Horses that are offered two sources of roughage are less likely to engage in stereotypic behaviors.
The simplest concept in the relationship between nutrition and behavior is that a deficiency of any nutrient can cause behavioral changes. Most of us remember learning that the signs of vitamin B deficiency are diarrhea dermatosis and dementia. In horses, this and other deficiencies can stimulate coprophagia. Whether re-ingestion of feces or ingestion of another, healthier animal's feces, whether or not the horse's health can improve by giving the upper gastro-intestinal tract a chance to absorb microbially derived nutrients is an unresolved question.
A horse cribbing: Note the minimal damage to the wooden surface. She is exerting about 50 pounds of force.
The usual effect of energy deficiency is less energy output. The horse will conserve its energy by reducing activities. This was well known by riding academies that kept their horses thin so the animals did not have enough energy to misbehave.
Conversely, horses can, and often do, ingest too much energy. There are two ways of dealing with an excess of calories: either burn more or store more fat. The old saying, "feeling his oats" describes the horse galloping and bucking as a means to use excessive energy. Corn often was surmised to make horses misbehave, but the cause likely is caloric; a scoop of corn has twice the calories as a scoop of oats.
We tend to feed monotonous diets to horses — three flakes of grass hay and two scoops of grain/molasses mixture. There is nothing nutritionally wrong with the diet, and we all know that the changes in diet are a risk factor for colic. Nevertheless, the diet is much less varied than that of a free-ranging horse who consumes dozens of different species of grass and forbs each week. It is little wonder that a horse might be dissatisfied. How do they express this dissatisfaction? Coprophagia, wood chewing and possibly cribbing, as well as locomotor misbehavior, such as stall walking or weaving. Horses that are offered two sources of roughage are less likely to engage in stereotypic behaviors.
Coprophagia and the other so-called oral vices probably are not caused by boredom; instead they are a result of real dietary needs.
For example: a group of three yearlings (a Plains Zebra and two Przewalski's horses) in a zoo were confined in a large indoor stall and allowed into an outdoor paddock every other day. The paddock had no vegetation; it was simply bare ground. It was raised above the sidewalk so that passersby at ground level were looking in at the animals eating feces, which upset the zoo visitors. Provisions of live oak branches redirected their foraging behavior to a more aesthetically pleasing substance. Coprophagia did not occur indoors, probably because the stall was bedded deeply with straw, and hay was available ad libitum. Freedom to exercise enriched the yearlings' environment, but the lack of anything to eat led to an abnormal behavior.
Another interesting finding was that horses stricken with equine motor neuron disease were more likely to be coprophagic than unaffected horses. The reason is that both coprophagia and equine motor neuron disease occur more frequently in horses kept in grassless enclosures. The discovery that equine motor neuron disease is a form of vitamin E deficiency explains why lack of fresh forage is an important risk factor.
Coprophagia is normal in foals, although the reason for the behavior remains obscure. Hypothetically, ingestion of feces may inoculate the cecum with micro-organisms, or — and this seems less likely — it might serve to teach foals which foods are safe to eat. Foals prefer their mother's feces to those of another mare. The behavior gradually decreases in frequency during the first few weeks of the foal's life, but it can be disturbing to a naï owner.
Wood chewing is perhaps the most destructive of nutritionally related abnormal behavior, but it is so common that it probably should not be considered abnormal. Lack of roughage appears to be a major cause, so ad libitum access to hay, and preferably another roughage source, will reduce the horse's motivation to chew wood. The behavior can result in loss of fence rails, trees and barn walls. Although many people work to prevent the problem by covering all horizontal surfaces with metal, horses are able to scrape their teeth on vertical surfaces, too, eventually eating holes in a wall. Most commercial repellents are effective only temporarily. Coating surfaces with creosote can help, but the horse likely still will be motivated to eat wood. It is always best to change the horse's motivation by meeting its needs rather than simply thwarting its attempts to reach a goal. One commercial product is not a repellent, but a food additive containing essential vitamins, micro- and macro- minerals and fatty acids. There are no scientific studies of its efficacy.
Horses chew many times — about 40,000 per day when grazing and when hay is available. What happens when a horse does not have the opportunity to chew? To answer this question, we fed horses a complete pelleted diet for three weeks and compared their behaviors to behaviors when they were fed free choice hay. The horse chewed only 10,000 times per day when eating pellets. There were two major differences in the behaviors of the horses when they were fed hay versus pellets:
Horses without hay spent a great deal of time — about 11 percent of their 24 hours — "grazing" on the wood chip bedding. They probably were looking for stray pellets, but they also were eating their manure and some of the shavings. When fed the hay diet, they "grazed" less than 1 percent of their time.
The other difference: Behavior was less consistent from hour to hour when the animals were fed pellets. Immediately after the pellets were fed, they ate for almost an entire hour; then eating fell to less than 10 percent of the day until the next feeding. In contrast, when hay was available, the horses spent 60 percent of their 24 hours eating except the last few hours before fresh hay was fed.
Not only does their behaviors change when horses are fed pellets, but the horses also appear to be motivated for long-fiber roughage or the opportunity to chew. While being fed a complete pelleted diet, they would press a switch 13 times to get access to hay; they did not press the switch at all for the pellets.
Physiologically, it is better to have a constant trickle of feed rather than large boluses of feed to avoid surges, as well as sudden changes in gastrointestinal motility. The best way to avoid these fluctuations is to feed free choice hay or maintain the horse on pasture. Another suggestion is to use diets fortified with fat. Fat has become a popular calorie source for horses. Substitution of fat for carbohydrates in the diet might lower the incidence of myopathies. In addition to its effect on health, it might also function to quiet the horse.
Cribbing is another equine behavior problem that may be affected by diet. Although the tendency to crib probably is inherited, introduction of grain to the diet is the usual precipitating factor. Horses crib mostly after they eat, in contrast to kicking the stall wall, pawing and neighing that precede feeding. One hypothetical cause of cribbing is gastric ulcers, and there is some evidence that an antacid supplement reduces cribbing, although we have not yet verified the claim. Cribbing was suggested to add more saliva and thus more bicarbonate to the stomach. Horses do not salivate when they crib, so little bicarbonate is added. Maintaining a horse on a diet of free choice hay and plain oats is the best dietary treatment. The role of opiates in cribbing is unclear. Opiates released by sweet taste might cause cribbing, or cribbing can release opiates. The experimental evidence favors the former hypothesis.
In conclusion, more attention should be paid to the horse's diet by the veterinarian and the owner. Increasing long-stem fiber and dietary variety will reduce the frequency of stereotypic behavior. Diet can be an important aspect of equine welfare.
Katherine A. Houpt earned her veterinary degree from the University of Pennsylvania in 1963 and her PhD in biology in 1972. She is a professor and director of the Animal Behavior Clinic at the College of Veterinary Medicine at Cornell University. She is board certified by the Animal Behavior Society and is certified as an applied animal behaviorist by the Animal Behavior Society. She is the author of the text "Domestic Animal Behavior," and her research is centered on equine welfare.