That is absolutely not true. If it were mine would be spontaneously combusting into flames. Corn is a very high energy feed. This is an exerpt from the book Equine Clinical Nutrition.
Corn, like other grains without hulls, is low in fiber and higher in both energy content and density than oats. It proves twice as much energy as an equal volume of oats (Table 4-5). Because of this, some feel corn has a tendency to cause obesity or to make a horse high spirited. If equal volumes of corn and regular oats are fed, this is true, since the horse is receiving twice as much energy from the corn. However, if equal amounts of energy, not equal volumes of grain, are fed, corn does not have any greater tendency to cause obesity or to make a hose too high spirited than other cereal grains. How spirited a horse acts is directly related to how good it feels and how much energy it has consumed compared to its needs.
Contrary to another popular belief, corn is not a "heatening feed." It is sometimes fed during the winter and not during the summer because of this mistaken belief. Corn is a "hot feed" when hot implies high energy, but not when it implies high heat production. The heat produced in the utilization of corn is one-third less than that produced in the utilization of oats. Only 41% of the gross energy in corn is given off as heat, as compared with 66% of the gross energy in oats. This is because corn is lower in fiber, and the greatest amount of heat produced in feed utilization is from microbial fermentation of fiber; therefore, the lower a feed's fiber content, the less heat produced in its utilization. However, because corn has a high energy density and because energy needs are increased during cold weather, it is a good winter feed.
Heat in grains implies "ENERGY", not hot heat.