Horses at maintenance need about 8 to 10% of their diet to be protein; this will only increase very slightly if exercising but will increase more significantly if breeding, lactating, or growing (up to 16% at times). Feeding excess protein will not create a problem unless the horse has existing kidney problems. High-protein diets will cause a high level of ammonia to be excreted in the urine. If horses have kidney problems and are consuming high levels of protein, it could create further kidney complications. However, healthy horses should not have a problem, but it is crucial that these horses be provided with free access to fresh water. Usually diets high in alfalfa or another legume hay will contain excess protein (alfalfa can have protein levels as high as 25%). Most grass hay will contain enough protein to be adequate for maintenance horses; however, this varies greatly, depending on quality and maturity of the hay.
Crude protein refers to the amount of protein that is in the feedstuff. It does not represent the amount of protein that is "digestible" in the horse. Digestibility of protein varies between roughly 65 and 85% depending on which feedstuff is being digested. It is called crude protein because it is a rough calculation, or the amount of nitrogen in the feed x 6.25.