Excess carbohydrates or fat causes obesity in the horse, which is detrimental to any animal. The rule of thumb for a horse being in good flesh is that you cannot see the ribs, but you can feel them if you rub your hands across them. Lack of sufficient energy in the diet is the primary cause of a thin horse, so give the horse more feed and increase the grain portion since it contains more energy per pound than roughage. Fat horses need less or no grain, as good quality roughage can meet the needs of most idle mature horses.

Feeding Horses Fats and Oils - Rhonda Hoffman, PhD, answers an EQUUS reader's questions about adding oil to horses' diets.

Dietary fat has been used to increase the energy density of horse feeds, to lower the bulk in the bowel and decrease fecal output, and to reduce health risks associated with high intakes of sugar and starch. Dietary fat also provides essential fatty acids, aids in the absorption of fat soluble vitamins, and may improve hair coat quality.

There are a number of advantages of adding fat, usually in the form of vegtable oil, to a horses diet. However for the average trail or show horse with no problems with nervousness or tying up and receive adequate amounts of forage, the extra trouble is difficult to justify. For race or show horses whose grain portion of the diet is approaching 50% of total intake by weight or horses that have special medical problems, vegetable oil can significantly improve a horse's health and performance. This report contains information on deciding whether adding fat to the diet of horses may benefit you and how to do it.