Poisonous Plants that Affect Cattle

Published on Thu, 03/31/2022 - 10:40am

Poisonous Plants that Affect Cattle.

 By Heather Smith Thomas.

 Some plants are toxic to cattle if eaten.  Different regions have different problem plants; the plains states and western states have some poisonous plants that don’t grow in humid eastern parts of the country, and vice versa.  Some can be safely eaten in small amounts or in certain stages of growth while others are toxic at all times.  Some parts of a plant may be more toxic than others; the toxin may be mainly in the roots, or seeds, etc.  Poisoning often depends on palatability (some plants are never eaten unless the cattle have nothing else to eat), stage of development, portion eaten, or growing conditions.

Kevin Welch, PhD, DABT, Research Toxicologist, USDA/ARS, Poisonous Plant Research Laboratory, Logan, Utah says that in the mountain West the two main plant problems for cattle are generally larkspur and locoweed.  In some regions lupine can also be a problem, causing deformed calves with fused joints if cows consume very much lupine during early pregnancy.  In other parts of the country other plants may be the most common problems.

It’s hard to determine how much of a certain plant the animal would need to eat in order to have adverse effects, because it can vary.  “With larkspur, for instance, there are many different species and each one has different toxicity.  Plants in different regions will also be different, even within the same species.  We think there is some environmental effect (such as growing conditions that year) but also genetic differences,” says Welch.  Some patches are deadly every year and it’s best to get rid of them or avoid grazing those areas during the time of year they might be a problem.

“Larkspur is potent enough, unless it’s a species/population that doesn’t contain the really toxic alkaloids, that it can be a problem every year.  How much the cow would have to eat, to be poisoned, can vary, however.”  If it’s a dry year and larkspur is the greenest plant in that pasture, cattle generally consume a lot of it.

“With most poisonous plants, livestock generally won’t eat them unless they are short on other feed.  In all the studies we’ve done, however, larkspur is one they will eat even if there is other forage available.  In flowering stage they seem to like it,” he says.

“We’ve also shown that as long as the animals don’t eat too much of it, they won’t die.  They might bloat and get sick and weak, and stop eating for a while.  They may go through a cyclical pattern; they’ll eat some and get sick and stop eating it, and then go back to it.  We’re not sure what causes them to eat too much at once and get a fatal dose,” says Welch.

The alkaloids in larkspur affect cattle more adversely than horses, sheep or goats.  Some producers use a flock of sheep to trample a larkspur patch before cattle are put into that pasture.  “The sheep won’t be affected,” he says.

“There are some drugs you can give to bloated, sick cattle but usually you don’t find the animals in time to treat them; usually it kills them so fast that you just find them dead.  There were probably several others in the herd that were poisoned but didn’t eat quite as much and survived,” Welch says.
There are areas in western states where lupine is a big problem for cattle and sheep.  “Lupine is a palatable plant and very high in protein.  If there’s a lot of lupine in a pasture (especially in early spring when lupine grows faster, ahead of the grass), cattle may eat a lot of it,” says Welch.  This can happen if cattle go out on spring range when the grass isn’t very good yet.  Lupine may be the most attractive plant.  

If the cows were bred early, they may already be in that 40 to 100-day window of gestation where toxic alkaloids adversely affect the fetus.  They give birth to malformed “crooked” calves the next year.  One recommendation in areas where lupine is a problem is to switch to fall calving.  Then it will be winter instead of spring when cows are in that susceptible stage of pregnancy.   They won’t be consuming lupine when it would be toxic to the fetus.

The toxic alkaloid inhibits movement of the fetus.  If the limbs aren’t moving, the joints fuse.  If the tongue stops moving, it may be in the way as the hard palate forms; the calf ends up with cleft palate or a hole in the top of the mouth.

“Dose is the issue with lupine.  For a cow to be poisoned, she would have to consume a lot of it, but the fetus is affected by a much smaller dose.  We generally don’t see adverse effects in the cows, but their calves may be malformed at birth,” says Welch.

Water hemlock is more deadly than poison hemlock.  “The toxin in water hemlock is similar to strychnine and the roots are very toxic, making water hemlock one of the most poisonous plants in North America.  The toxin in poison hemlock is different; its mechanism is similar to the toxin in lupine, so it can also cause crooked calves.  Cattle don’t seem to eat poison hemlock very often, however,” he says.

They look similar and are both part of the parsnip and carrot family but water hemlock grows in wetter soils.  “Water hemlock and poison hemlock may be similar in size, depending on the species, but poison hemlock can grow in drier areas.  It may grow out on the foothills or in the fields and right up to the ditch but not in the water, whereas water hemlock needs more moisture.  It’s usually in an area covered with water most of the spring and early summer,” he says.

Poison hemlock can grow on the ditch banks and encroach into hayfields and get harvested with hay.  Since it remains toxic when dried, cattle may consume enough in the hay to be poisoned.  “This is especially true with todays’ harvesting and feeding methods, when hay is mixed with other components of diet—processed in a feed mix or with silage or corn or some other kind of grain.  Then cattle are unable to sort it out.  In this situation, they generally wouldn’t get a big enough dose to cause problems, but sometimes when cattle are fed fresh green-chopped hay containing poison hemlock, they may be affected,” says Welch.

Some toxins are less deadly when the plant dries out, but others are stable, no matter how old the hay might be.  Even in last year’s hay it could be just as toxic as the day it was harvested.

