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.
There are also reported cases of coprophagy in horses older than 6 months of age consuming diets that provide very adequate levels of protein, fiber, and vitamins and minerals. These reports appear to be more common in the springtime in stabled horses and are reported most frequently in young stallions. Boredom could be the reason coprophagy appears more in confined horses than pastured horses. Parasite infestation is a concern in these animals, so routine facility cleaning and a good deworming program are important.
Geophagia refers to eating dirt and is reportedly fairly common in feral horses. Geophagia refers not to horses taking in dirt while grazing close to the ground or eating grain off the ground, but is a behavior where horses actively bite into the ground with the intent of eating dirt. Researchers have proposed that horses eat dirt in search of salt or minerals, but soil analyses shows no consistent mineral profile of consumed versus nonconsumed soils; in fact, the soils tested varied tremendously in mineral content. Domestic horses consuming diets containing plenty of salt and mineral have also been seen consuming dirt.
Anecdotal evidence indicates dirt eating might be more common in stallions than mares or geldings, but no studies of gender effects on geophagia have been reported. Although geophagia is generally harmless, consuming sandy soil can cause colic or diarrhea. Some horses are more prone to eating sand than others, even when eating the same diet under the same conditions as horses that don't consume sand. Grazing sparse pastures on sandy soil, consuming grain from the ground, and being thin and/or young are all factors that contribute to increased sand ingestion.
If owners observe individual horses exhibiting unusual eating behaviors such as coprophagy or geophagia, they should evaluate that animal's diet for nutritional balance, forage availability, and the general environment for potential causes of the behavior. A veterinary exam to detect parasite infestation or other health issues could also be warranted. If the diet is adequate, the horse is healthy, and other factors are not at play, then it might be a simple case of boredom. Decreasing time spent in confinement, providing a companion, or increasing exercise might help alleviate the problem.