Poisonous Plants in Hay: Which Weeds Stay Toxic After Baling

By the XES Netting team — bale net-wrap manufacturer · Updated August 6, 2026 · ~13 min read

Every hayfield has a fenceline, a ditch bank, or a wet corner where something grows that your cattle have walked past all summer and never touched. That's the part that catches people out. On pasture, livestock are picky — they sort, they graze around the bad stuff, and the weed sits there year after year doing no harm. Then you mow that headland, it goes through the conditioner, it dries with the crop, and it comes out the back of the baler as an anonymous brown stem in the middle of a round bale.

At that point the animal has lost the one defense it was relying on. It can't sort. And with a handful of these plants, drying doesn't help — the toxin is still there in the bale in February. This guide covers which weeds actually stay dangerous after baling, which ones genuinely fade, how to tell the two hemlocks apart (a mix-up that shows up constantly on the farm forums), and what to do if you already have bales with something in them.

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Why a bale is more dangerous than the pasture it came from

The single most useful idea in this whole subject is that selection stops at the bale.

USDA-ARS says it about poison hemlock in one line: "Fresh leaves are unpalatable, so livestock seldom eat hemlock when other feed is available." That's why a patch can stand in a grazed paddock all season without killing anything. The same agency says the same thing about milkweed ("Animals usually do not eat milkweed unless good forage is scarce") and about nightshade ("Nightshades are generally unpalatable and are not grazed by livestock except under the stress of overgrazing or in contaminated hay and grain").

Read that last clause again. The exception the USDA writes into its own guidance is hay. Oregon State University Extension puts the same idea in the form of an instruction: "Never include poisonous plants in hay. Drying may improve palatability and animals fed only small amounts of hay may be hungry enough to eat the weeds." Ohio State's beef team reaches it from the other direction — "poisonings are more common when the plant tissue is unavoidable in hay."

Baling takes away three things at once:

• Choice. A cow at a bale ring is eating a compressed mat, not a pasture. She cannot walk around a stem that's pressed into the middle of it.
• Smell and taste cues. Curing knocks back the volatile compounds that make a plant obviously repulsive. The toxin and the warning flavor are not the same molecule, and they don't fade at the same rate.
• Dose control. Grazing spreads intake over hours. A hungry animal at a fresh bale can eat a large amount very fast — which matters enormously for the plants where lethality depends on how much goes in per hour.

One HayTalk member described exactly what that looks like in practice, and it's the most sobering post in the whole thread:

"We lost our 3 goats (a buck and 2 milking does) from a single round bale that had milkweed in it…"

"Goats won't touch milkweed in the wild but apparently will in hay. Only took about 24 hours…"

— Ox76 · HayTalk thread 98953

Goats — the animal everyone assumes will eat anything and survive it — refused the plant on pasture and ate it out of a bale. One bale, three animals, about a day. That is the whole argument in a single anecdote.

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The plants that stay toxic after drying

This is the list worth memorising, because for these species "I'll just let it dry down" is not a plan.

Poison hemlock (Conium maculatum)

Poison hemlock is the one that has spread hardest along road ditches, fencelines, and creek bottoms across much of the country in the last two decades — which is to say, along exactly the edges you mow. It's a biennial in the carrot family that reaches six to eight feet at maturity: hollow stems with distinctive purple blotches, hairless leaf stalks, fern-like leaves, umbrella-shaped clusters of small white flowers, and an unpleasant mouse-urine odor when a stem or leaf is crushed.

The USDA Agricultural Research Service is unambiguous about the dried plant: sheep, cattle, swine, horses and other domestic animals "are poisoned by eating small amounts of green or dried plant." The toxins are coniine, gamma-coniceine and related piperidine alkaloids, and they are still active in cured hay.

The quantities are not large. USDA-ARS reports that sheep may be poisoned by as little as 100 to 500 grams of green leaves, and cattle by 300 to 500 grams — a few handfuls. University of Kentucky specialists put it in a more useful unit: cattle have died from eating 0.2 to 0.5% of their body weight in green hemlock. Signs usually appear within thirty minutes to two hours, and animals die from respiratory paralysis in two to three hours.

