Horses need water, energy from carbohydrates and fat, protein with specific amino acids, at least a dozen minerals, and several vitamins. The balance between these nutrients shifts depending on the horse’s age, workload, and reproductive status, but the foundation is the same: plenty of forage, clean water, adequate salt, and a mineral profile that matches what the forage lacks.
Water Comes First
Water is the single most critical nutrient. A 1,000-pound horse at rest in mild weather drinks 6 to 8 gallons per day. Bump the temperature to the mid-80s and that jumps to 11 to 14 gallons. Add moderate exercise on a hot day and the range climbs to 19 to 24 gallons.
Cold weather creates a sneaky problem. Horses drink less when it’s cold, yet their bodies still need the same amount or more. Dehydration in winter is common and easy to miss. During periods of heavy sweating, offering 5 liters of water every two hours with electrolytes helps replace what’s lost through sweat. Horses that don’t drink enough lose performance, develop impactions, and recover slowly from exercise.
Energy: Fiber, Starch, and Fat
Horses get most of their energy from carbohydrates, which come in two forms. Fibrous carbohydrates (the structural parts of plants like hay stems and seed coats) are fermented by microbes in the hindgut. Nonfibrous carbohydrates (starches and sugars, found mainly in grains) are partially broken down by the horse’s own enzymes and absorbed in the small intestine. Fiber should always form the backbone of the diet because it keeps the gut microbes healthy and the digestive tract moving.
A 1,000-pound horse at maintenance needs roughly 15 megacalories of digestible energy per day. That number rises steeply with work: light exercise bumps it to about 20, moderate exercise to 23, and heavy exercise to 27. Lactating mares have the highest demands of all, needing up to 32 megacalories in the first month after foaling. For growing horses, a 12-month-old at 700 pounds needs around 19 megacalories daily to support both maintenance and bone development.
When forage and grain alone can’t supply enough calories, fat is the safest way to close the gap. Horses can tolerate up to 20% of the diet by weight as fat, though keeping it at 10% or below is the practical recommendation. Fat is calorie-dense, produces less digestive heat than grain, and is especially useful for hard keepers, performance horses, and heavy-milking mares. Common sources include vegetable oil, rice bran, and stabilized flaxseed.
Protein and Amino Acids
Protein matters less for energy and more for building muscle, repairing tissue, and supporting growth. A mature horse at maintenance needs about 1.26 grams of crude protein per kilogram of body weight per day. For a 1,100-pound horse, that works out to roughly 630 grams daily. Horses with calmer temperaments and lighter workloads can get by on slightly less (1.08 g/kg), while more active horses benefit from closer to 1.44 g/kg.
Not all protein is equal. Lysine is the first limiting amino acid in equine diets, meaning it’s the one most likely to be in short supply. The optimum lysine intake for a mature horse at maintenance is about 54 milligrams per kilogram of body weight per day, or roughly 4.3% of total crude protein intake. Threonine is the second limiting amino acid, with requirements estimated at 53% to 62% of lysine intake. When either of these is deficient, the horse can’t fully use the rest of the protein it consumes, no matter how much total protein is in the feed. Soybean meal, alfalfa, and commercial ration balancers are common ways to improve the amino acid profile of a forage-based diet.
Calcium, Phosphorus, and the Ratio That Matters
Calcium and phosphorus are the two minerals horses need in the largest quantities, and their ratio to each other is just as important as the total amount. The ideal is about 2 parts calcium to 1 part phosphorus. Anywhere from 1:1 to 6:1 is acceptable, but a diet that contains more phosphorus than calcium (an inverted ratio) should always be avoided. Excess phosphorus blocks calcium absorption in the gut and can lead to weakened bones over time.
Broodmares and growing foals need the most careful attention here. Inadequate phosphorus slows growth and causes improper bone formation. Pushing foals for rapid growth without properly balanced calcium and phosphorus increases the risk of developmental joint disease. Grass hay tends to have a narrower calcium-to-phosphorus ratio than alfalfa, so horses on an all-grass diet sometimes need a calcium supplement or the addition of some alfalfa to correct the balance.
