In the horse, as well as companion and zoo industries, it’s not uncommon to observe feeding decisions based on anecdotes, folklore, fables or even human data that are not compatible or skewed from the known fundamentals. There are six major classifications of nutrients and depending on the animal there are approximately 45+ specific nutrients that are required in the diet. In addition there are components of the diet that are not “required nutrients” but are critical considerations in the nutritional management of the horse. The Nutrition Fundamental Series will address the fundamentals of nutrition.

Proteins, what are they?

Proteins provide the amino acids or building blocks that the horse requires for a variety of body functions and are part of most cells in the horse’s body. There are amino acids that can be made by the body and there are amino acids that can not be made. Those amino acids that can not be made or not made fast enough by the body are essential and must be included in the diet; those amino acids that can be made in the cells/ tissues are non-essential, thereby do not need to be supplied by the diet.

10 fundamental facts about proteins

1. Proteins are organic compounds made of amino acids, containing carbon, hydrogen, oxygen, and nitrogen; amino acids may also contain a very small amount of phosphorus and/or sulfur.
   
   
2. For each different protein there are specific amino acids and specific number of amino acids which are joined in a specific order. Different proteins have different functions in the body.
   
   
3. Protein requirements are highest in young growing animals and that requirement will decline gradually as the animal reaches maturity. Horses that produce foals, milk, or work will require higher levels of protein in their diet compared to adult horses that are fed for maintenance.
   
   
4. There are approximately 200 natural occurring compounds that have been classified as amino acids; however there are only 20 - 23 amino acids that are more prevalent in most proteins; that is, those that occur naturally in plant and animal tissues.
   
   
5. The first studies (which as one would expect was with rats) identified and classified amino acids as essential or nonessential; these first studies established that there are 10 amino acids required or needed in the diet thus are essential.
   
   
6. The 10 amino acids that must be supplied in the diet of the horse are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
   
   
7. What’s interesting is that only the amino acid lysine has been clearly established as required by the growing horse; however studies with other similar type animals provide the nutritional logic that horses require these essential amino acids.
   
   
8. The amino acid lysine is also the one most often lacking in quantity for growing horse rations; hence the importance of feeding plant protein sources that are high in lysine.
   
   
9. There is a sulfur containing amino aid that is critical to the diet, which is methionine (most likely the second most important amino acid). Other amino acids that contain sulfur that you may read about are cysteine, cystine, and taurine.
   
   
10. On feed labels you will notice that protein content is listed as crude protein. That’s because the laboratory procedure used is an indirect measurement of protein thus is a rough or crude estimate of the amount of protein.

Proteins, what are requirements?

Protein requirements of the horse are influenced by what the horse does for a living and the quality of the protein source. To the average horse owner, identifying requirements can be mind baffling. A nutritionist considers how much (that is the grams of protein) to feed a horse compared to a horse owner who evaluates the feed’s protein content relative to percent of protein. The fear of not providing enough protein leads to the “more-is-better” feeding practice. Protein is expensive thus it’s economically wasteful to feed more than is needed. There is also concern that feeding excess protein to animals leads to more nitrogen in the waste products and thus the nitrogen waste can contaminate our watersheds.

One method established to determine protein requirements is the relationship of protein and energy. Body weight and production level (reproduction, growth, work & maintenance) will influence the amounts of protein a horse consumes but the protein – energy ratio has an established relationship. For example a 1200 lb western pleasure horse requires more protein (895 grams) than a 900 lb western pleasure horse (670 grams) but for each horse the amount of protein has the same relationship with each unit of energy; that is, for every one unit of energy (the energy unit is called Mcal) that is fed, the horse requires 50 grams of protein. So a 1200 lb western pleasure horse requires 17.9 Mcal of energy; 17.9 Mcal X 50 grams/Mcal = 895 grams of protein per day. Of curse your question now is how do you determine energy requirements? …That’s for the Nutrition Fundamental Series on energy.

Proteins, what are sources?

Most protein feedstuffs fed to animals today are essentially by-products but by-product is NOT a BAD word! The oil (or fat) is extracted from the plant seed and the meal remaining is the rich source of protein. There are variations in the seed composition of protein grains but generally, plant protein feedstuffs are high in protein content, low in fiber, and have an inverted calcium-phosphorus ratio; that is, it has more phosphorus than calcium. Some plant protein sources are more concentrated in a particular or group of amino acids than others; hence the importance of being familiar with the composition of major protein feeds. For example, soybean meal is the richest plant source of lysine and lysine is considered the most important amino acid. 

All plants contain protein but there are plants that will provide a lot more protein than others. For example soybeans, cottonseed and flaxseed (>22%) are classified as plant protein sources and thus contain high levels of protein. Other plant sources, classified as grains and energy sources can be grouped as modest or low sources of protein. For example the grains oats, barley and corn contain low levels of proteins (9 – 12% crude protein).