There's a second, quieter risk that catches breeding herds. Sub-lethal amounts eaten by pregnant animals can cause skeletal deformities and cleft palate in the offspring. USDA-ARS gives the susceptible windows as 40 to 70 days of gestation in cows, and 30 to 60 days in sheep, goats and pigs. A crooked-legged calf born in spring can trace back to hemlock in a winter bale, and almost nobody makes that connection.

This is exactly where the most common — and most dangerous — piece of forum folk wisdom shows up. From an AgTalk thread on poison hemlock:

"In hay I wouldn't worry much about it, by the time it dries it turns into a nasty brown crunchy stick…"

— Lb94 · AgTalk thread 1161326

It's an honest observation and a reasonable-sounding inference — the dried stem really does look like inert rubbish. And there's a grain of truth in it: coniine is somewhat volatile, so total alkaloid content really does drop as the plant cures.

Here's the trap. University of Kentucky's weed scientist, veterinary toxicologist and ruminant extension veterinarian spell out what actually happens next, in a line worth reading twice: "Although toxicity is reduced during drying due to volatility of coniine, animals will eat much more dried poison hemlock than fresh because palatability is greatly improved."

So the plant gets somewhat less toxic per mouthful and dramatically easier to eat — and the second effect swamps the first. Worse, the drop isn't uniform: coniine is the dominant alkaloid in mature plants and seed, and Kentucky notes that seeds and dried plant material carry the highest coniine concentrations of all. Their conclusion is one sentence: "avoid feeding animals hay or grain known to contain poison hemlock." The University of Tennessee's beef and forage team says the same thing more bluntly — do not risk feeding hay from fields where poison hemlock was present, even if it was mowed down.

Nightshades (Solanum spp.)

Black nightshade, silverleaf nightshade, horse nettle, and buffalo burr are the ones you'll meet. USDA's handbook is blunt on two points at once: "Drying does not destroy the toxin," and these plants "often grow as weeds in hay and silage crops and small grains where they can be harvested with the crop and then fed to livestock."

The toxins are steroidal glycoalkaloids — solanine and relatives. The dose is smaller than most people expect: Penn State Extension estimates a 1,200-pound cow could show toxicity symptoms after eating as little as a pound of a nightshade species.

Poisoning doesn't always kill; chronic exposure shows up as emaciation, rough hair coat, poor appetite and constipation, which is easy to blame on the hay just being poor. Acute cases show labored breathing with an expiratory grunt, salivation, weakness, trembling and increased heart rate.

Worth knowing: potato vines belong to this group too, so potato haulm or cull piles near a forage field are the same problem.

Hemp dogbane (Apocynum cannabinum)

Reddish stems, opposite leaves, milky sap, long slender seed pods — and constantly mistaken for milkweed. USDA's handbook gives it the clearest sentence in the whole document: "Leaves are poisonous at all times, even when they are dry." The toxin is thought to be cymarin, a cardiac glycoside once used as a human heart stimulant.

Dogbane is a case where forum experience and the literature genuinely diverge, and it's worth being honest about that. Several experienced growers report animals simply leaving it:

"Our sheep leave dogbane alone but eat common milkweed."

— Bobolink_B · HayTalk thread 103891

Both things are true. Animals with a choice usually avoid it, so a light infestation in a big diverse bale often passes without incident — which is why so many people have fed dogbane-y hay for years with no problem. The risk isn't the average bale; it's the hungry animal, the small pen, the thick patch that got baled concentrated into one section, and the fact that drying gives you no protection at all.

Milkweed (Asclepias spp.)

Milkweed's toxins are cardenolides — cardiac glycosides chemically similar to digoxin, which cause heart arrhythmias and cardiac failure. USDA-ARS specifically calls out the hay route: "Poisoning also may occur if animals are fed hay containing large amounts of milkweed," and its management advice is one sentence long — "Hay contaminated with milkweed should not be fed to sheep or cattle."

Toxicity varies a lot by species. The narrow-leaved whorled milkweeds are considerably more toxic than the broad-leaved common milkweed most people picture. For the more toxic species, USDA-ARS reports an average sheep eating 30 to 100 grams of green leaves is likely to die.

