Protein Requirements for Horses

Protein requirements for horses are actually pretty simple once you understand what a horse does with protein. His body needs it to build and repair muscle, grow hair and hooves, make enzymes and hormones, and maintain pretty much every structure. The more he's building—because he's young, working hard, or lactating—the more protein he needs. The less he's building—because he's resting, older, or idle—the less he needs.

The problem is that many horse owners underestimate what their horse actually does and therefore underestimate what he needs. A horse that's working three or four days a week needs more protein than you'd think if you just look at idle time. A young horse that looks full-grown physically is still building into his teenage years. A mare producing milk is running an incredible metabolic bill.

Because most hay diets fall short of protein requirements for anything beyond a resting adult horse, understanding exactly what your horse needs helps you make better feeding decisions. You might think your horse needs the same diet as he always gets, but if his situation has changed—he's started working, he got older, he became a mom—his protein needs changed too.

Feeding for actual needs instead of guessing means your horse maintains better condition, has better energy for work, grows stronger if he's young, and just generally thrives better. It also prevents the slow, gradual decline that happens when a horse is chronically under-fed for his actual situation.

This guide breaks down what different categories of horses need, how to tell if what you're feeding meets the requirement, and what to feed if your current diet falls short.

Protein requirements are usually expressed as a percentage of total diet dry matter or as grams per day. Percentages are easier to work with practically. A resting adult horse needs 8 to 10 percent protein in his total diet. If he eats 20 pounds of hay daily, that hay needs to average 8 to 10 percent protein. If the hay is 6 percent protein, he's protein deficient.

When you add grain, the total diet changes. If a horse eats 15 pounds of grass hay at 7 percent protein plus 5 pounds of grain at 14 percent protein, his total diet is roughly 9 percent protein. That satisfies a resting adult but not a working horse. Working horses need 10 to 12 percent total diet protein, so you'd need to add more grain or switch to hay with higher protein.

Young growing horses need 12 to 14 percent total diet protein, with particular attention to amino acid balance, especially lysine. A growing horse that's only getting 8 to 10 percent protein is nutrient deficient relative to his needs, even if the percentage seems okay for an adult.

Performance and working horses benefit from 12 to 14 percent protein, depending on intensity. A horse doing light work can sometimes manage on 10 to 12 percent, but if he's in training or competition or heavy work, the higher end is better. Work tears down muscle; adequate protein rebuilds it.

Breeding mares in late pregnancy need 10 to 12 percent protein starting in month six of pregnancy, moving to 12 to 14 percent in the final months. Lactating mares need 14 to 16 percent protein because milk production is energetically expensive. Nursing a foal might be the most nutritionally demanding thing a mare ever does.

Senior horses sometimes benefit from 10 to 12 percent protein because their digestion is less efficient and maintaining muscle is harder as they age. Some old horses drop condition partly because they're not getting adequate protein for their aging digestive system. Higher-quality protein that's more digestible often helps senior horses more than younger horses.

Hard keepers and horses on poor-quality forage need protein percentages at the higher end of normal for their category, even if they're not working or young. A hard keeper that's resting might need 10 to 12 percent protein instead of 8 to 10 percent just to maintain any condition.

The amino acid profile matters, not just the percentage. Lysine is the limiting amino acid in most horse diets—the one that runs out first. If your diet meets the protein percentage but is short on lysine, the horse still can't use the protein effectively. Quality grains and legumes are higher in lysine than grass hays.

Calculate what your horse is eating and what he actually needs. It's not complicated. Write down everything he gets daily—hay type, amount, grain type, amount, supplements. Then estimate or measure the protein percentage of each. Your grain bag will have this; hay you can estimate if it looks good quality or measure through hay testing. Add it up and see if the total percentage meets his needs.

For a resting adult horse on grass hay: if the hay is 6 to 8 percent protein, he meets requirements with just the hay. If it's lower than that, he's deficient.

For a working horse on the same hay: add 4 to 6 pounds of a 12 percent protein grain daily to bring the total diet to 10 to 12 percent protein. The exact amount depends on the hay quality and the grain quality.

For a young growing horse: feed hay that's decent quality (8 to 10 percent protein ideally) plus a grain formulated for growing horses. Growing horse grains are usually 12 to 14 percent protein with balanced amino acids. Amount depends on the horse's size and growth stage, but usually 3 to 5 pounds daily for a growing horse.

For a mare in late pregnancy: switch to higher-protein hay if possible—alfalfa or alfalfa mix—or add grain to bump her total diet to 12 to 14 percent. She should be eating well and look solid going into foaling, which requires adequate protein.

For a lactating mare: feed the highest protein diet you reasonably can. Alfalfa hay plus grain formulated for mares, totaling 14 to 16 percent protein, is appropriate. Some mares need even more if they're hard keepers or producing lots of milk.

For a senior horse: feed hay that's better quality than minimum—not moldy or stemmy—plus grain if he's struggling to maintain condition. A senior concentrate formulated to be easily digestible helps some older horses more than regular grain.

For a hard keeper of any age: don't try to manage on straight hay. Add grain and possibly a ration balancer to meet the protein requirement. Accept that this horse will cost more to feed than an easy keeper. It's the cost of ownership.

Change anything gradually. If you're adding grain or switching hay, do it over a week or two so his digestion adjusts. His bacteria are used to fermenting what he's been eating; sudden changes disrupt that.

Have your hay tested if you're unsure about quality. A simple forage test costs $20-40 and tells you the protein percentage so you know exactly what you're working with instead of guessing.