Manufacturing allows us to process feedstuffs so that they deliver a more concentrated source of protein than the whole seed before processing. For example grinding soybeans into a meal form and extracting most of the oil produces a concentrate source of protein (44%) and is the feedstuff that we know as soybean meal; …and another reminder soybean meal is an important source of the essential amino acid lysine. For plant eaters, such as the horse, a combination of plant protein sources are used in mixed diets to ensure that there is a balance of the required amino acids provided. Using several plant feeds including at least two protein rich sources in a ration to balance for amino acid requirements is referenced as mutual supplementation. 

Animal sources of protein are considered the most complete proteins because of their amino acid composition; animal protein sources are used in large animal diets but are more expensive and perhaps less is used today because of federal restrictions and the general concern of the public with known diseases such as mad cow’s disease. Protein sources from milk are the most common animal protein source used in horse’s diets.

Listed are common plant protein-feeds used in rations; do not feed as an only feed or as a top dressed of a balanced formula.

Soybean Meal: Soybean meal has two commercial protein standards. The 44% crude protein standard contains 2.8 % lysine and the 48% crude protein standard contains 3.1 % lysine. As indicated already, soybean meal has the highest content of lysine compared to other plant proteins and is usually a less expensive source of protein available for horse feeds. Soybean meal is produced by extracting the oil from soybeans and the left over meal is cooked and ground. Raw soybeans do contain a compound that will interfere with protein digestion. However since soybean is heated to extract the oil, this trypsin inhibitor is destroyed; so no worries with feeding soybean meal or roasted soybeans.

Canola Meal: Canola meal is approximately 33% crude protein and 2.1% lysine. Studies with growth suggest good results compared to soybean meal.  Canola meal is a by-product and is produced when the oil is removed from canola.

Flaxseed Meal: Flaxseed meal is approximately 32% crude protein, low in lysine (1.2% lysine) and is more expensive than other plant protein sources that are fed to horses. Horse owners associated a quality hair coat with flax but have confused the meal form with the whole ground seed. The oil (or fat) in the whole seed is what provides the hair coat benefits. To produce flaxseed meal the oil is removed from the flax. Flaxseed meal contains 1.5% fat compared to 33% fat in the flax seed. Flaxseed meal is also noted for being a concentrated and natural source of the mineral selenium.

Cottonseed Meal: Cottonseed meal is approximately 38% crude protein and is also low lysine (1.6% lysine). The meal is cost effective and may be used for horses. Cottonseed contains a toxin substance that interferes with digestion and high feeding levels may have an adverse effect on growth of young animals. No studies however have directly linked this adverse effect to foals and adult horses can tolerate a certain level of the gossypol toxin. Studies with other animals suggest cottonseed meal may decrease sperm production thereby there are concerns with feeding cottonseed meal to breeding stallions. Gossypol is removed from cottonseeds during the heating/extraction process with ethyl ether. Reports from studies have suggested that cottonseed with less than 0.2% gossypol had no toxicity effects. In livestock feeds, the general rule is that cottonseed meal is less than 25% of the total protein provided in the feed.

Distillers Dried Grain: Distillers grains are approximately 28% crude protein and also low in low lysine (0.8% lysine). This by‑product is produced from the distilled liquor and ethanol industries. With the production of ethanol for gasoline, there is an abundant supply of this feedstuff which is in part why more distillers dried grain is showing up in animal feeds.

There are other protein sources used in horses’ formulas including sunflower meal, peanut meal, brewer’s dried grains and corn gluten meal; these protein sources are less common, or more expensive, or are poorer quality when compared to the list that has been provided.

Summary of Selective Nutrient Content of Protein Sources

Protein Source	% Crude Protein	% Lysine (amino acid)	% Crude Fat	% Calcium	% Phosphorus
Soybean meal	44.0 48.0	2.8 3.1	1.4 1.1	0.4 0.3	0.6 0.7
Canola meal	33.0	2.1	2.8	0.6	1.2
Flaxseed meal	32.0	1.2	1.4	0.4	0.8
Cottonseed meal	38.0	1.6	1.5	0.2	1.1
Distiller dried grain	28.0	0.8	6.5	0.1	0.4
Dehydrated skimmed milk	33.0	2.5	1.0	1.3	1.0

Number are based on as-fed or approximately 90% dry matter

Proteins, digestibility

For horses, protein feeds are enzymaticly digested in the foregut and the amino acids are absorbed and assimilated by the gut for use. The quality of the protein source is very important but not all amino acids that make up the protein are equally digestible. Studies with growing horses have indicated that protein from soybean meal, canola meal, and milk by-products provide a better complement of amino acids compared to other protein sources. Milk by-product (such as dehydrated skimmed milk) sources are obviously from animals (usually cows) and although animal protein sources are considered superior to plant protein sources they are more expensive and depending on the source can influence palatability.

Final thought

In general when evaluating a commercial formula label, soybean meal should be the first plant protein source listed because of the high lysine content. If the company is following ingredient listing protocol then that would suggest that soybean meal is at least 50% or more of the protein sources that were added to the formula. You may want to review the subheading “Proteins, what are sources?” to refresh your memory of plant protein sources.

Lysine content of a protein source is the number one criterion in selecting a protein source to be incorporated in a formula for horses. Also remember…; more-is-NOT-better”. Protein is the most expensive ingredient of the diet therefore it’s economically wasteful to feed more than is needed.