In general, feeding a mature adult horse 2% of its body weight in long-stem, good-quality forage each day comes close to meeting most of a horse’s energy and protein requirements for maintenance and a desirable body condition score, but that isn't always easy in drylots. For horses that are still growing, in regular training, or need more nutrients to maintain body condition, you should follow manufacturers feeding directions on the feed bag of your choice. It's also wise to consult your veterinarian and/or equine nutritionist to tailor a feeding program to each horse and his or her particular needs.
If you don't have access to a pasture, the first thing you need to do is a hay analysis. A hay analysis lets you know exactly what nutrients are available to your horse from a specific batch of forage. If there are changes in the type of hay (legume vs. cool-season grass vs. warm season grass vs. cereal grain hay) offered, the cutting of hay, or source of forage, then you’ll want to get a new analysis to enable appropriate adjustments to the feeding program. Hay analysis is relatively inexpensive and can save much more on feed costs, especially when feeding hay year-round.
During some times of the year, local hay can be in short supply. If this is the case where you live, you can purchase byproduct feedstuffs (e.g., cottonseed hulls, soybean hulls, distiller’s dried grains, oats, etc.) to provide bulk roughage in the diet, as well as other nutrients. Use of these substitutes is mostly recommended for mature adult horses rather than young, growing horses. Due to wide variations between different byproducts, as well as between batches of the same feedstuff, test the feed for specific nutritional values. Consult your veterinarian and nutritionist to help fine-tune your use of these substitute feed products. Other forage substitutes are available in the form of hay cubes, alfalfa pellets, beet pulp, and compressed hay.
Neurologic or muscle disease and immune deficiency problems can occur when horses don’t consume adequate vitamin E, a nutrient normally found in green grass. Most modern commercially available equine-formulated feeds contain vitamin E on the guaranteed analysis label. If such commercial products are fed, there may be no need to supplement further. I recommend that you ensure your horses take more vitamin E—either with a supplement or commercial feed—as vitamin E is vital when horses have no pasture access and/or are growing, breeding, lactating, or in heavy exercise.
If you have concerns about vitamin deficiencies, consult your veterinarian and/or an equine nutritionist. Be sure to provide plenty of fresh, clean water, and salt, as these are always important to good horse health no matter the environment.