Protein Quality
Crude Protein
Crude Protein, abbreviated CP, is a required metric in the guaranteed analysis for horse feeds (1), as well as feeds for other animals. Protein is a major component of blood, muscle, organs and enzymes in a horses' body (2). Indeed, after water, protein is the next most abundant substance in the composition of a horse's body (3). But, what is Crude Protein? Essentially, it is the nitrogen content multiplied by 6.25 (4). It is not a direct measure of the proteins within a feed sample. Protein is a major source of nitrogen. There can also be non-protein nitrogen sources. Examples of Non protein nitrogen sources would include urea, nitrates, nitrites as well as non-protein bound amino acids (5). So what does CP tell us? Not a lot really, just that there are sources of nitrogen in the feed which is assumed to be protein-sourced. However, a review of the ingredients should give some indication of likely sources of nitrogen and consequently protein.
Ingredients
Alfalfa hay, oil seed meals, cereal grain processing by-products, distillation by-products and amino acid supplements are some common ingredients that contribute protein and amino acids sources of nitrogen. Many of these ingredients will also provide other nutrients such as fiber, fat, vitamins and minerals, but it is the protein content that we are interested in here. Proteins are long chains of amino acids. The length of the chain and amino acid composition will differentiate one protein from another (4). In the horses' digestive tract, proteins are broken down into composite amino acids which are absorbed and then used by the horse to build needed proteins. It should be noted that excess amino acids are not stored in the horses' body; rather they are metabolized into glucose and or fat (5).
Protein
Protein is a significant and abundant substance in a horse's body. Stripped of fat and water, a horse's body is 80% protein (2). Proteins are components of blood, muscles, organs and enzymes (2). However, protein is not a singular substance. Muscle protein is not the same as organ protein, neither of which are the same as protein in blood and enzymes. Proteins are a series of amino acids linked together end to end like a chain; length and sequence of types of amino acids differentiate one protein from another in form and function (4). Indeed, in the digestion of protein, enzymes break the protein down into its constituent amino acids which are then absorbed to provide a pool of amino acids that a horse can use to make needed proteins.
Protein Quality
Proteins are a class of organic substances; differences in amino acid sequence, and number of amino acids (length) differentiates one protein from another in form and function. Consequently, proteins will vary in their respective nutritional value to the horse. The term, "protein quality," refers to the nutritional value, and is a function of a proteins' constituent amino acids and its digestibility. Of the 20 primary amino acids that a horse utilizes to make most proteins, half can be manufactured by the horse (non-essential amino acids) but the other half can only be obtained from diet (essential amino acid) (4). The more a feedstuff can provide those essential amino acids, the greater its nutritional quality; provided the proteins can be digested into component amino acids and be absorbed by small intestine.
Essential Amino Acids (4) = Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine and Lysine
Limiting Amino Acids
For horses, lysine is the first limiting amino acid, followed by methionine then threonine (2) (3) (4) (6). What that means is that when lysine is used up, protein synthesis shuts down. Unlike sugar that can be stored as glycogen, or fat that can be stored in adipose tissues, amino acids are not stored. Unused amino acids are metabolized for energy (5). Such is the importance of limiting amino acids. Some manufacturers include lysine, methionine and threonine in the guaranteed analysis. The amino acid constituents of a protein are key to the quality of a protein.
Amino Acid Profile
What is the optimal or ideal profile of amino acids, and what feed ingredients provide that ideal profile? So far, there isn't any research that can point to an ideal profile. There are ingredients that are considered good protein sources, largely based on the amount of lysine that the ingredient provides (2). Alfalfa, milk protein and soybean meal are considered good protein sources, while corn gluten, cottonseed meal and cereal grains are not considered good sources (7). Table 8-8, Amino Acid Contents of Some Horse Feed Ingredients and Forages in the National Research Councils, Nutrient Requirements of Horse (6th edition) provides the content of ten select amino acids in a variety of feed ingredients. The NRC also provides the amino acid profile of horse muscle tissue, expressed as ratios to lysine: lysine, 100; methionine, 27; threonine, 61; isoleucine, 55; leucine, 107; histidine, 58; phenylalanine, 60; valine, 62; and arginine, 76.
