Lysine: The Most Important Nutrient You're Not Thinking About.

Have you ever looked at a feed tag and wondered "What the heck is lysine? And what is it doing on my feed tag?" As it turns out, lysine plays a critical role in muscle growth and repair. Lysine might be one of the most important nutrients that you’re not thinking about.

Lysine is an essential amino acid, meaning horses cannot synthesize it on their own and must obtain it through their diet. Of the 20 naturally occurring amino acids, 10 are essential in horses, lysine being the most critical. It is considered a limiting amino acid, meaning the amount of lysine available in the diet directly limits how much protein the body can build, regardless of how much total protein is being fed.

When you look at a feed tag, the first number you'll see in the nutritional analysis is crude protein percentage. I'm going to tell you to skip it, scandalous! I know. Crude protein is simply a measure of how much nitrogen is in the feed, not whether your horse can actually use it for muscle building and repair. The number I want you to look for is lysine percentage, because that is what tells you whether your horse has what they need to build and maintain muscle.

A quality grass hay will typically provide around 2 g/lb. of lysine, while a legume hay like alfalfa will provide approximately 4 g/lb. A 1,000 lb. adult horse at maintenance requires roughly 25 g of lysine per day. If you are noticing difficulty building topline or muscle mass, poor hoof quality, or a dull coat, lysine deficiency is worth investigating before adding more protein to the diet.

If your horse is not meeting their daily lysine requirement through forage alone, there are a few easy ways to address it. Alfalfa hay is the most accessible and often cost-effective option because even adding a small amount of alfalfa (2-3 lbs/day) to a grass hay diet can meaningfully improve lysine intake. A quality ration balancer is another excellent option, as most are formulated with lysine as a priority and will list it clearly on the tag. Standalone lysine supplementation is also available and can be useful for horses with specific dietary restrictions or those in heavy work with higher demands. If you feel your horse is deficient in lysine, work with a nutritionist to assess whether their daily requirement is being met.

With the marketing currently towards mineral balancing, the importance of lysine often gets overlooked but it plays a key role in what we like to see in horses: a beautiful topline, strong muscle mass, shiny coat, and healthy hooves. The good news is that lysine deficiency is one of the easier deficiencies to address once you know how to spot it. So next time you pick up a bag of feed skip the crude protein and go straight to the lysine.