Amino acid levels are a good indication of the digestibility and overall quality of protein. This is why I recommend using feeds that guarantee lysine levels. A 12-percent protein feed with a guaranteed lysine level of 0.6 percent will give you better results than a 14 percent protein feed with a lysine level of 0.4 percent. It is the amino acid content of the feed, not the total protein level of the feed, that will determine how well your horses use the protein they ingest. When specific ingredient listings are used, you can evaluate the actual protein sources.
Soybean meal offers the best quality from plant protein sources.
Milk protein sources, such as whey or dried skim milk, are good protein sources, but are normally used only in feeds for young horses. Many feeds contain added amino acids such as lysine and methionine. A feed product that uses soybean meal with added lysine will provide a high-quality protein source to your horses. Not all mineral sources are equal in digestibility. I would select the feed with copper sulfate and zinc sulfate since mineral sulfates tend to be more digestible than mineral carbonate or oxide forms.