Sheep and goats are the animals at greatest risk, which lines up precisely with Ox76's three goats. Another HayTalk grower summarised the mechanism in one line:

"Milkweed toxins remain active even after the plant is dried down."

— Hayjosh · HayTalk thread 98953

There's also a real commercial dimension here. In a 2026 HayTalk thread, a grower with twelve acres of rye going up as baleage found it loaded with milkweed — and his buyer was feeding both cattle and goats. Cattle might tolerate that crop. Goats are the species that dies. If you sell hay, the animal species at the other end changes the risk profile completely, which is worth being upfront about when you list a load. (Our guide on how to sell hay covers describing a crop honestly.)

Jimsonweed, yew, and white snakeroot

Jimsonweed (Datura) is toxic to all classes of livestock and to humans, via tropane alkaloids that survive drying. Signs include dilated pupils, subnormal temperature, restlessness, bloat, muscle twitching, incoordination, paralysis, rapid heart rate and respiratory paralysis. Seeds are the most concentrated part, so a late-cut crop that has set seed is worse than an early one.

Yew (Taxus) barely belongs in a hay article except that it keeps killing cattle anyway — because it arrives as hedge clippings tipped over a fence, or as brush mixed into feed. USDA calls it "probably one of the most toxic of all poisonous plants," notes that "the foliage is readily eaten by livestock," and puts the toxic dose at roughly 0.5% of body weight in ruminants. Dried yew is still lethal. If you take yard waste from neighbors, this is the one to refuse.

White snakeroot matters because it's the one that can travel into people. Its toxin, tremetol, passes into milk — that's the "milk sickness" of frontier history. It remains toxic in dried hay, and it grows in exactly the shaded, moist woodland edges that get clipped into late-season cuttings.

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The plants that genuinely do get safer

Being accurate in both directions matters. If you tell people everything is deadly forever, they stop listening. Some plants really do lose their punch, and one experienced HayTalk grower had the nuance right:

"For many poisonous plants, the toxicity decreases on drying out or curing."

— Hayman1 · HayTalk thread 98953

That's correct as a general statement. It's just not a species-independent rule, which is the whole reason identification comes first.

Buttercup

Buttercup's active irritant is protoanemonin, and it is genuinely unstable. University of Kentucky's equine group traces the chemistry: the plant stores ranunculin, which hydrolyses to the blistering agent protoanemonin when cells are damaged — including when buttercups are cut and dried in hay — and protoanemonin then converts to anemonin, which is not a vesicant. Dried buttercup is therefore "expected to lose toxic potential fairly rapidly," though Kentucky is careful to add that specific research confirming this has not been published. Ohio State's beef team is more categorical: the toxin "is not a problem or concern in hay."

The practical read: buttercup grazed fresh can blister mouths and cause scours; buttercup in cured hay is not a poisoning worry. What it is is a signal — and a seed-spreading risk, since baling a mature stand moves buttercup seed to wherever you feed. Buttercup in hay tells you far more about a wet, thin, overgrazed field than about toxicity.

Water hemlock — a special case

Water hemlock deserves its own note because the name confusion is genuinely dangerous. USDA-ARS calls it "the most violently toxic plant that grows in North America," and its toxin, cicutoxin, causes grand mal seizures and death sometimes within fifteen minutes.

But the toxin is concentrated in the tuberous roots — the chambered rootstalk that oozes brown or straw-colored liquid when cut. USDA states that "leaves and stems lose most of their toxicity as they mature," with the important exception that green seed heads remain poisonous. Since a mower doesn't normally lift roots, mature water hemlock stems in a bale are a much smaller risk than poison hemlock stems in the same bale. A grower on AgTalk had this right:

"I wouldn't worry too much about a few stalks getting into hay."

— junk fun · AgTalk thread 1170788

Grazing that same plant is a different story entirely — animals dig at exposed roots, and losses are fast:

"I've lost 3 cows that grazed it. Kills them dead fast. I would not feed it"

— chrisws89 · AgTalk thread 1170788

Wilted cherry and the cyanide group — a timing problem, not a drying problem

Wild cherry, black cherry and chokecherry work differently again, and the difference is when, not whether.

