A horse’s protein needs can’t be defined solely by a percentage. A 1,100-pound horse at maintenance needs 660 grams of crude protein a day, which he can meet with just 22 pounds of a 6.6% protein grass hay. For grain, 11 pounds of a 13% protein grain will also meet the needs, although few horses need this much grain.
A typical diet for a horse in light work is about five pounds of grain and 25 pounds of hay per day. Five pounds of a 10% protein grain provides 227 grams of protein, while a typical 7% protein grass hay gives him 796 grams. This is 1,022 grams, 55% more than minimum needs.
For working horses, if your base diet is adequate in protein, you’ll likely cover added protein needs simply because you have to feed him more to be sure he gets enough calories to support the work. For example, a 1,100-pound horse in moderate work needs 984 grams of crude protein a day. If you increase his 10% protein grain to 10 pounds to add calories and leave hay the same, his protein intake is 1,249 grams/day, 27% over his need.
However, your horse also needs to consume adequate levels of essential amino acids. The horse’s body isn’t capable of making these nutrients, so they must be present in the diet at a certain minimal level. They often aren’t.
There are at least 10 amino acids considered essential, although little is known for most about the actual required levels in horses. We do know the lysine requirement for a 1,100-pound adult horse is about 0.3% in the diet. This means 23 grams per day at maintenance and 34 grams per day with moderate work. The requirement of sulfur-containing amino acids, especially methionine and cystine, is about 0.22.
High-quality, fortified commercial grain mixes contain about 0.6% lysine and 0.45% methionine/cystine. To meet the lysine requirements from this source you must feed at least 10 pounds per day at maintenance and 15 pounds per day for moderate work.
Not many people feed this much grain, and doing so would be overkill on calories and total protein. Hay isn’t the answer either, since grass hays are a poor source of essential amino acids. This is why most horses can benefit from a high-quality protein supplement or a supplement containing key amino acids, such as lysine and methionine.
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