For a horse to gain weight, he needs to take in more calories than he burns (such as through normal metabolism or exercise). The source of those calories can be anything: hay, grain, vegetable oils, or, yes, even bread. Bread is mostly wheat flour, a source of starch, which is digested to glucose in the small intestines and absorbed. The increase in blood glucose gives some horses a short-lived “sugar rush” of excitable energy, and for others it potentially causes metabolic disturbances such as insulin resistance. Further, if the starch isn’t digested in the small intestine, it will continue through the digestive tract to the large intestine, where it is rapidly fermented. This may cause excessive acid and gas production, leading to colic or even laminitis. Furthermore, bread is not nutritionally balanced for a horse, potentially lacking in nutrients such as protein, calcium, and zinc, as well as vitamins A, D, and E. Therefore, it shouldn’t be fed to replace a commercially formulated grain mix. Some breads’ high-starch/low-fiber content is not ideal, and if any bread is to be offered, I would recommend higher fiber breads. Also, any additives to the bread (such as chocolate or poppyseeds) may cause positive drug test results.
So while a horse may consume bread as a calorie source, there are safer and more effective ways to accomplish weight gain. Vegetable oil is an excellent source of calories and doesn’t have any glucose to cause the aforementioned potential negative effects (though it doesn’t have any vitamins and minerals either). Introducing high-fiber feeds such as beet pulp (without molasses), rice bran, and even additional hay (or hay cubes) can also be effective ways to add calories to your horse’s diet. Consult an equine nutritionist if you are having problems getting your horse to gain weight.