Although many people think high-protein feed provides more energy, that's not the case. Horses can convert protein to energy, but the process is inefficient-it produces lots of body heat and by-products that increase the ammonia smell in urine. The best diet provides only as much protein as your horse needs to build and repair body tissues.

What your horse needs:
Protein needs vary at different stages of a horse's life. Growing horses and pregnant mares need extra for tissue building; an old horse (over 18) may need extra protein, because the ability to digest it often declines with age. Here are percentages to look for on feed labels. The numbers are averages and assume your horse is getting grass hay. (If you feed alfalfa, which has a high protein content, the percentage of protein in your horse's feed can be less.)

How to provide it:
The simplest way to get the right protein level is to use a feed formulated for your horse's stage of life. That way, he gets all the other nutrients he needs, too.

Most adult horses aren't likely to be protein-deficient. Oats and many other grains typically contain 10 to 13 percent protein. Growing pasture grass typically has 12 percent or more.

Mature grass hay may have as little as 5 to 6 percent protein, so if your horse eats primarily mature hay, and little or no grain, he may not be getting enough protein.

Boost the percentage by adding some alfalfa to your horse's ration or using a high-protein feed. Some specially formulated feeds, designed to be fed in very low amounts (such as 1 or 2 pounds a day), contain 25 to 30 percent protein.

Elaine Pascoe is a contributing editor for Horse & Rider magazine and is based in Roxbury, Conn.