Intact cherry leaves are relatively safe. Damage them — a storm-broken limb, frost, or a mower — and enzymes release hydrogen cyanide from cyanogenic glycosides. USDA's handbook states that "wilting, frost, or other damage to the plant enhances toxicity," and that "wilted leaves, as well as fresh leaves, are poisonous." Roughly 0.25% of body weight in leaves can be fatal, and it has to be eaten fast — within thirty minutes to an hour — for the dose to overwhelm the animal.

The classic disaster is a cherry limb blown into a pasture and cattle finding the wilting leaves. In a hay context, the risk window is a conditioned cherry branch going straight into a wet windrow and being fed soon after. Once the material is fully cured and stored, cyanide levels fall substantially — this is one where fully dry, well-stored hay genuinely is lower risk than the wilted intermediate stage.

The same chemistry drives prussic acid in sorghum, sudangrass and johnsongrass, which is a bigger deal for most hay producers than cherry ever will be. We cover that in detail in prussic acid and nitrates in baled forage.

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Quick reference: does drying make it safe?

Plant	Still toxic in hay?	Toxin	Most at risk
Poison hemlock	Yes — green or dried	Piperidine alkaloids (coniine)	All stock; pregnant females (birth defects)
Nightshades / horse nettle	Yes — drying does not destroy it	Glycoalkaloids (solanine)	Cattle, horses, sheep, swine
Hemp dogbane	Yes — "poisonous at all times"	Cardiac glycoside (cymarin)	Sheep especially
Milkweed	Yes	Cardenolides	Sheep and goats
Jimsonweed	Yes (seeds worst)	Tropane alkaloids	All classes, incl. humans
Yew	Yes — clippings kill	Taxine	Cattle, horses
White snakeroot	Yes — passes into milk	Tremetol	Cattle; and people drinking the milk
Cocklebur	Mainly the seedling stage	Carboxyatractyloside	Swine, cattle, sheep
Wilted cherry	Risk peaks while wilting; falls once fully cured	Cyanogenic glycosides → HCN	Cattle, sheep
Water hemlock	Roots are the danger; mature stems much less so	Cicutoxin	Grazing stock (root exposure)
Buttercup	No — irritant breaks down on curing	Protoanemonin	Grazing stock only

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Poison hemlock vs. water hemlock vs. Queen Anne's lace

These three get confused constantly, and the confusion is not harmless. In the AgTalk thread that started as a poison hemlock ID question, the original poster called the plant "poison hemlock, also known as water hemlock" — and another member corrected him immediately:

"Looks like yours might be Water Hemlock, plant is not poison, but roots are, Poison Hemlock whole plant is poison."

— WCWI · AgTalk thread 1161326

That correction matches USDA-ARS exactly, which notes that "poison-hemlock is sometimes confused with western water hemlock, a more deadly species, because the names are similar" — and that the two "cause different types of poisoning."

All three are white-flowered members of the carrot family with umbrella-shaped flower clusters. Here's how to actually tell them apart in the field:

Feature	Poison hemlock	Water hemlock	Queen Anne's lace
Stem	Smooth, hollow, purple blotches	Stout, often purple-streaked, chambered base	Hairy, solid, green
Root	White taproot (parsnip-like)	Chambered, oozes brown/straw liquid	Pale taproot, carrot smell
Leaf veins	Fern-like, finely divided	Side veins end at notches, not tips	Fern-like, finely divided
Where it grows	Ditches, fencelines, disturbed ground	Wet seeps, stream banks, wet meadows	Dry roadsides, old fields
In hay	Dangerous — toxic dried	Lower risk (roots rarely baled)	Harmless

The single fastest tell for poison hemlock is the smooth stem with purple splotches. Queen Anne's lace has a distinctly hairy stem and no purple. That lookalike problem is the reason people walk past hemlock for years without ever registering it.

If you can't confidently identify a plant, photograph it — stem, leaf, flower and root — and send it to your county extension office or a state diagnostic lab. Do not taste-test anything in the carrot family.

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What to do if it's already in the bale

Most people find this article after the fact. Here's the honest decision path.

1. Identify the plant, not the category

Everything downstream depends on species. "A weed in the hay" is not actionable; "poison hemlock in the hay" is. Cut open a bale, pull the foreign stems out, and get a positive ID.

