Protein Demystified

It’s all about amino acids

Protein is a major component in most tissues in the horse. Proteins are made up of individual amino acids that join together to form chains. The length of the protein chains varies from protein to protein, as does the combination of amino acids making up the chains.

When horses consume proteins in their diet, the proteins are broken down and absorbed as amino acids. These amino acids are then circulated in the blood and reconstructed into new proteins as needed.

In horses, protein is digested and adsorbed in the foregut (stomach and small intestine). The proteins that escape absorption in the foregut are utilized by the microbes (bugs) in the hindgut. Although the bugs benefit from this protein, it does not add to the horse’s circulating pool of amino acids. A certain percentage of any protein will pass undigested into the hindgut.

As we strive to provide a balanced diet for our horses, the requirement we are working to fulfill is for amino acids, not proteins. A good quality protein will provide the necessary amino acids in the correct ratios to meet the horse’s needs.

Essential amino acids explained

There are 22 primary amino acids and 10 of those are considered “essential.” Essential amino acids are those that cannot be synthesized by the body in adequate enough amounts to meet the demand; they have to be provided in the diet. The 10 essential amino acids are: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

In order for a protein to be synthesized, all the necessary amino acids must be present at once. The amino acid whose supply runs out first and “limits” protein synthesis from proceeding is considered the “limiting amino acid.” For horses, lysine is the first limiting amino acid. In other words, it is the one most lacking in the equine diet; therefore, it is important to supply adequate levels of lysine in your horse’s diet. Growing horses, third-trimester broodmares and lactating mares have the highest lysine requirements.

Protein source is important

Since lysine is the first limiting amino acid in the horse, the protein source utilized in the equine diet should be high in lysine, especially for growing and reproducing horses.

  • Soybeans are high in lysine and historically have been the ingredient of choice when formulating a good quality horse feed
  • Soybean meal (the high protein part of the grain that has had the oil removed) provides excellent lysine levels
  • Canola meal (not to be confused with rapeseed) is another source that provides adequate lysine for growing horses
  • Animal sources, such as milk proteins, can also be considered
  • Grains and grasses are typically low in lysine

Other sources of protein

The follow sources of protein can be found in equine feeds and supplements. Although they do provide lysine, they typically do not provide adequate levels for growing or reproducing horses.

  • Cottonseed meal
  • Linseed meal (not linseed oil)
  • Brewers grains
  • Distillers grains
  • Alfalfa
  • Corn gluten

When you are reviewing your horse’s feeding program be sure to check out what source of protein is provided by your feed. It may make a difference.


  • Barb B.

    How is the % of protein determined? Is it per lb. of total feed? I.E. if my horse is getting 20 lbs. of feed per day, is 10% protein 2 lbs. of alfalfa? This confuses the heck out of me.

    • Karen @ KPP

      You are not the only one who is confused by this! Let’s see if we can make it easier for you to understand how to determine the percentage of protein in your horse’s total diet. You have to do some math, but it isn’t too hard.

      I will use my horse’s diet as an example.

      My gelding is fed 20 lb of grass/alfalfa hay. When I had the hay tested, the crude protein came back at 12%.

      He eats 4 lbs a day of a commercial feed that contains 10% protein.

      Here is how you work the numbers:

      First I determine how much total feed my guy is eating. That is pretty easy: 20 lbs of hay + 4 lbs of grain = 24 lbs in the total diet.

      Then I determine how many pounds of protein each part of the diet is contributing. I do this by multiplying the total pounds of the particular feed by its percent protein.

      The hay portion of the diet consists of 20 lbs of hay that is 12% protein.

      20 lbs of hay at 12% protein equals 2.4 lbs of protein per day.

      How did I get that number? When doing the math you have two options:

      1) You can use this formula: 20 pounds times 12 divided by 100

      2) Or you can move the decimal of the percent over to the left two places and simplify the formula to 20 pounds times .12.

      Use whichever formula is easier for you to keep straight.

      The concentrate portion of the diet contributes 4 lbs at 10% protein.

      4 lbs of grain at 10% protein equals .4 lbs of protein.

      (The math: 4 lbs x 10 ÷ 100 = 0.4 or 4 lbs x .10 = 0.4)

      Add the two numbers together to get the total amount of protein your horse is consuming per day: 2.4 lbs (from hay) + .4 lbs (from grain) = 2.8 total lbs per day.

      Next I figure out what percentage of the total diet is protein. To do that I divide the total pounds of protein provided by the total pounds in the diet.

      2.8 lbs of protein divided by 24 lbs of total diet = .1166

      To convert .1166 into a percent I have to multiply by 100 and I get 11.66%

      My horse’s diet provides him 11.66% protein.

      If I decide to add a supplement or any other ingredient to the diet, I can use the same calculations to figure out how much protein it is adding to the diet, if any.

      For example, suppose I decide to add a vitamin and mineral supplement that is 24% protein. Sounds like a lot of protein, right? Let’s see how it affects the total protein in the diet.

      The directions recommend I feed 4 oz per day. That is equivalent to .25 lbs (16 oz in a pound).

      .25 lbs of supplement times 24% protein divided by 100 = .058 lbs of protein per day provided by the supplement.

      Remember to update your totals:

      New amount of feed in the total diet is 24.25 lbs. New amount of protein in the diet is 2.858 lbs.

      2.858 lbs protein ÷ 24.25 lbs of total feed = 11.78

      Now the percent of total protein in the diet is 11.78%.

      Hope this helps!
      — Karen at KPP

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