Table 8-8, unfortunately, doesn't include valine in the amino acid profiles of feed ingredients. However, I believe I can still use that table and the amino acid profile of horse muscle tissue to determine relative protein quality.
Amino Acid Profile Variance
Using horse muscle tissue as an, "ideal profile" reference, I can calculate how much each feed ingredient deviates from that ideal by amino acid. The sum of the deviations, or variance, in absolute value (I don't want the positive and negative deviations to cancel each other out) would provide a singular value of how closely a given ingredients amino acid profile matches the reference. The lower the value, the closer to the ideal reference. I call this value the Amino Acid Profile Variance. How did the "good" and "poor" protein sources fare?
Amino Acid Profile Variance for Ingredients Considered Good
Alfalfa meal, 20% CP = 242
Soybean meal, solvent extracted = 110
Whey protein, dried = 153
Amino Acid Profile Variance for Ingredients Considered Poor
Cereal grains - using Wheat Middlings, 9.5% fiber as example = 306
Corn gluten feed = 585
Cottonseed meal, solvent extracted 41% CP = 360
The Amino Acid Profile Variance for the "good" sources are lower than those for the "poor" sources. I also calculated the Amino Acid Profile Variance for three other common horse feed ingredients:
Beet pulp = 76
Brewers' yeast = 71
Rice bran = 220
This metric, however, only addresses the amino acid content. It doesn't address how much protein the ingredient provides. Having to feed an enormous amount of certain ingredient to get enough protein has its own issues of practicality and cost. Table 8-8 also provides the % crude protein (CP) data. Using % CP to represent the relative protein content, a scatter plot of Amino Acid Profile Variance and % CP should improve the evaluation of good and poor sources of protein.
I have read in multiple places where Soybean meal is considered the "gold standard" of plant protein sources. It certainly shows up favorably in my analysis; relatively high protein content and an amino acid profile the approaches the horse muscle tissue reference profile. The plot also agrees with the assertion that cottonseed meal and grains are not good sources. Beet pulp is a good example of an ingredient that has a very good amino acid profile, but a low crude protein content, which would indicate that a large amount would need to be fed to meet dietary protein requirement. While soybean meal, and other oil seed meals, are specifically used as a protein sources, that may not be the case for the other ingredients. Alfalfa and beet pulp, for example, have value as sources of digestible fiber (energy), bulk and other macronutrients.
Horse Feeds
Crude protein is a required metric in the guaranteed analysis portion of a commercial feed label (1). That metric, though, says nothing about the quality of the protein. A review of the ingredients, looking for good sources of protein like alfalfa, whey, brewers' yeast and soybean meal at the top of the list, will provide a better assessment of the protein quality. Knowing what ingredients are good and poor sources in a list of ingredients should also set your expectations of whether there should be additional amino acid supplementation in the list of ingredients. For example, as grains have a poor quality protein, a grain based feed should have amino acid supplements in the ingredient list and/or a good source such as soybean meal.
Amino Acid Profile Variance vs % CP for several other ingredients in Table 8-8 in the NRC's Nutrient Requirements of Horses (6th edition):
References:
(1) AAFCO. (2017). 2017 Official Publication. Association of American Feed Control Officials, Champaign, IL. USA
(2) Pagan, J.D. Protein Requirements and Digestibility: A Review. Kentucky Equine Research, Inc. Versailles, Kentucky, USA
(3) Frape, D. (2010). Equine Nutrition and Feeding. (4th Ed.). Wiley-Blackwell, West Sussex U.K
(4) NRC. (2009). Nutrient Requirements of Horses. (6th Ed.) National Academy Press, Washington D.C.
(5) Wu, G. (2018). Principles of Animal Nutrition. CRC Press, Boca Raton, FL
(6) Horsefeedblog.com.(2017, January 10th). Why are Amino Acids for Horses Important. Horsefeedblog.com. Retrieved from www.horsefeedblog.com
(7) Kentucky Equine Research. (2004, October 27th). An Introduction to Protein Supplements for Horses. Kentucky Equine Research. Retrieved from www.ker.com