equine nutritionist and owner of Summit Equine Nutrition, in Gilbert, Arizona.

Evaluating the Horse

The first thing to do is evaluate the horse to see where he falls on the Henneke body condition score (BCS) chart, says ­Mueller.

Ideally, the horse should score between a 4 and 6, meaning you can feel but not see his ribs. His withers, neck, and shoulders should be rounded, and the withers should have a layer of fat over them. The horse has a crease down the back, but it is not pronounced. The backbone, tailhead, and hip bones also have some fat cover.

If your horse is ribby (and a veterinarian has ruled out underlying health issues causing weight loss), he needs more calories in his diet, says Mueller. If the horse is angular over his topline (withers, back, loin, top of the hip, and croup region) and/or is sunken in around the neck, he needs to build muscle. Developing the topline is important because it plays a vital role in how a horse performs and handles when ridden. Adjusting the protein and amino acids in the diet can help.

Adding Calories for More Fat Cover

Each horse requires a minimum DE per day for maintenance (to stay the same weight). Different activity levels increase daily DE requirements. You can find these values in the National Research Council’s (NRC) 2007 Nutrient Requirements of Horses, although many horse owners simply consult their veterinarians or nutritionists about their horses’ nutritional needs. These experts can evaluate what nutrients each horse is taking in with his base diet to see if that diet is ­appropriate or needs to be changed.

For a horse to gain weight, it takes an increase of about 20 Mcals of DE above maintenance to gain 1 kilogram (2.2 pounds). However, this varies depending on grain composition and energy sources. To move up one score on the Henneke scale, says Thunes, a horse needs to gain 16-20 kilograms (35-44 pounds), but this varies depending on the horse’s weight. Assuming your horse needs 20 kilograms to move up the scale, he must consume a total of approximately 400 Mcals above maintenance needs.

Thunes presents two possible scenarios for adding more calories:

  • You could feed the horse an additional 5 pounds of grass hay, which will provide about 4-4.5 Mcals per day. Therefore, it would take around 100 days for the horse to move up one BCS.
  • You could feed the horse the minimum serving of a higher calorie feed, such as a senior or performance feed (about 6 pounds is a common minimum daily serving, says Thunes). Then it might only take 45 days to move up one BCS as the horse consumes around an additional 9 Mcals per day (this could differ based on individual feed formulation).

The NRC has created a table (see below) showing how much time it takes for a horse to go from 4 to 5 on the Henneke scale, based on how much additional DE he’s consuming above maintenance.

Estimated increase in digestible energy (DE) intake necessary
to change the condition score of a 500-kg (1,100-lb) horse from 4 to 5

Time Period to Accomplish Gain DE Above Maintenance (Mcal/d) Percent Increase in DE Above Maintenance
60 days 5.3-6.7 32-41%
90 days 3.6-4.4 22-27%
120 days 2.7-3.3 16-21%
150 days 2.1-2.7 13-16%
180 days 1.8-2.2 11-14%

Assumptions: 1 unit of change of condition score requires 16-20 kg of gain, and 1 kg gain requires 20 Mcal DE above maintenance.

Bulking Up: Does Your Horse Need to Gain Weight, Muscle, or Both?
When changing a horse's diet, Thunes takes a forage-first approach, which is the safest way to put on weight. | Photo: iStock

Forage Comes First

When changing a horse’s diet, Thunes takes a forage-first approach, which is the safest way to put on weight. Because ­alfalfa provides more calories per pound than grass hay, she says owners can switch up to 25% of their hay to alfalfa.

“If this does not do the trick, feeds using fiber sources like beet pulp and soybean hulls are a good option, as these are still fiber but higher in calories,” she says.

Mueller has found that owners are more willing to change their feed or their supplements than their forage, simply because of availability and/or growing conditions. “A more cost-effective route may be to spend a little more money on your hay versus spending a lot more money on feed, if you have another hay type or quality available to you,” he adds.

Mueller says he might suggest owners in regions where bermuda hay is ­prevalent switch to a cool-season grass hay (e.g., orchardgrass, bromegrass), which will provide more calories per pound.

