A lot of horse feed contains corn and it is a good feed for many cases. Corn in mixed feeds is usually cracked or rolled to increase digestibility, but some folks feed corn directly out of the field and this is generally not a good idea without testing because corn can be infected with certain toxins, including fumonisin. Fumonisin is produced by the fungi fusarium that can infect corn before harvest. Kentucky Equine Research staff indicates any stress such as weather or insect damage can make the corn plants more vulnerable to the fungus and increase the chance that the corn will be toxic. If horses ingest the fungal toxin, they can develop a neurological disease called equine leukoencephalomalacia, also called moldy corn poisoning. This disease is quite serious as it is almost always deadly and no treatment is available. Corn contaminated by the toxin cannot always be identified by looking at the corn and testing is required to determine if the corn is infected or not. Corn used at feed mills is always tested to make sure it is negative for fumonisin toxin but if you buy corn out of the field, you should have it tested before feeding it to your horses. So never feed corn directly out of the field without having it tested for fumonisin.
On the plus side, Catherine Whitehouse from Kentucky indicates corn is quite palatable as most horses like it more than any other grain except for oats. If horses have good teeth, they can break the corn kernels easily by chewing to make the corn more digestible, but that's not be a good option for older horses with dental problems. Corn is high in energy ― it has more energy than oats when you feed the same volume ― but does not cause horses to be any more susceptible to weight gain or excitability.