“With water hemlock, we’ve found that if cattle eat above-ground portions (stalk, leaves and flowers) they usually don’t eat enough to kill them,” says Welch. They may just take a few bites as they graze or travel through the area.  

“We have done studies that show the green seeds are potent enough to poison a cow.  Thus it depends on the stage of growth of the plant, but the root is always extremely poisonous.  Water hemlock often grows in marshy areas or along a stream or ditch bank.  When cows are walking in and out of those areas they may dislodge the plants, exposing the roots.  If they eat the roots they generally die,” he says.  

Molds And Fungi
If conditions are wet, molds and fungi may grow on forage plants or in hay or grain after harvest if it is damp or gets rained on.  Aflatoxins can cause liver damage in cattle, and some of the other molds may cause abortion or other problems.  Mushrooms are fungi, and some of them can also be deadly or cause illness in livestock if consumed.  “It all goes back to dose.  Small quantities may not cause problems,” says Welch.

“Some of the molds that grow on corn can be deadly.  Make sure grains get adequately dried, and stored in a cool, dry area.  In much of the West molds aren’t quite as big a problem because humidity is low in this dry climate.  Some plant toxins, however, are actually produced by symbiotic fungi, like the endophyte fungi on some types of fescue grasses.”
“With locoweed poisoning, it’s also due to a symbiotic relationship with a fungi.  Locoweeds Astragalus and Oxytropis contain the toxin swainsonine, produced by an endophytic fungus that grows inside the plant.  One of our researchers, Dr. Daniel Cook, has done a lot of research on this,” says Welch.

“Dr. Cook has shown that if you can find plants that don’t have this particular fungus, they don’t produce the adverse effects caused by locoweeds.  Selecting plants without the fungus works great for a cultivated species like fescue, planted in a pasture, but out on the range we don’t have control over what’s out there.  Visually, these plants look the same, so there is no way to look a locoweed to know whether it contains the toxin,” he says.

“The bad thing about swainsonine is that it makes locoweed toxic all the time, at every stage of growth.”  Even when the plant is mature and dry, out on winter pasture, it is still toxic to livestock.  The toxic effects are cumulative.

“Locoweed is notorious for chronic poisoning; the animals have to eat it for several weeks before the neurological lesions cause them to start acting crazy,” says Welch.  Then they seem to seek it out and eat more of these plants.

Other Astragalus species may accumulate excessive amounts of selenium, and too much selenium in the diet of animals can cause loss of hair and hoofs.

Ornamental And Garden Plants
Some plants found in yards, gardens and barnyards can be very toxic, such as yew, oleander, etc.  “Even certain garden plants can be problematic.  Rhubarb leaves contain oxalates that are a problem for livestock.  The toxin in yew is potent and short-acting.  It only takes a very small amount and the animal dies quickly,” Welch says.

Other Toxic Plants
Some plants like brackenfern create problems if livestock consume small amounts over time.  “They have to eat it for a while before signs of poisoning are seen.  Yellow star thistle and knapweed are a big problem for horses and not so much for cattle,” says Welch.

Frosted, wilted chokecherry leaves are toxic for cattle.  “They contain cyanide, which is deadly in small amounts.  Some plants also accumulate nitrates under certain growing conditions.  Some grasses and cereal grains when frosted will have an increase in nitrate concentration.”  A grain crop that was fertilized but didn’t grow very well because of drought may have a high level of nitrates, and if cut for hay it could be deadly.  This is definitely a situation where the dose makes the poison.  Above certain levels you can’t safely feed that forage to pregnant animals, and at higher doses you can’t feed it to anything, unless you mix and dilute it with other forages.

“You need to know the nitrate levels, and if you mix it you need to mix it very well, or you may end up with a hot spot.  There might be one or two animals that eat that portion and become poisoned,” says Welch.

One toxic type of alkaloid contained by a number of different plants worldwide, is pyrrolizidine alkaloids that cause liver damage.  “Once the liver is damaged, the animal suffers from photosensitization as a secondary problem.  If too much of the liver is damaged, the animal will die,” he says.

Plants that contain pyrrolizidine alkaloids include several species of Senecio, such as tansy ragwort, as well as groundsel, rattleweed, comfrey and others.  “Grazing animals usually don’t readily eat them, but livestock may consume them in harvested feeds when mixed in with hay or grain.  Some of these will grow in grain fields and their seeds get harvested with the grain.  This can be a problem for humans as well, if the grain is contaminated.”

He says comfrey is also an issue in human health.  “This was commonly used as tea, but if people drink it every day it begins to accumulate and cause enough damage to the liver that they start to have health problems,” says Welch.  

Just because something is “natural” doesn’t mean it is safe.  “Our lab is a perfect testament to that.  There are many toxic plants that humans and livestock should avoid.”

The problem of toxic plants is complex; it’s impossible to give blanket statements about certain plants or dosage.  If producers have questions about a plant or possible toxicity, the website for the Poisonous Plant Research Lab (Logan, Utah) can be a good resource.  “There is a short overview showing where the plant grows, what it looks like, which animals it affects, how it affects them, and what you might be able to do for them once they are poisoned,” he says.

Useful information can also be gleaned from Bulletin 415, available through (https://www.ars.usda.gov/ARSUserFiles/oc/np/PoisonousPlants/PoisonousPla...)

People can also contact the lab by phone: (435)752-2941 or by e-mail: daniel.cook@usda.gov (Research Plant Physiologist) or kevin.welch@usda.gov (Research Toxicologist).