Having your hay analyzed is a great idea. It is the only way to determine the actual nutrient content of your hay. It is important to know this so that you can be sure your horse is consuming an adequate diet. You may or may not need to feed grain depending on the quality of your hay. The better the quality of the hay you feed, the less grain you will need to feed. This can be a significant savings.
When discussing hay quality or looking at your hay sample analysis results the acronym ADF is often heard or seen. ADF stands for Acid Detergent Fiber and is the percentage of the plant material in the forage that is difficult for your horse to digest. This indigestible part contains cellulose, lignin, and silica.