Protein is not a popular subject. Most "nutrition-talk" revolves around carbohydrates -- sugar and starch, to be specific, because they impact metabolic conditions that are a very real concern for many horse owners. We also talk about fat -- types of fat, essential fatty acids, omega 3s, you know the terms - because horses require a daily supply of essential fatty acids and they also benefit from fat to fulfill high energy needs for weight gain and exercise.
But protein? Just check the "percent crude protein" and figure it's enough, right? Not necessarily. There's a lot more to it than that. To guide you, let's start by looking at what happens to the protein in your forages and feeds, when your horse eats it.
Proteins in the feed are digested down to amino acids. There are 22 individual amino acids -- "building blocks" your horse's cells put together to create new proteins. There are literally hundreds of proteins in his body, all of which rely on not only enough total protein, but enough amino acid variability.
Forages have protein, but their variability is limited; they have lots of some amino acids and not much of others. If a single type of grass as hay or pasture is the only protein source in your horse's diet, the pool of amino acids available to your horse's body will be deficient in several amino acids, making it difficult for him to stay healthy.
Imagine a bowl full of red, blue, yellow and green beads. You want to make a long necklace with a very specific color pattern. As you progress in stringing this necklace, you notice that you've run out of yellow beads. Uh oh -- now you cannot make the necklace you planned. You either get more yellow beads, or you end up with a bracelet instead of a necklace!
Protein molecules are like long, beaded chains of amino acids, in a very specific order, depending on where the protein is located. Muscle protein looks different than joint proteins. Hemoglobin in red blood cells, looks different that digestive enzymes. The DNA within each tissue's cells dictates the order of amino acids needed to produce that specific protein. If there are enough amino acids available, the protein can be created. If not, then that tissue goes without.
And what about all those unused amino acids -- those red, blue and green beads? Can't they be saved for later in the hope that you'll feed more "yellow beads?" Unfortunately, no. Instead, they get destroyed and cannot be used for protein synthesis. They can be used for energy, glucose production, or stored as fat, but that doesn't meet your horse's protein need.
Horses in a wild setting travel for miles each day, grazing on a vast assortment of feedstuffs -- grasses, legumes, flowers, fruits, berries, seeds, leaves, edible weeds, shrubs, and bark, offering a mixture of nutrients, including proteins. Can we duplicate this in a domesticated setting? Not usually, unless you have many acres of untouched land. Therefore, our goal should be to improve the horse's protein quality of the diet by offering more protein-rich feeds.
We need to pay attention to the amino acid profile of the entire diet. Of the 22 different amino acids, your horse's body is only able to make 12. The remaining ten are considered essential, meaning the body cannot produce them, or cannot produce them in adequate quantity. Therefore, they must be in the diet. The 10 essential amino acids (EAAs) are methionine, arginine, threonine, tryptophan, histidine, isoleucine, leucine, lysine, valine, and phenylalanine.
We do not know the specific requirements of each EAA for horses. The only one that has been evaluated is lysine, because it is considered "limiting." This simply means that the amount of proteins produced will be limited by the level of lysine. If lysine is low, it's like not having enough yellow beads (going back to our beaded necklace analogy).
There are two other limiting amino acids: methionine and threonine. Exactly how much the horse requires is unknown, but we do have an idea of the levels relative to the lysine content. The general thinking among equine nutritionists is that there should be 2 to 3 times more lysine than methionine, and threonine content should be about the same as lysine.
Most animal proteins are higher in quality than those found in plants. This means that they contain more than enough amino acid building blocks to build tissues for vital organs as well as peripheral, non-vital tissues. But horses do not naturally consume animal protein sources, so we have to get a little creative by mixing several plant protein sources so that they ultimately reflect the amino acid profile of an animal source.
Most grasses have a similar amino acid profile. Cool season grasses, such as timothy, brome, orchardgrass, rye, fescue, and Kentucky bluegrass, tend to have more amino acids than warm season grasses, such as the popular Bermuda and Teff. To improve the protein quality, you can add a legume such as alfalfa (lucerne), clover, and perennial peanut grass (grown in some southern areas of the US).
Adding alfalfa to grasses will certainly help, but many horse owners choose to avoid it. Or even if you do include it, the EAA content may not be sufficient for your particular horse. For example, feeding 18 lbs of grass hay plus 4 lbs of alfalfa may meet the EAA need of an average horse on light activity, but it may not if the horse has any compromised health issues.