2. Estimate how much, and where

Weeds are almost never evenly distributed. They came from a ditch bank, a wet corner, or one headland pass — which means a specific, identifiable subset of your bales carries nearly all the risk. If you know which round of the field the contaminated windrow came from, you can often quarantine ten bales instead of condemning two hundred.

3. Match the risk to the animal

Species matters enormously. Sheep and goats are far more sensitive to milkweed and dogbane than cattle. Horses are more sensitive than ruminants to several plant toxins and have no rumen to buffer them. Pregnant females carry the birth-defect risk from hemlock. The same bale can be an acceptable risk for a dry beef cow and a fatal one for a milking doe.

4. Separate it before you decide anything

This is the step people skip, and Penn State Extension is emphatic about it: "If you have hay from a field that has weeds you know or believe to be poisonous, the first thing to do is to keep that hay separated from the rest of your supply. Once it is mixed with the rest of your feed, it will be impossible to separate."

There's a second reason to isolate suspect bales that has nothing to do with poisoning. Penn State points out you're also "introducing that seed into a new area of your operation through feeding or manure." Feed a hemlock-seedy bale in a clean paddock and you've just planted next year's problem, complete with free fertiliser.

5. Dilute only where the guidance actually allows it

Dilution gets talked about as a universal fix. It isn't — it's species-specific, and it's worth being precise about where it's genuinely sanctioned.

USDA's handbook explicitly permits it for nightshade: contaminated forage "can be fed if it is diluted (mixed) with nightshade-free forage," using an on/off feeding strategy, with animals "kept under close surveillance and immediately removed from the contaminated feed if signs of poisoning appear." Forum practice matches. One grower who chopped a lot of haylage put it as directly as possible:

"Dilution is the solution in this case."

— Mitchco · AgTalk thread 1161326

But note what that guidance is not. University of Minnesota Extension frames the mixed-ration approach as a tool for weedy forage — hay with anti-quality components — and closes with the caveat that weedy forages "should be fed with caution and only after proper identification to rule out poisonous plants." That's the opposite of a license to dilute a confirmed toxic weed.

Two more hard limits. Dilution only works if animals genuinely can't select the bad material out, which is far more true of a mixed TMR than a bale ring where a cow can pick through at leisure. And for the worst offenders the option isn't offered at all: USDA-ARS gives no dilution allowance for milkweed — it simply says contaminated hay should not be fed to sheep or cattle — and Kentucky and Tennessee both say to avoid feeding hemlock-containing hay outright. When contamination is dense, the cheapest bale on the farm is the one you use for erosion control or bedding rather than feed.

6. Don't pass the problem to the neighbors

If you sell hay, contaminated bales are a liability question, not just an agronomy one. Disclose or don't sell. A buyer running goats or horses is taking a materially different risk than one running dry cows, and they can't see a brown stem in the middle of a wrapped bale any more than the animal can.

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Preventing it next season

The permanent fix is agronomic, and it's not complicated — it's just not glamorous.

• Walk the edges before you mow. Fencelines, ditch banks, the wet corner, around the field entrance. Ten minutes on foot before first cutting is the highest-return scouting you'll do all year, because it's where nearly all of these plants live.
• Leave the bad patch standing. The cheapest control measure ever invented is not mowing something. Swing wide around a hemlock patch and lose a few square yards of low-quality headland hay.
• Spray at the right growth stage. USDA-ARS recommends 2,4-D plus dicamba on poison hemlock before the bud stage; hitting rosettes in fall or early spring works far better than spraying five-foot flowering plants. Our weed control in hay fields guide covers herbicide choice, timing, and the aminopyralid carryover problem.
• Keep a thick stand. Almost all of these are opportunists in thin, bare, or overgrazed ground. An AgTalk grower made the point better than any extension bulletin, warning against blanket-spraying hemlock with glyphosate: "while Roundup kills it great, don't. It also kills what works the best in control which is a good stand of grass." A bare spot is a hemlock nursery. (See also foxtail in hay, which follows exactly the same thin-stand pattern.)
• Mind the timing on sprayed plants. USDA-ARS warns that water hemlock stems and leaves increase in palatability right after herbicide treatment, and advises keeping animals away from treated plants for three weeks. Never mow a freshly sprayed poison patch straight into a windrow.
• Never accept yard waste. Yew, oleander, and rhododendron clippings tipped over the fence by a well-meaning neighbor have killed a lot of cattle. One HayTalk member described losing calves after a landscaper dumped oleander in with lawn trimmings.

