I’m quite confident that our current livestock species are not large enough to eat entire oak trees. But autumn is the time for acorns, sometimes lots of acorns, and folks are worried about their potential toxicity. And rightly so.

Lane woody
Lane Livestock Services / Roseburg, Oregon
Woody Lane is a certified forage and grassland professional with AFGC and teaches forage/grazing ...

Oak toxicity can be a real problem in some situations and not all of these involve acorns. Oaks (all in the genus quercus) are wonderfully common trees.

There are roughly 60 species of oaks in North America – more than 350 worldwide – and they live in nearly all our environments, from the familiar woodlands in the East to the stately live oaks in the South, to the dry gambel and shinnery oaks in the open range country, to the famous oak savannas in California, and even amid the towering conifer forests of the Pacific Northwest. Oaks can be large, massive trees or twisted shrubs – and toxicity problems can occur with most of them.

Toxicity usually occurs when livestock eat young oak leaves, shoots or sprouts in the spring or large amounts of acorns in the fall, especially fresh, green acorns. The mature leaves are low in toxicity and are also unpalatable.

But sometimes weird situations occur, like a fresh blowdown where young, succulent leaves are suddenly near the ground. Or a very heavy acorn crop that can attract cattle because the rest of the pasture has little feed in it. Or individual animals can develop a craving for acorns, or part of a herd can choose to graze under oaks, or even worse.


A few years ago in California, more the 2,700 cattle died when a late spring snowfall covered their normal forage and forced cattle to browse the spring buds in the oaks. And nearly all types of livestock can be affected.

Cattle and sheep are most commonly reported, probably because they are the species spending most of their time grazing in unusual places, but there are plenty of reports of oak toxicity in horses and hogs.

The danger with oak

Oaks are associated with being sturdy and dependable – as in “solid like an oak” – but the dark side is that they contain “hydrolyzable tannins.”

I know that you’re thinking: What’s the problem with tannins? Aren’t tannins good for protecting against bloat (as in birdsfoot trefoil) and possibly for controlling parasites (as in sericea lespedeza)? It’s true that the tannins in some plants are beneficial, but oak tannins are not.

There are two basic types of tannins: (1) condensed tannins and (2) hydrolyzable tannins. They both impart a strong astringent taste to whatever contains them, but it’s the condensed tannins that are associated with positive effects.

Condensed tannins are built around a flavone structure, and they can link to proteins to form things like bypass protein and leather (which is really a bypass protein that bypasses everything).

In contrast, many hydrolyzable tannins are built around the compound “gallic acid” and are known as “gallotannins.” Once consumed into the digestive tract, hydrolyzable tannins are broken down (hydrolyzed) into smaller units such as gallic acid, resorcinol and pyrogallol.

These compounds can cause gastrointestinal inflammation (gastritis), and the body dutifully tries to eliminate them through the urine. Unfortunately, these compounds also cause profound damage to the kidney, and therein lays some very bad news.

Symptoms usually begin a few days after a toxic dose and will last three to 10 days or so. Animals go off feed and suffer abdominal pain and constipation followed by dark scours. They become emaciated and dehydrated, show excessive thirst, urinate frequently and strain to urinate.

There is no fever. As the kidney damage worsens, the animal becomes jaundiced, fluid accumulates around internal organs, and the kidney becomes enlarged and pale. I won’t describe the veterinary lesions in excruciating detail here, but suffice it to say that the damage is so extreme the animal will probably die of kidney failure. In fact, once an animal begins showing symptoms, its chances of survival are less than 20 percent.

The good news, however, is animals require a lot of oak tannins to cause these problems. Oak tannins aren’t as powerful toxins as, say, cyanide or the alkaloids in poison hemlock. For example, livestock must consume at least 50 percent of their diet as oak buds or young leaves before they become ill.

Even with acorns, animals must eat a lot of acorns to obtain enough tannins to cause symptoms. Although it’s not common, this could easily occur in a year with a very heavy acorn drop if the animals have nothing else to eat. But small amounts of oak buds and acorns are harmless and can even add value to a diet because they have some nutritional value.

Avoiding the attraction

After reading the literature, I found the best way of dealing with oak toxicity is to get the animals away from the oaks and feed them something else. That makes sense. Even just adding something palatable to the diet like alfalfa hay will help, except for those individuals addicted to the oak leaves or acorns.

Some books recommend feeding a supplemental pellet containing 10 to 15 percent calcium hydroxide (“slaked lime”). The hope is that the calcium hydroxide will bind to the tannins to prevent their breakdown in the gut. I’ve never tried this technique, but there it is. One other technique is to call the veterinarian.

One oak-associated syndrome that I’ll just mention is something called “acorn calf disease” – a sporadic problem in North America and Australia in which calves are born suffering from dwarfism, deformities and loose joints. But the reason I haven’t emphasized this problem is that, in spite of its name, it is not caused by acorns.

Early field observations linked this syndrome to acorn consumption, especially in California, and some older veterinary literature will still report it that way. But we now understand acorn calf disease is a congenital problem associated with drought conditions and feeding certain types of silage during pregnancy, and possibly a fungus.

But scientists still don’t know the specific guilty agent or the mechanism of this syndrome.

In fact, there’s still a lot we don’t know about oak toxicity – such as what causes the variations in tannin content in those oaks – genetics, weather, plant stress, annual cycles, etc. – or how those tannin-binding salivary proteins actually work, or which treatments are truly effective when animals show symptoms.

But scientists have actually been working on this problem longer than you think. Remember the classic 1925 silent movie The Gold Rush with Charlie Chaplin playing the Tramp? Particularly the famous cabin scene where the starving Tramp tries to cook and eat one of his boots. Well, that was actually a study on tannin digestibility, and I suspect he was trying to develop an assay for the level of tannin-binding salivary proteins.

And you remember the scene because it has stood the test of time, like an oak.