Acorn Toxicity in Horses

Acorn toxicity

It’s been a banner fall season for acorns. The deluge started a few weeks ago and all of a sudden we noticed Curly and Zelda were spending a lot of time under the oak tree in the back pasture. It’s a glorious tree and I never gave it a second thought, other than to admire it, until the horses started to chow down on the acorns.

Most years, an oak tree produces 25-30 pounds of acorns. In a “mast” year, a single oak can produce up to 250 pounds of acorns. All reports are that we are not in a mast year, but no one told the tree in our pasture.

Although Zelda and Curly have access to lots of grass, they appear to find the acorns irresistible.

What’s the big deal about horse’s eating acorns?

While acorns may sustain squirrels, chipmunks and deer for the winter, the tannins and gallotannins found in the leaves and acorns are toxins that are poisonous to horses.

According to Tufts Veterinary School, “Together, the oak toxins affect the kidneys and the gastrointestinal system as direct toxins and can also cause impaction colics in horses that eat a very large amount. Typical symptoms include depression, inappetence, colic, diarrhea, ventral edema, apparent straining to urinate, and red-brown urine. The rare horse with oak toxicity often needs sustained fluid therapy for kidney damage, as well as supportive care for the colic or diarrhea.”

When I called my vet to find out if the acorns would hurt Curly and Zelda, her enigmatic response was that most horses don’t eat enough acorns to be a problem because acorns are bitter. However, there was no way to know how many acorns the mares actually were eating. Or how many acorns it would take to cause a problem. Many of the symptoms are hard to track if your horse lives out 24/7 — especially the color of their urine.

Acorn Removal

Removing the acorns is tricky because there are so damn many of them. Eventually, we bought a nut rake, which works reasonably well, even if the task does appear to be Sisyphean. You roll the basket over the acorns and the wires separate enough to trap the acorns in the basket. You don’t actually need to collect the tops, only the nuts themselves are appealing to horses, but even with selective raking, after two hours I’d only made a dent in the supply. I wish the squirrels and chipmunks would do a better job of eating them; they made a beeline for the piles that I created, I guess they’re too lazy to pick them up individually. Complicating the matter is that the pasture is filled with poison ivy, so after collecting the acorns, it’s essential to not touch any of them.

From what I’ve read, this year has been unusually problematic, with horses that never showed any interest in the toxic treats suddenly acting like addicts. I wonder if the drought we had all summer made the acorns particularly tasty? From what I’ve read, the taste of the acorns and their relative toxicity varies from tree to tree and from year to year. So, just because your horse never expressed interest in acorns in the past, doesn’t mean that she won’t in the future.

Certainly, there were oak trees at our previous barn, but none of the horses ate them in the past. This year, even though they have plenty of grass, they circle back to the acorns after their grain, almost like they were desert. Perhaps they are different enough to be intriguing. Nutritionally, they are 41% carbohydrate; 24% fat; and 6% protein.

In October of 2020, Dr. David Marlin polled his followers on Facebook to gather more data.

The survey questioned owners about:

➡ How often horses/ponies eat acorns
➡ How likely they are to become ill from eating acorns
➡ What steps owners take to reduce ingestion of acorns

The Results:

➡ 2173 owners who had OAK TREES in or overhanging their paddocks took part
➡ 33% said their horses/ponies ate acorns OFTEN, EVEN WHEN THEIR WAS PLENTY OF GRASS
➡ 14% of horses that ate them became ill
➡ 53 horses/ponies were reported to have died as a result of eating acorns
➡ Around 1 in 3 owners did not take any steps to stop or limit horses/ponies eating acorns
➡ The most common management to limit or prevent horses/ponies eating acorns was to fence off areas around Oak trees (44%)

From this data it appears from these results that some horses/ponies are at risk of becoming ill (14%) and a significant number reportedly died due to acorn ingestion (2.5%). It may be that some horses/ponies are more sensitive to acorns and/or that some trees/years are more toxic. It’s also worth considering that damage to the GI tract, liver and kidneys may be cumulative and take place over several months and that when horses/ponies eventually become ill, it’s not necessarily then linked to acorn ingestion.

Since acorn toxicity is not a risk that Curly’s owner and I are willing to take, the mares are having a vacation from that field. Luckily, they have access to an acorn free pasture (and it’s giving the acorn infested field a chance to recover from the summer drought). Now that the last of the acorns have fallen, I’m out raking most days and hoping that the rodents and deer get a hustle on.

What about you? Have you ever had problems with your horses eating acorns? What did you do about it?

5 thoughts on “Acorn Toxicity in Horses

  1. My Arab, Jordan, would eat acorns. It didn’t hurt him at all. But he was eating green ones.
    The veterinarian who did the research may not have understood that oak acorns go through a process once they fall.
    Realize that there are two main types of oaks…white oaks, like the ones I have here in WA, and red oaks, the most common of oaks. The easiest way to tell the difference is that red oaks have pointed ends , almost fanglike to their leaves and white oaks have rounded ends.
    At first the acorns are green. This is when a horse is most likely to eat them. It takes about a day off the tree for the acorn to develop the tannins that are toxic. The tannins serve to keep critters like deer and squirrels from eating them, but the aforementioned animals didn’t get the message. A brown acorn doesn’t hurt them in the slightest. When a squirrel harvests an acorn, it bites the tip off to keep it from sprouting before it stashes it. Deer, pigs, jays, (of all types), squirrels and their kin..a lot of animals eat acorns.
    Even Indians ate them. The tribes in California…before they were ””assimilated” into white man culture or just wiped out, used to harvest California live oak acorns to eat. They would crack them open and then flush the resulting meat several times to leach the toxins out.

    I wish I had your acorns. Here in the west our oaks are suffering from smoke inhalation and drought. We haven’t had a mast year in years.

    1. I do love the oak trees and I’m perplexed that Z and Curly are so enamored of them this year. However, horses are really good at finding ways to make themselves sick (or injured). I’m just glad we figured it out before one of them got ill. Curly is 30 and prone to choke, so her owner is very careful about what she eats. Zelda is just a glutton.

    1. There seem to be many horses that are okay after eating acorns, but the ones that are not can suffer mightily. I’m still trying to figure out why this year is so different. Our horses have been in fields with acorns before, but I’ve never seen them eating them with such enthusiasm.

  2. I had a mare that foundered a few yrs ago after eating lots of acorns. She eventually had to be euthanized. She was on a diet of grass hay and was an easy keeper. She was the only horse of the 3 I had that went after the acorns. I’ve always wondered….

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