It is very important to ensure that your horse is fed the correct portion of food by weight, not by volume!
You should ask yourself several questions: what is the age of your horse? What is the physiological stage or use of the horse? (These are easily characterized and have specific nutritional requirements. The use of the horse or activity level is more difficult to discern. The categories of activity used are maintenance or working).
What is your horse’s body weight and condition?