Protein isn’t just for bodybuilders. It’s the foundation of your horse’s muscles, skin/hooves, and enzymes. We know that muscle has a high percentage of protein. We know that we need adequate protein in hay. But what do we really know besides that?
Protein is integral to body systems. Its function can be broken down into two different parts – structural and signaling.
The major reservoir of protein exists in the muscle. When the horse loses weight or is very ill, the first thing we notice is a loss of muscle. The muscle is sacrificed to provide essential amino acids to keep vital organs functioning and create glucose for energy.
◆︎ Protein digestibility
Protein molecules are very large. Because the large molecule cannot cross the intestinal barrier, enzymes cleave them into their constituent amino acids. This occurs in the stomach and small intestines. Amino acid absorption occurs almost entirely in the small intestine. Amino acids that are not absorbed in the foregut travel to the hind gut. There, they will be fermented and utilized by the microbial population – and not absorbed by the horse.
This fact is important if you feed a mature hay, like late-cut timothy. The proteins could be locked within indigestible lignin. This could result in lower protein available to the horse.
◆︎ Hay or feed compositions
Crude protein measures the total nitrogen of the feed. For normal horses, there is a wide range of protein that is suitable. When horses have had significant muscle loss (think illness, PPID, or starvation), attention to protein content is imperative.
◆︎ Is hay enough?
We’ve all heard of theTop 3 limiting amino acids: lysine, methionine, and threonine. Horses that have had significant muscle loss frequently need more of these than a horse at maintenance work. If your hay is marginal, it could be prudent to add additional protein.
◆︎ Not all protein sources are the same.
When investigating the choices for additional protein, it’s important to remember that the amino acid profiles may differ.
Whey protein: This is an excellent source of leucine, which is a signaling molecule to create more muscle. It also has rapid foregut absorption. It contains less arginine and phenylalanine compared to plant sources.
Soy protein: Present in many high protein feeds. It has excellent lysine content. However, it does not have adequate methionine for optimal hoof and coat health.
Pea protein: This is highly digestible and is hypoallergenic. It has a good lysine profile. It lacks sufficient methionine and cysteine.
Rice Protein: This has a good methionine and cysteine profile. It is highly deficient in lysine. It may also contain the heavy metal arsenic.
Collagen: This is high in glycine, proline, and hydroxyproline. It does not contain tryptophan, and is deficient in branched chain amino acids, particularly leucine.
Knowing that the profiles differ, it’s important to read the labels to determine if there is a sufficient blend of proteins. Alternatively, you can use a high protein feed, plus add the appropriate amino acids to make it more suitable.
◆︎ Special consideration for horses with PPID
One of the concerns that we have for our horses with PPID is muscle wasting. Classical presentation includes loss of topline, swayback, and potbellied appearance. These horses have lost muscle due to the catabolic breakdown from the PPID. To help these horses, they need a higher quality of protein, closer to 12% - 14%. Additionally, they would benefit from a high leucine content. Leucine is a signaling molecule (via the mTOR pathway) to build more muscle.
◆︎ Parting thoughts:
With the plethora of choices available at the feed store, knowledge is key to making good decisions when feeding for our horses.
We do the best we can for our horses. Understanding our horse's needs is a part of that.
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Disclaimer: Statements regarding dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease or health condition.
©Joan Kulifay, MSc. 2026