Can a horse have too many amino acids? Is there a maximum level?

Amino acids are the building blocks of protein. As with anything, too much protein can cause problems. It is best to feed your horse at the recommended level of protein for his age and stage in life.

When a horse is fed more protein or amino acids then they need, the horse excretes the excess as ammonia and urea molecules in the urine. In order to clear all the extra protein, a horse will increase his water intake and urinate more. Larger quantities of this noticeably stronger-smelling urine can lead to respiratory problems when a horse is kept up in a stall. Excess protein can also interfere with calcium absorption in young horses: Studies show that when 25% more protein than needed is fed to young horses, it can negatively impact growth rates.

The best way to avoid problems is to feed the recommended amount of a high-quality protein to your horse. Below are the widely accepted rules of thumb to estimate the required protein in the total ration for different classes of horses. The total ration includes the following: pasture, hay, grain, and supplement.

• Growing horses: feed 14% to 16% protein
• Lactating mares, older horses: 12% to 14%
• Mature horses: 8% to 10%

Studies have shown that workload does not significantly increase protein requirements, as long as a horse’s digestible energy requirements (DE) are met.

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.

The National Research Council (NRC) estimates the daily lysine needs of mature 1,100-pound horses to be:

23 grams of lysine for horses that are idle or in light work

46 grams for horses in intense work

More is needed in growing horses, depending on their age.

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 the amino acid lysine.

The following 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

If you are feeding your mature horse a well-balanced diet that includes good quality forage and the recommended amount of a fortified balancer pellet or a commercial feed, your horse should be getting adequate levels of protein that include all the essential amino acids he or she needs.