Salt and Electrolytes
Salt is the one mineral almost every horse needs beyond what forage provides. An 1,100-pound horse on a cool day requires about 10 grams of sodium and 40 grams of chloride, which translates to roughly 30 grams of table salt, or about two tablespoons. When temperatures rise or exercise increases, that requirement can double or triple.
During extreme sweating, a 1,100-pound horse benefits from 30 grams of salt (or twice that amount of a mixed electrolyte product) added to water every two hours, along with a small amount of sugar to aid absorption. A free-choice salt block covers baseline needs for most horses at rest, but horses in regular work often don’t lick enough from a block to replace what they lose in sweat. Top-dressing loose salt on feed is a more reliable approach for working horses.
Trace Minerals
Beyond the major minerals, horses need small but critical amounts of several trace minerals. Iron requirements for performance horses sit at 400 to 500 milligrams per day, and most forages easily meet or exceed this, so iron deficiency is rare. Over-supplementing iron is actually the more common problem.
Selenium is a biological antioxidant that works alongside vitamin E. The total diet should supply roughly 0.3 parts per million, which translates to about 2.5 to 3 milligrams per day. In regions with selenium-deficient soil (the Pacific Northwest and parts of the Northeast in the U.S., for example), supplementation is essential. In selenium-rich areas, adding more can push horses toward toxicity. A regional forage analysis is the best way to know where your hay falls.
Copper, zinc, and iodine round out the key trace minerals. Iodine deficiency causes thyroid enlargement (goiter), and interestingly, so does iodine excess. Choosing a commercial mineral supplement that provides at least 2 milligrams of iodine daily is a reasonable baseline. Copper and zinc are typically supplemented together because they compete for absorption; most fortified feeds and ration balancers account for this.
Vitamins: What Horses Make and What They Need
Horses synthesize most B vitamins (thiamin, riboflavin, niacin, biotin, and others) through microbial fermentation in the hindgut. As long as the horse has a healthy gut and eats adequate forage, outright B-vitamin deficiencies are uncommon. The exception is biotin for hoof quality. Horses with poor hoof horn don’t typically show deficient blood levels of biotin, yet research consistently shows that supplementing 15 to 25 milligrams per day improves hoof condition. It takes months to see results because the new, stronger hoof wall has to grow down from the coronary band.
Vitamin E is the fat-soluble vitamin most likely to fall short, especially in horses without access to fresh pasture. Current recommendations call for 1 to 2 IU per kilogram of body weight per day, which works out to roughly 500 to 1,000 IU for a typical riding horse. Fresh green grass is the richest natural source; hay loses vitamin E rapidly after cutting. Horses on a hay-only diet or kept in stalls often need a supplement. Natural-source vitamin E (d-alpha-tocopherol) is absorbed more efficiently than the synthetic form.
Vitamin A comes from beta-carotene in fresh forage, and vitamin D is synthesized in the skin with sunlight exposure. Horses that live outdoors and graze green pasture rarely lack either one. Horses kept indoors on stored hay may need supplementation, which most commercial feeds already provide.
How Needs Change With Age
Senior horses face a specific challenge: their ability to digest and absorb nutrients declines with age. Studies show that older horses have lower apparent digestibility of crude protein compared to younger horses, so their diets often need 12% to 14% crude protein to compensate. Supplementing lysine and threonine helps seniors maintain muscle mass that would otherwise waste away.
Fiber remains essential, but it needs to be highly digestible. Senior feeds are formulated with processed fiber sources (like beet pulp and soy hulls) that are easier to ferment than mature grass hay. Horses with dental problems that can’t chew long-stem hay well benefit from soaked hay cubes, chopped forage, or complete senior feeds that replace hay entirely. The crude fiber content of a senior diet should stay above 10%, but the source matters more than the percentage.
Putting It Together
For most horses, the practical starting point is simple: high-quality forage fed at 1.5% to 2% of body weight per day, free-choice water, and loose salt. From there, a forage analysis tells you what’s missing. Horses in work, growing foals, pregnant and lactating mares, and seniors each need targeted adjustments in energy, protein, and minerals. A ration balancer or fortified feed fills gaps in trace minerals and vitamins without adding unnecessary calories. The goal isn’t to pile on supplements but to match what the horse actually needs based on its forage, its workload, and its life stage.