Most horses need between 0.5 and 2 pounds of soybean meal per day, depending on their size, age, and workload. Soybean meal is the most nutritionally valuable plant protein source available for horses, delivering 44% to 48% crude protein along with the highest lysine content of any common oilseed meal. The exact amount you feed depends on how much protein your horse already gets from hay and grain, and how much more it needs.
Why Soybean Meal Works Well for Horses
Soybean meal stands out because of its amino acid profile, particularly its lysine content. Lysine is the first limiting amino acid in equine diets, meaning it’s the one most likely to be deficient and the one that caps your horse’s ability to use all the other protein it eats. While other oilseed meals like cottonseed meal and linseed meal are low in lysine, soybean meal delivers roughly 2.9% lysine on a dry matter basis. It also provides meaningful amounts of methionine, threonine, and tryptophan.
You’ll find two common types at feed stores. Standard soybean meal (with hulls) runs about 44% crude protein and 6% to 7% fiber. Dehulled soybean meal hits 48% crude protein with only about 3.5% fiber, plus slightly more lysine and energy per pound. Either works fine for horses, though the 44% version is more widely available and less expensive.
How Much to Feed by Life Stage
Soybean meal is a supplement, not a complete feed. You’re adding it to an existing diet of hay (and possibly grain) to fill a protein gap. The amount depends on what that gap looks like.
Adult Horses at Maintenance
A mature horse doing little to no work and eating decent-quality grass hay typically needs around 8% to 10% crude protein in its total diet. Most mixed grass hays fall in the 7% to 10% range, so many idle horses don’t need soybean meal at all. If your hay tests low in protein, adding 0.5 to 1 pound of soybean meal per day is usually enough to close the gap for an average 1,100-pound horse.
Weanlings and Yearlings
Growing horses have the highest protein demands relative to their body size. Weanlings need concentrate feeds at a rate of 1 to 1.5 pounds per 100 pounds of body weight per day, and those concentrates should contain around 14% to 16% crude protein. Yearlings follow a similar guideline of 1 to 1.5 pounds of concentrate per 100 pounds of body weight. Soybean meal is commonly blended into grain mixes at 10% to 20% of the concentrate to hit these protein targets. For a 600-pound weanling eating 6 to 9 pounds of concentrate daily, that translates to roughly 0.5 to 1.5 pounds of soybean meal mixed into the grain.
The lysine piece matters most here. Young horses building bone and muscle tissue need adequate lysine to actually use the protein they consume. Soybean meal’s lysine advantage over other protein sources makes it especially well suited for growing horses.
Performance and Working Horses
Horses in moderate to heavy exercise need more protein to support muscle repair and development. Research on exercising horses has used concentrate mixes containing soybean meal as the primary protein source, with the total concentrate running about 14% crude protein. These horses consumed roughly 5 pounds of grain mix alongside 17 to 18 pounds of hay per day. Within that grain mix, soybean meal typically makes up 15% to 25% of the blend, putting daily soybean meal intake in the range of 0.75 to 1.25 pounds for an average-sized horse in training.
For heavily muscled breeds or horses in intense conditioning programs, some owners push toward 1.5 to 2 pounds of soybean meal daily, but this only makes sense when the rest of the diet is genuinely low in protein. Excess protein doesn’t build extra muscle. It gets broken down and excreted, putting unnecessary strain on the kidneys and increasing water consumption and urination.
Broodmares
Lactating mares in the first three months after foaling have protein needs that rival those of growing horses. Adding 1 to 1.5 pounds of soybean meal to the daily ration helps support milk production. Mares in late pregnancy benefit from 0.5 to 1 pound per day as the foal’s growth accelerates in the final trimester.
How to Calculate Your Horse’s Protein Gap
The most accurate approach is to get your hay tested. A basic forage analysis costs around $20 to $40 and tells you exactly how much crude protein your hay provides. Once you know that number, the math is straightforward.
Say your 1,100-pound horse eats 20 pounds of hay per day that tests at 8% crude protein. That’s 1.6 pounds of protein from hay alone. If the horse needs about 1.4 pounds of crude protein daily (a common estimate for light work), the hay already covers it. But if the horse is a yearling needing closer to 2 pounds of total daily protein, you’re short by 0.4 pounds. Since soybean meal is roughly 44% protein, about 1 pound of soybean meal delivers 0.44 pounds of protein, neatly filling that gap.
Without a hay test, a conservative starting point is 0.5 to 1 pound per day for any horse that seems to need more protein, then adjusting based on body condition, coat quality, and muscle development over the following weeks.
Feeding Tips and Practical Limits
Soybean meal is palatable and most horses eat it readily when mixed into their grain. You can top-dress it on a small amount of grain or pellets, or blend it into a custom concentrate mix. It tends to be dusty, so adding a splash of water or oil helps it stick to the grain.
Concentrates of all types, including soybean meal, should make up no more than about 30% of your horse’s total diet by weight. For most horses, the forage portion (hay and pasture) should remain the foundation. Introduce soybean meal gradually over 7 to 10 days, starting with a quarter of your target amount and increasing every few days. Sudden changes in protein intake can cause loose manure and digestive upset.
There’s no established toxic dose of soybean meal for horses, but feeding more than 2 pounds per day to an average-sized horse rarely makes nutritional sense. If your horse needs that much supplemental protein, it’s worth re-evaluating the base diet rather than piling on more soybean meal.
Make Sure It’s Properly Processed
Raw soybeans contain trypsin inhibitors, compounds that block protein-digesting enzymes in the gut. When these inhibitors are active, they form stable complexes with digestive enzymes, reducing the amount of free enzyme available to break down protein. This defeats the whole purpose of feeding a protein supplement.
Commercially sold soybean meal has been heat-treated (steam-processed at 110 to 130°C) to deactivate these inhibitors. This is standard in the feed industry, and any soybean meal you buy from a reputable feed supplier has already gone through this step. Never feed raw or roasted-at-home soybeans to horses. The processing matters, and getting it wrong means your horse absorbs significantly less protein from the meal while potentially developing digestive problems.
Soybean Meal vs. Other Protein Sources
Cottonseed meal is sometimes cheaper, but it contains gossypol, a compound that can be toxic at higher intakes and limits how much you can safely feed. It’s also lower in lysine, methionine, cystine, and tryptophan. Linseed (flaxseed) meal provides omega-3 fatty acids but has a weaker amino acid profile than soybean meal. Alfalfa hay is another good protein source (typically 15% to 20% crude protein) and can reduce or eliminate the need for soybean meal, though it adds more calcium to the diet.
For pure protein supplementation with the best amino acid balance, soybean meal remains the standard in equine nutrition. It’s widely available, cost-effective, and well-researched. If your horse has specific dietary sensitivities or you want to avoid soy for other reasons, alfalfa pellets or cubes are the most practical alternative.