There are many good equine feeds available and yet it seems some people continue to feed other foods to horses, and one of those is bread. Apparently some people feed lots of bread to their horses, especially in Europe, and the folks at Kentucky Equine Research published some information on this subject at equinews.com. Assuming the bread is made of wheat flower, yeast, salt and water, there is nothing in the bread that is toxic to horses and can add calories to the ration. Although there is no harm in feeding small amounts of bread intermittently, it is not very nutritious and provides few nutrients. Wheat is deficient in vitamins A, D, and E as well as zinc, copper, and selenium; all of these nutrients are required in the horse's diet.
Anytime you feed anything to an animal that is not formulated for it, it is possible to shift the nutrient balance of the total diet. An example would be feeding bread, which is low in calcium because of the wheat, but wheat is not low in phosphorus. The larger amount of phosphorus interferes with calcium absorption, further contributing to the calcium deficiency. Without enough calcium in the horse's diet, the horse's body will start pulling calcium from the bones and weakening them; the more bread that is fed, the greater the chance this will occur. Bread is also too low in protein for most horses and if you are just feeding hay, a protein supplement will be required. Bread is also high in starch and if your horse has equine metabolic syndrome and is insulin resistant, bread could lead to laminitis and founder. So although many items like bread are not toxic to horses, it is still not a good idea to feed these items as there are much healthier choices for equine feeds.