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Frequently asked questions

Does drying hay make poisonous plants safe?

For some plants, yes; for several important ones, no — and for poison hemlock the answer is worse than a simple no. USDA-ARS states animals are poisoned by "small amounts of green or dried" poison hemlock, and University of Kentucky specialists note that while drying does reduce toxicity somewhat as coniine volatilises, "animals will eat much more dried poison hemlock than fresh because palatability is greatly improved." For nightshade, USDA states drying does not destroy the toxin at all; hemp dogbane leaves are "poisonous at all times, even when they are dry." Buttercup genuinely does lose its irritant on curing, and fully cured cherry leaves are lower risk than wilting ones. You have to identify the species before you can answer the question.

Can cattle eat around poison weeds in a bale like they do on pasture?

Much less reliably. On pasture, animals sort and avoid — that's why a patch of hemlock can stand in a grazed paddock for years. In a bale the plant is cut, dried, and pressed into a mixed mat, so selection is far harder and the taste cues that drove avoidance have faded. Feeding a mixed ration reduces sorting even further, which cuts both ways: it lowers the chance of an animal getting a big dose at once, but it also means they can't avoid a small dose.

How much poison hemlock does it take to kill a cow?

USDA-ARS reports that cattle eating 300 to 500 grams of green leaves may be poisoned, and sheep as little as 100 to 500 grams. Signs usually appear within an hour, and death from respiratory paralysis follows in two to three hours. Sub-lethal doses during gestation — 40 to 70 days in cows — can cause skeletal deformities and cleft palate in the calf.

Is milkweed in hay dangerous to cattle and goats?

Yes, and it stays toxic dried. USDA-ARS specifically warns that poisoning "may occur if animals are fed hay containing large amounts of milkweed" and advises that milkweed-contaminated hay should not be fed to sheep or cattle. Goats and sheep are at highest risk. One HayTalk producer lost three goats within about 24 hours to a single round bale containing milkweed — animals that ignored the same plant growing in their pasture.

What's the difference between poison hemlock and water hemlock?

They're different plants with different toxins. Poison hemlock (Conium maculatum) is toxic throughout the whole plant, green or dried, via piperidine alkaloids — so it's the bigger hay risk. Water hemlock (Cicuta) concentrates cicutoxin in its chambered tuberous roots; USDA notes its leaves and stems lose most of their toxicity as they mature, though green seed heads stay poisonous. Water hemlock is more violently toxic overall but far less likely to end up dangerous in a bale, because you don't usually bale roots.

Should I still feed a bale that has a few poison weed stems in it?

It depends on the species, the density, and the animals. For low-level nightshade contamination USDA's own guidance permits feeding diluted with clean forage on an on/off basis under close surveillance. For milkweed, USDA-ARS says contaminated hay should not be fed to sheep or cattle. For dense poison hemlock, don't feed it. When in doubt, quarantine the suspect bales, get the plant identified, and call your veterinarian before feeding rather than after.

What are the first signs of plant poisoning in cattle?

It varies by toxin, but common early signs are staggering or an unsteady gait, trembling or muscle twitching, excessive salivation, dilated pupils, labored breathing, and animals hanging back from the group. Cyanide poisoning shows bright cherry-red blood and rapid distress; nitrate poisoning shows chocolate-brown blood and blue-tinged membranes. Any sudden death or neurological sign in animals on new hay warrants pulling the feed immediately and calling your vet — most of these toxins act far too fast for a wait-and-see approach.

This article is general information for hay producers and livestock owners, not veterinary advice. If you suspect plant poisoning, contact your veterinarian immediately — most of these toxins act within hours.

Featured photo: Conium maculatum, South Side Park by Cbaile19, released under CC0 1.0, via Wikimedia Commons.