Are acorns poisonous to horses?

There are many unknowns about acorn toxicity in horses, but it’s wise to take whatever preventive measures you can to minimize your horse’s exposure. | ©

Q: I have received conflicting information in regard to acorns being dangerous for my horses. Is it unsafe to have them in my pastures?

CYNTHIA GASKILL, DVM, PhD, Diplomate ABVT (American Board of Veterinary Toxicology)

A: Acorns grow on oak trees, which are widespread across North America. Although acorn poisoning has been widely documented in cattle, very few cases have been confirmed in horses. However, in the rare instances in which horses have been known to eat large amounts of acorns—or any other part of the oak tree—in a short period of time, they have sometimes experienced severe and even fatal reactions.

Symptoms of acorn toxicity include depression and loss of appetite followed by digestive-tract issues, such as colic, gastric upset and diarrhea (often bloody). Kidney and possibly liver damage may also occur as well as bowel obstructions and ruptures. The more acute the symptoms, the higher the likelihood of death. Horses experiencing a gradual onset of symptoms—over a number of days rather than hours—have a better prognosis.

Ingesting a small amount of acorns is probably not a risk to most horses. Unfortunately, we don’t know how many it takes to add up to a toxic dose because of several varying factors. The concentration of toxic compounds, called tannins, in oak acorns, bark, leaves, twigs and sprigs varies depending on the time of year—and even from year to year. It is higher in younger acorns, buds and branches than in more mature ones, so it may be more dangerous for horses to eat them in the spring than in the fall. Acorns also seem to have higher levels of tannins in certain years, which, unfortunately, we don’t know enough about yet to predict.

Sensitivity to acorn toxicity varies among individuals, so it’s hard to know which horses might be most vulnerable. Exceptionally inquisitive horses—especially young ones—who like to put everything in their mouths, may be more at risk of overeating acorns than older horses, who are less likely to experiment with different foods.

In general, horses don’t find acorns and other parts of the oak tree particularly desirable. Fresh immature leaves are more palatable than mature ones, so they may be more tempting in the spring. In general, though, most horses still prefer to eat grass and good-quality hay. They’re unlikely to eat acorns in large amounts unless their caloric intake from other sources is limited. An example of a high-risk scenario would be a dry-lot (grass-free) paddock with several oak trees, limited hay and a large number of hungry horses competing for that hay. A big fall storm that knocks branches and acorns to the ground could exacerbate this situation.

Because of these unpredictable factors, it’s wise to take whatever preventive measures you can to minimize your horses’ exposure to acorns. All of the more than 60 oak species in North America should be considered potentially toxic. So trim any trees adjoining your fence lines to prevent both branches and acorns from falling inside the pastures. If you have oak trees in your pastures, consider building fences around them, far enough away so that your horses can’t reach the branches by leaning over them. Promptly remove fallen branches and acorns after heavy windstorms. Most importantly, supplement inadequate forage with plenty of good-quality hay.

Here in Kentucky, we have horses and oak trees everywhere and yet acorn-poisoning cases are still extremely rare. Being aware of potential hazards in pastures and taking precautionary measures, such as the above recommendations, are great steps that all owners should make to protect their horses.

Cynthia Gaskill, DVM, PhD, Diplomate ABVT, is an associate professor of veterinary toxicology at the University of Kentucky’s department of veterinary science and is the head of toxicology at its Veterinary Diagnostic Laboratory. The lab studies the effects of drugs and poisons—both natural and man-made—on all species, from horses and cattle to exotic zoo animals. Dr. Gaskill has also adopted several retired Standardbred racehorses.

This article originally appeared in the September 2015 issue of Practical Horseman.

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