A typical 1,100-pound horse at maintenance (no work, not breeding) needs roughly 630 grams of crude protein per day. That number shifts significantly based on age, workload, and reproductive status, ranging from as low as 540 grams for an easy keeper to more than double that for a mare producing milk. Understanding where your horse falls on that spectrum helps you choose the right hay and grain combination without under- or overfeeding.
Maintenance Protein for Adult Horses
The National Research Council (2007) breaks maintenance protein into three tiers for an adult 1,100-pound horse doing no work:
- Minimum: 540 grams per day
- Average: 630 grams per day
- Elevated: 720 grams per day
Most owners should aim for the average figure. The minimum is a true floor, not a target, and chronically feeding at that level leaves little margin for individual variation. The elevated level suits horses that are harder keepers or under mild metabolic stress. In practical terms, a horse eating 2% of its body weight in decent-quality grass hay (around 10 to 12% protein) will land close to the average requirement without any grain at all.
How Exercise Changes the Numbers
Working horses need a higher percentage of protein in their total ration, not just more calories. The minimum crude protein concentration in the overall diet rises with intensity:
- Light work (trail riding, light schooling): 10% crude protein
- Medium work (regular training, lower-level competition): 11% crude protein
- Hard work (racing, upper-level eventing, polo): 12% crude protein
Because working horses also eat more total feed to meet their energy needs, the actual grams of protein consumed climb from both the higher percentage and the greater intake. A horse in hard work eating 25 to 28 pounds of feed per day at 12% protein takes in considerably more total protein than the same horse at rest. The key is that the protein percentage in the ration needs to keep pace with the calorie increase, or the horse may lose muscle condition even while maintaining weight.
Growing Horses Need the Most Concentration
Young horses have the highest protein requirements relative to body size because they are building muscle, bone, and connective tissue at a rapid pace. A weanling (around six months old) needs approximately 15% crude protein in its total ration, with concentrate mixes formulated at 16% or higher depending on the type of hay offered. When weanlings eat grass hay, which is lower in protein, the grain mix needs to be 16 to 18% protein to compensate. With legume hay like alfalfa, 14 to 16% in the grain is sufficient because the hay is already contributing more protein.
Yearlings require less than weanlings but still more than adult horses. The total ration should contain about 14% crude protein at 12 months of age, and concentrate mixes are typically formulated at 14% or slightly above. Feeding a 14% concentrate even though the minimum ration requirement is closer to 12% gives you flexibility to adjust hay quality without risking a shortfall. As the horse matures past 18 months, protein needs gradually taper toward adult maintenance levels.
Broodmares: Gestation and Lactation
Pregnant mares can get by on a standard maintenance diet for roughly the first eight months of gestation. It is in the final 90 days that protein needs jump, increasing by about half a pound of crude protein per day above maintenance. That additional protein supports the rapid fetal growth that happens in the last trimester, when the foal gains roughly 60% of its birth weight.
Lactation is far more demanding. A mare producing milk needs roughly double the protein of a maintenance horse because of the large amount of protein leaving her body in the milk every day. Mares that don’t receive enough protein during this period show measurably decreased milk production, which directly slows foal growth. This is the stage where upgrading to a higher-protein hay (such as alfalfa) or adding a concentrated protein source makes the biggest practical difference. Peak lactation occurs in the first three months after foaling, so the dietary increase should begin right around birth, not weeks later.
Senior Horses May Need More
The NRC notes that the precise effect of aging on protein requirements is still unclear, but there is evidence that older horses digest protein less efficiently. One study found that aged horses (averaging 26 years old) had a crude protein digestibility of only 67%, compared to 73% in younger horses. That gap means a senior horse eating the same diet as a younger horse absorbs less usable protein from each pound of feed.
Dental deterioration compounds the problem. Worn or missing teeth make it harder to chew hay thoroughly, and poorly chewed forage passes through the gut with less of its protein extracted. For older horses showing muscle loss along the topline or hindquarters, increasing dietary protein by a percentage point or two, or switching to more digestible protein sources, can help offset the decline. Senior feeds are typically formulated with this in mind, often running 12 to 14% crude protein with softer, easier-to-chew pellets or extruded forms.
Why Protein Quality Matters as Much as Quantity
Total crude protein is only part of the picture. Horses, like all animals, don’t actually need “protein” in a generic sense. They need specific amino acids, the individual building blocks that make up protein. In a typical horse diet built around grasses and cereal grains, lysine is the first limiting amino acid. This means lysine runs out before the other amino acids, and once it does, the horse can’t use the remaining amino acids efficiently for building muscle or other tissues. Threonine is the second most limiting amino acid.
This is why two feeds with the same crude protein percentage can produce very different results. A diet heavy in corn and grass hay may technically meet a crude protein target but fall short on lysine, while a diet incorporating soybean meal or alfalfa delivers more lysine per gram of protein. When lysine or threonine is deficient, the horse can show signs of protein deficiency even though the total crude protein number on paper looks adequate. The NRC provides specific lysine recommendations alongside crude protein for this reason, though the broader amino acid picture in horses is still less well understood than in other livestock species.
Protein Content of Common Forages
Your hay choice is the single biggest lever for hitting protein targets, since forage makes up the majority of most horses’ diets. Legume hays like alfalfa, perennial peanut, and clover typically range from 16 to 22% crude protein. That high concentration makes them excellent for lactating mares, growing horses, and hard-working athletes, but potentially excessive for easy keepers at maintenance.
Cool-season grasses such as timothy, orchardgrass, and tall fescue fall in the 10 to 16% crude protein range. The wide spread depends heavily on when the hay was cut and how it was fertilized. Early-cut, well-fertilized timothy can test at 14%, while late-cut, unfertilized timothy might come in under 8%. This is exactly why hay testing matters. A $15 lab test tells you the actual protein content of your hay rather than forcing you to guess based on averages. Without that number, you’re either leaving your horse short or buying expensive supplements it doesn’t need.
What Happens When Protein Is Too Low
A horse that doesn’t get enough protein, or enough of the right amino acids, will eventually show visible changes. The topline is usually the first area affected: the muscles along the spine, over the croup, and behind the withers flatten or develop a sunken appearance. Coat quality declines, becoming dull or slow to shed. Hoof growth slows. In growing horses, protein deficiency stunts growth rates. In lactating mares, milk production drops, and foals raised on protein-deficient mares gain weight more slowly than expected.
These signs develop gradually, which makes them easy to attribute to other causes like aging, poor-quality hay in general, or inadequate calories. If a horse is maintaining weight but losing muscle definition, protein quantity or quality should be one of the first things you evaluate.
Risks of Feeding Too Much Protein
Overfeeding protein is wasteful and comes with its own set of problems. Horses can’t store excess amino acids the way they store fat. Instead, the body strips the nitrogen off the unused amino acids and excretes it through urine, feces, and sweat. This produces higher concentrations of ammonia in the stall, which is the sharp smell you notice in a poorly ventilated barn. Research has shown that horses fed protein in excess of recommendations produce significantly more ammonia in their feces and higher nitrogen levels in their urine, with measurable effects on air quality in the barn environment.
For the horse, excess protein means more water consumption to flush the nitrogen waste through the kidneys, leading to wetter bedding and higher blood urea nitrogen levels. Over time, the elevated ammonia in a confined space can irritate the respiratory tract of both horses and the people who care for them. There is also a financial cost: protein is the most expensive component of most feeds, so overfeeding it raises your feed bill without any performance benefit. Meeting the requirement accurately, rather than dramatically exceeding it, is better for your horse, your barn air, and your budget.