We expect our horses to eat grass, hay, grain, and maybe treats like carrots or apples. We don’t expect them to eat things like dirt or manure, but sometimes they do. While this type of behavior can be both offensive and worrisome, is it actually harmful to the horse?

The term “pica” refers to persistent eating of non-nutritive substances for at least one month’s time. Many mammals, including humans, are known to do this, and it is most common in younger animals. In some cases specific nutritional deficiencies might trigger unusual cravings, such as a long-term phosphorus deficiency causing cattle to eat bones or significant amounts of dirt. In horses these behaviors are not defined as stereotypic stable vices because they appear to represent a normal physiologic or foraging response.

Unusual oral behaviors in horses include coprophagy and geophagia. Coprophagy, eating manure, is normal in young horses from 5 days to 2 months of age. Foals typically eat their mothers’ manure but occasionally consume their own or an unrelated adult’s feces. This practice is more common in foals confined to stalls than those on pasture, and is uncommon after 6 months of age. Researchers and veterinarians speculate that the coprophagy in foals is a mechanism for populating the digestive system with bacteria and protozoa necessary for a fully functioning cecum. These microbes are required for effective fiber digestion, which is necessary for a foal to fully utilize a grass or hay diet as he grows and consumes more forage and less mare’s milk.

Scientists have not identified a nutritive motivation for coprophagy in foals, but mature horses eating protein-deficient diets will often begin eating their manure as well. In these cases coprophagy ceases when adequate protein is provided. Horses in starvation situations or those without adequate forage (consuming less than 1.3 pounds of hay per 100 pounds of body weight) have also been observed to eat manure