Young horses need protein based on their growth stage. A weanling or yearling is growing rapidly and needs the highest percentage—12 to 14 percent total diet. A two-year-old that's near finishing height but still muscling up needs 12 to 14 percent. A three-year-old that's full-grown but still building can sometimes manage on 10 to 12 percent. The key is that any young horse in any growing phase needs more protein than a resting adult.

Racehorses or performance horses in hard training need 12 to 14 percent protein because the work is intense and recovery is critical. A horse competing or training seriously can't afford to be protein deficient; it affects recovery, muscle development, and performance.

Weekend warriors—horses ridden occasionally or lightly—often get away with 10 to 12 percent protein, but only if they're not hard keepers and the forage quality is decent. Don't shortchange even a light-work horse on protein; you'll see condition issues develop over time.

Breeding stallions benefit from adequate protein year-round but especially during breeding season. Reproductive function is energetically expensive. A 10 to 12 percent protein diet is reasonable for a stallion, moving higher if he's actively breeding.

Weanlings and foals nursing from their dams get mother's milk, which is incredibly protein-rich. Once they're eating hay and grain supplementally, that grain needs to be formulated for foals with appropriate protein and minerals. Weaned foals need even more attention because they've lost the nutrient-dense milk and have to get everything from forage and grain.

Older horses that are thin despite eating adequate quantity might be protein deficient or might have a digestive problem that reduces protein absorption. Either way, higher-quality, higher-protein feed often helps more than just feeding more of the same thing.

Easy keepers can meet all their protein requirements on decent hay if they're resting. These horses don't need grain unless they need the calories or you want to give supplements mixed in grain. But don't assume an easy keeper doesn't need grain if he's working; his protein requirements go up even if his calorie requirements are easy to meet.

Hard keepers need protein at the higher end even when resting. These horses have fast metabolisms and struggle to maintain condition. Meeting their protein requirements is essential; giving them less to save money backfires because they lose condition and create other problems.

Underestimating what your horse does is the biggest mistake people make. A horse that's ridden three times a week isn't a resting horse; he's a working horse. A horse that looks full-grown at three years old still has development happening; he's not a mature horse yet. A mare nursing a foal is running a massive metabolic bill; she's not a resting horse. Be honest about what your horse actually does.

Feeding straight hay to a working horse and wondering why his condition drops is a common scenario. Hay alone often doesn't meet the protein requirement for any horse doing work. You need to add grain or switch to higher-protein hay. Don't expect a horse to thrive on inadequate nutrition.

Assuming that more calories will fix a protein deficiency doesn't work. A horse can eat plenty of calories and still be protein deficient if the protein percentage is low. Adding another flake of hay when the problem is that the hay is only 6 percent protein doesn't solve anything. You need higher-protein feed, not more of the same feed.

Ignoring hay quality leads to chronic underfeeding. Moldy, stemmy, or weathered hay has much lower protein than good hay. If your horse is on poor-quality hay, he's probably deficient in multiple nutrients, not just protein. Testing the hay or getting better hay matters more than the quantity of poor hay.

Changing a young horse's nutrition randomly or trying to save money on a growing horse's feed creates permanent problems. Young horses have one chance to build their bodies right. Cutting corners during growth shows up as weak bones, poor hooves, and joint problems that follow them for life. Invest properly in young horse nutrition.

Lactating mares that are under-fed for protein produce less milk or milk of lower quality. The foal suffers, growing poorly and missing critical nutrients. This affects the foal's long-term development and potential. Mares deserve good nutrition when they're nursing.

Senior horses that lose condition often get written off as "just old" when the real problem is inadequate protein for an aging digestive system. An older horse doesn't need to decline; better nutrition often brings them back into good condition and energy.

Write down your horse's age, what he does, and his current diet. Use that to figure out what category he falls into and what protein requirement that category has. Then look at what he's actually eating and calculate whether he's meeting it. This takes maybe 15 minutes and clarifies a lot.

If you're buying new hay, ask the seller about it or get it tested. Knowing the protein content tells you whether it will meet your horse's needs on its own or if you need to add grain. Visual assessment—"it looks good"—is not the same as knowing the actual protein percentage.

If you're adding grain to a hay diet for the first time, pick a grain formulated to complement the hay you're using. If the hay is grass hay, you want a grain that provides the amino acids and minerals that grass hay is short on. Your grain store should be able to recommend something appropriate.

Start where you are. If your horse is currently under-fed, improve gradually rather than making massive dietary changes all at once. Better to add a couple pounds of grain per week and adjust than to double his grain overnight and risk digestive upset.

If your horse is doing more work than he used to, increase his protein percentage slightly. You don't need to double his grain, but an extra pound or two makes a difference when the work level changes.

Monitor his condition monthly. Body condition should stay stable or improve slightly as you adjust feed. Coat quality, muscle tone, hoof quality, and energy level are all indicators that his nutrition is adequate. If any of these decline, something's wrong—either the feed isn't as good as you thought, or his needs have changed.

Don't get stuck on a fixed feeding program. A young horse's needs change as he matures. A horse's needs change if work level changes. A horse's needs change as he ages. Reassess periodically instead of assuming that what worked five years ago is still right.

If your horse has special needs—he's a hard keeper, he's recovering from illness, he's an older horse losing condition—consider consulting with a nutritionist. Many grain stores have nutritionists available, and some vets specialize in equine nutrition. It's worth paying for expertise rather than guessing and dealing with problems.