Oat hay is a reasonable forage for mature horses. It is made from oats and, depending on the stage of maturity at harvest, the hay may be a very good source of digestible energy; however, if it was very mature at harvest and most of the grain has shattered and is gone, the feeding value will be reduced. You may find your horses eat the seed heads and leaves and leave the stems behind, increasing the waste. For horses that are mature idle horses, the oat hay can make up a significant portion of the forage in the diet. A concern with oat hay is nitrate levels. If the hay was harvested after a frost or the grain crop was hurt by drought, the nitrate level may be high. While horses seem to tolerate a higher level of nitrate than do cattle you may want to have the hay tested for nitrate and other nutrient levels as well. Good quality oat hay with a reasonable amount of grain in the seed heads can make up a significant portion of the daily roughage your horses are fed. It is worthwhile to have the hay tested to determine the nutrient value; then plan your feeding program based on that. When sampling the hay, use a forage probe and take 20-25 core samples from the lot of hay, mix those cores, and submit for an analysis. You should contact the Cooperative Extension Service in your area for help in sampling the feed. They may have a forage probe you can borrow, and they should be able to give you advice on where to have the sample analyzed.