Q. My horse maintains his body condition well on forage alone, but I’m worried he doesn’t have enough protein in his diet. Can horses get enough protein from hay alone?

A. There is a common misconception that grass hays (which are the type of hays most commonly fed to horses) don’t provide horses with adequate protein. However, most mature horses only need about 10% crude protein in their diet. If fed an adequate amount, grass hays can often fill this need.

Mature Horses’ Protein Requirements

According to the Equi-Analytical feed database, on average, grass hay provides 10% crude protein on an as-fed basis. A 500-kilogram (roughly 1,100-pound) horse at maintenance has a crude protein requirement of 630 grams. If fed 2% of his body weight (10 kilograms or 22 pounds) of this grass hay, the horse will receive 1,000 grams of protein—plenty to fill his needs. The same horse in very intense work requires about 1,004 grams of crude protein each day, so this hay at this intake can meet a broad range of requirements.

Of course, protein content varies by hay type, and legumes provide significantly more than grass hay while grain hays provide less. If we do the same calculation feeding the same horse the same amount of an oat hay with 7.5% crude protein, the protein intake is only 750 grams. This is still enough for the nonworking horse but not enough to support a hard-working horse’s needs.

If the amount of hay is restricted to 1.5 percent of body weight (as it might be if a horse needs to lose weight) in the previous scenarios, the grass hay will provide 750 grams of crude protein (still okay for the horse at maintenance) while the oat hay will only provide 563 grams—below even maintenance requirements. In these cases, it’s not that the hay cannot provide enough protein, but, rather, how the hay is being fed that dictates whether it is enough.

More Protein, Please

Protein content becomes more of a concern when feeding lactating broodmares and growing young horses, especially those less than a year old. If our 500-kilogram horse was a broodmare in the second month of lactation, her crude protein requirement would be 1,530 grams each day. A 6-month-old weanling with an expected mature weight of 500 kilograms would require 676 grams of protein each day. The latter amount seems easy to meet until we remember that this weanling likely only weighs about 215 kilograms (about 475 pounds)—even if he’s eating 2% of his current body weight, he would only be consuming about 430 grams of protein from the grass hay and 322 grams from the oat hay. This is why most lactating broodmares and young horses need additional supplemental protein and nutrient sources in their diets.

Other Considerations: Protein Availability and Quality

Another consideration is the hay’s maturity of the hay. As plants mature, their structural carbohydrate levels increase and protein levels tend to drop. So, more mature hay will typically have lower protein levels than that same hay harvested earlier. That structural carbohydrate might also make it hard for the horse’s body to access the protein.

The structural carbohydrate requires microbial fermentation in the hindgut, which is located behind the small intestine. However, protein is digested and absorbed in the small intestine. Therefore, if the protein in bound up within complex carbohydrate it might not be fully available to the horse, even though on paper the amount of crude protein appears adequate.

The Bottom Line

At the end of the day, horses do not actually have a requirement for crude protein. What they require are the amino acids that make up protein. Some amino acids are essential and must be provided in the diet as the horse’s body can’t produce them. The horse can make other amino acids, which is why they’re considered nonessential in the diet. And, the greater the proportion of essential amino acids there are in a protein, the better the protein quality. Therefore, it is possible that a hay could provide enough protein but it might not be the best quality.

That said, because we typically feed significantly more total crude protein in the diet than required, most of the mature horse’s essential amino acid needs will be met. If your horse is having problems with issues such as poor hoof, skin, or coat quality; poor topline development; or slow wound healing, he might require a source of better-quality protein than your hay is providing. In such a situation a high-protein ration balancer might be a good option. An equine nutritionist or your veterinarian can help you determine the best solution for your individual horse.