Since most people feed by the “scoop” rather than by weight, if is often easy to overestimate the amount of feed you are providing. Weighing out your feed will help. Additionally, be sure to consult the feeding directions on the back of the bag. Most feeds are formulated in such a way that a minimum feeding rate is required to provide for the nutrient and calorie requirements of a particular weight, lifestage and lifestyle. For example, you may be feeding your horse 5 year old 1000 lb. horse in light work one scoop of Strategy twice a day but he is losing weight. You decide to weigh out one scoop of Strategy and find that your scoop holds one pound of Strategy, therefore you have been feeding 2 lb./day total. Strategy is formulated to be fed at a minimum rate of 3-4 lb. /day to an adult 1000 lb. horse in light work in order to provide the basic nutrients and calories a horse of that description requires. So even though you thought you were providing an adequate amount of feed, it turns out that based on weight, you weren’t and you need to increase the volume fed.
Source: Purina Mills Equine Specialist