Add Fat and Carbohydrates Next

If you can’t achieve weight gain with pasture or hay and other fiber sources alone, Thunes recommends adding something more calorically dense to the diet or substituting some of the forage with a new feed. “Concentrate feeds that are higher in fermentable fiber, fat, and/or starch are going to be more calorically dense than most hays,” she says.

Mueller always asks owners about their horses’ temperaments, because calories from a starch source can hype up a horse, which might not be desirable for an already-excitable animal. He adds that he might, however, use a higher starch- or sugar-based carbohydrate source of calories to add weight to a horse that does high-intensity bursts of speed in competition—roping, barrel racing, short-distance racehorses, etc.—because they need this type of energy source.

For more excitable horses, instead of starch Mueller adds a fat source such as oil. He prefers flaxseed or soy oil versus canola or corn oil because the latter two have poorer omega-3 to omega-6 ratios, which can increase inflammation in the horse’s body. When he wants to provide the essential omega-3 fatty acids docosahexaenoic acid and eicosapentaenoic acid, Mueller says fish oil has its benefits.   

When supplementing fat, a horse might hit a maximum intake threshold with liquid fat if he doesn’t like consuming a lot of fluid in his feed. Therefore, Mueller sometimes uses an extruded fat ­supplement. 

In addition, he might suggest a blend of fat and carbohydrate calories, because pure fat-based calories are pricey.

Developing Muscle

While muscle development happens bodywide in horses, it is easier to see and measure using the horse’s topline, says Mueller. To evaluate a horse’s muscle development, he uses a topline evaluation system that scores muscle quality along the topline from an A to a D. He says this system acts as a supplement to the Henneke body condition scoring system.

To add or develop muscle you must evaluate your horse’s current dietary protein levels and sources before increasing intake or changing protein sources. Mueller says he might opt to add a higher-protein feed or supplements or make a change in hay.

“Alfalfa is one of my protein levers,” he says. Alfalfa’s benefit is that it can up protein and add calories for a horse that needs both.

Feeding the Horse's Topline
RELATED CONTENT | Feeding the Horse’s Topline

Crude protein requirements for horses vary. An adult horse at maintenance only requires 10% protein. Performance horses are often fed a product with 14% crude protein, while halter horses might get upward of 16%. Researchers have not determined a maximum amount of protein that can be fed.

When choosing a protein source, says Mueller, the amino acid profile is key because amino acids are protein’s building blocks. While horse owners might only see the crude protein content listed on a feed label, they can ask the manufacturer more about the amino acid profile.

Thousands of amino acids exist in nature, but only 20 have dietary benefits for horses. Ten of these are essential amino acids, meaning the horse can’t make them and, thus, must consume them in his diet.

“The ratio of these amino acids and the amount of those amino acids are what determine your protein quality,” Mueller says. “You need to focus on the quantity and quality of the amino acids. Are you getting enough, and are you getting the right amounts of specific amino acids that will truly bring about the best muscle development in the horse?”

A limiting amino acid is an essential amino acid that sometimes doesn’t appear in adequate amounts in a feed. A horse cannot synthesize adequate protein if he doesn’t consume enough limiting amino acid. New protein will only be synthesized to the amount of the limiting amino acids. If the horse runs out of limiting amino acids, he can’t make use of the other amino acids he has consumed.

Lysine is the only amino acid that has a specific listed requirement in the NRC guidelines. For a 500-kilogram (1,100-pound) adult horse at maintenance, NRC recommends 27 grams of lysine per day, but this amount changes based on the horse’s age and use. For instance, a lactating mare in the first month after foaling needs 85 grams of lysine per day.

Soybean meal and other seed meals are great lysine sources, says Mueller, and show up in many feed ingredient lists.

Thunes says whey protein that provides the branched-chain amino acids, especially leucine, will help support muscle development, as well.

For some horses Mueller might recommend a diet ration balancer with high protein or purified amino acid supplements. He says the issue with supplements, however, is the lack of research behind them, so much of the information he has gathered is from field observations.

Take-Home Message

When a horse needs added bulk, in fat or muscle or both, it’s crucial you make the right changes to supply the necessary dietary components to see improvements. When in doubt, an equine nutritionist or your veterinarian can help guide you through these changes. This can help eliminate expensive and unrewarding blind guesses and trial and error.


Written by:

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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