Adding whole foods to your horse's diet will not only improve the overall protein quality, but can add valuable vitamins, antioxidants, trace minerals, and fatty acids that your horse might not otherwise consume. Here are some examples:
1) Dehulled soybean meal. This is the most commonly added protein source to commercial products. Economical and rich in protein (47%), it is easy to see why it is used to boost the protein content of many feeds and ration balancers. But there are several potential problems with soy:
2) Hemp seeds. High in protein (32%), they contain two main proteins: albumin and edestin. Both have significant amounts of all EAAs. Some other aspects of hempseeds:
3) Flax seeds. With 18% protein, they make a good choice to include in the diet (make sure they are ground). But their real claim to fame is their essential fatty acid content which duplicates those naturally found in fresh, healthy pasture grasses. (Remember, the word, "essential" means that they cannot be made by the body and must be in the diet.) Adding flax will, therefore, serve two benefits: provides necessary essential fatty acids, and offers a source of protein to boost overall protein quality in the diet.
4) Chia seeds. They are comparable to flax seeds in their protein content and nearly identical to flax in their essential fatty acid content. In fact, you can feed either ground flax seeds, or chia seeds, depending on your budget and your horse's preference.
5) Split peas and pea protein isolate. Peas that are dried and split are a tasty way to add protein and crunch to the diet. They can be fed raw, but it is good to soften them a bit by soaking them in warm water for a few minutes. Though the protein content is high (24%), it doesn't compare to the protein content of pea protein isolate, with 75% protein. I recommend adding pea protein isolate to the diet for horses who require extra protein due to aging, growth, intense exercise needs, pregnancy, and lactation.
6) Coconut (copra) meal. A good source of protein (20%), it is low in sugar/starch, and high in fat, from coconut oil, making it a good choice for a horse who is underweight or is heavily exercised. Keep in mind that the fatty acid content of coconut oil does not include essential fatty acids, necessitating supplementation from an additional fat source (such as flax or chia).
7) Pumpkin seeds. A tasty treat, supplying 34% protein, and a variety of vitamins and minerals, including a high amount of magnesium. They can be fed raw, hulled, or with the shells on. When fed raw, they contain active digestive enzymes that are helpful for gastrointestinal tract.
8) Whey. Whey is a protein found in milk and is highly concentrated (80% protein). Because it is animal, and not plant, it is of very high quality. It can contain some lactose, and adult horses are lactose intolerant; therefore, they may develop loose manure.
9) Other feedstuffs:
Please note: Whenever you add a new feed to your horse's diet, it is important to start slowly, taking two or three weeks to allow the hindgut microbial population to adjust.
Since each whole food has a difference density, the information below provides the volume measure equivalent to 4 ounces by weight of each product along with the protein grams.
According to the National Research Council, protein requirements vary based on mature size, activity level, age of growing horses, and breeding status. On average, a 1100 lb (500 kg) adult horse at maintenance, will require a minimum of 630 grams of crude protein per day. As exercise increases, values can increase to approximately 1000 grams/day. Growing horses require more, and pregnancy and lactation can double the maintenance requirement.
But, and this is important... these values do not take into consideration that the amino acids in forages are not highly absorbed. The level of absorption is referred to as its biological value (BV). The BV of pasture grasses and hays ranges from 45 to 80 percent.
That means that the NRC numbers may need to be increased by 20 to 55% to get a clear estimate of how much your horse is realistically absorbing. Here are some points to consider:
For your horse's diet to contain quality protein, consider how many protein sources you are feeding. Adding one or more whole foods to hay and/or pasture will accomplish this goal. This will boost the essential amino acid content, allowing for every tissue in the body to get what it needs to thrive. Variety is key!
 Speedi-Beet from Emerald Valley Natural Health is an excellent, non-GMO source of beet pulp. https://emeraldvalleyequine.com/product/speedi-beet/
 Nutrient Requirement of Horses, 6th Revised Edition. National Research Council of the National Academies. 2007. Table 16-3 Daily Nutrient Requirements of Horses with mature body weight of 500 kg.
 Tucker, DVM, G., Chronic protein deficiency in horses. https://theequinepractice.com/protein/. Also Nutrient Requirement of Horses, 6th Revised Edition. National Research Council of the National Academies. 2007. Chapter 8, Feeds and Feeds Processing, page 172.
For permission to reprint this article, in part or in its entirety, please contact Dr. Juliet Getty directly at Gettyequinenutrition@gmail.com.