Proteins are formed from amino acids. Many of the amino acids are chemically formed (synthesized) within the horse's body, but some are called essential amino acids because they have to be obtained through food that is ingested. Horses differ from humans in some regards, but the basic amino acids that combine to form the protein structures of the body such as muscle are found in plant and animal sources. Legumes like soybeans contain the amino acids that are essential in human diets (we can't synthesize them) and that we get from animal meats, so it is important for vegetarians to include plant sources like legumes in their diets, since other plants don't contain these essential amino acids.
So, yes, plants contain proteins (amino acids) and horses are designed to utilize the amino acids supplied in the forages they eat, and they don't require animal proteins in their diets.
Add......here is some information on protein needs in horses that is from a well respected veterinary sight, so the information in it is accurate. As you'll see, many forages are poor sources of important limiting amino acids like lysine and methionine, so it pays to have some knowledge on this when designing your horse's feeding program.
http://www.thehorse.com/ViewArticle.aspx?ID=9707
So, yes, plants contain proteins (amino acids) and horses are designed to utilize the amino acids supplied in the forages they eat, and they don't require animal proteins in their diets.
Add......here is some information on protein needs in horses that is from a well respected veterinary sight, so the information in it is accurate. As you'll see, many forages are poor sources of important limiting amino acids like lysine and methionine, so it pays to have some knowledge on this when designing your horse's feeding program.
http://www.thehorse.com/ViewArticle.aspx?ID=9707