Toxic Forages?

A grazier’s education in three species

Our informal education in toxic forages began one midsummer day twenty years ago. That spring, a new plant species had shown up in the back pasture. Its straight, thin, reddish stems and oval, white-veined leaves oozed a white, sticky sap when broken, and the plant was colonizing the pasture at the edge of the woods. In fact, it was spreading rapidly, and by July it covered an area about 40 yards in diameter. It had all the appearance of being invasive, and we were beginning to be concerned. 

Our friend Elizabeth, a forage specialist, came to visit, and we showed it to her. She wasn’t familiar with it, so she took a sample back to the office, and a couple of days later, she called.

“It’s Apocynum cannabinum — ‘hemp dogbane,’” she said. “It’s invasive and extremely toxic.”

“Toxic?” we protested. “The cows have been gorging on it for weeks!”

Elizabeth was unruffled. “It’s listed as ‘toxic,’” she repeated. “It says right here that three to six leaves will kill a 1,200-pound cow.” She paused. “I wouldn’t worry too much, though; so long as your cows can’t read the identification chart, they should be okay.” 

She laughed and hung up. Thus began our rough-and-ready education in toxic forage species — a vital part of our ongoing adventure in small-scale pasture management.

Graze Native Pastures

Learning to identify pasture species is interesting, fun — and important. That’s because native forages — a huge assortment of plants that are abundant, diverse and volunteer — have so much to offer. Our own pastures, which are primarily native grasses and forbs, with a sprinkling of blackberry cane, seedling trees and multiflora rose, are a case in point. There’s a lot of diversity out there, and we want to be able to identify and address those plant species, because if we’re going to use this natural resource, we need to be comfortable with whatever is growing. 

Hemp Dogbane

Let’s go back to that first encounter with hemp dogbane. After we talked to Elizabeth, we looked it up ourselves. Every pertinent website we could find — and there were dozens, from U.S. and state extension bulletins to spraying protocols from agriculture chemical corporations — assured us that the plant was extremely toxic. But we, along with all the intensive graziers in our area, had observed the eagerness with which animals ate this new invasive. So, what was going on, and why did the forage guides list it as a deadly poison? It took us about six months to find the answer: misidentification. 

According to a bulletin from Penn State, authored by forage specialist William Curran, while “much of the literature on hemp dogbane claims that it is poisonous to livestock…these claims were based on an early investigation in which oleander (Nerium oleander) was mistaken for hemp dogbane…. Animals find fresh hemp dogbane distasteful, but can eat it in hay without suffering ill effects.” Problem solved! The only part he seems to have gotten wrong is thinking livestock find it distasteful; our cows just love it. 

Meanwhile, all over northern Appalachia, landowners are spraying pastures with glyphosate, dicamba and 2,4-D, or leaving them ungrazed, for fear of this deadly
“toxic” plant species.

White Snakeroot

After our hemp dogbane experience, we were better prepared when a visitor to the farm pointed out the next “deadly” plant species. She had attended a wildflower identification tour at a nearby state park and was ready to share her new education. Pointing to the froth of pretty white flowers growing along the tree line, she warned us, “Your dairy cows are eating white snakeroot. It’s poisonous. Animals that eat it die, and their meat and milk are poisonous. That’s what killed Abraham Lincoln’s mother.” 

We checked it out, and, sure enough — if you look up white snakeroot (Ageratina altissima), every site will tell you the same thing: Nancy Hanks Lincoln died of milk sickness, the result of drinking the milk of cows that grazed white snakeroot. There’s a whole legend about it, in fact, assembled out of many decades of guesswork. Actual data is harder to pin down.

Laboratory assays show that white snakeroot does indeed contain tremetol, a chemical compound that, in large doses, can be toxic; actual feeding studies, on the other hand, turn out to be inconclusive. Given a big enough dose of the stuff, some animals get sick, and some even die, but results are far from consistent. Still, the USDA lists the toxic dose as “3-4% of the animal’s bodyweight in a 2-3 day period,” which means that a 1,000-pound cow needs to eat 20-30 pounds in a short period of time. She’d have to search a fairly large area to find so much, and it’s unlikely she would want to; it doesn’t appear to be very palatable, and the scientists who did the studies had to administer their dosage with a stomach pump. 

While our own cows do graze white snakeroot, they do so only lightly, and we’ve been drinking their milk for decades with no ill effects, for them or for us.

Horse Nettle

Look up the spiny pasture plant called “horse nettle” (Solanum carolinense) and you’ll find dozens of sites informing you that it’s poisonous. That’s hardly surprising since, along with tomatoes, peppers, and eggplant, it’s a member of the nightshade family. The wild plant contains a chemical called solanine that is toxic in large quantities. It’s common enough in pasture and hay meadows, but where you really see a lot of it is on land that has been heavily overgrazed. Livestock mostly avoid it.

But perhaps, instead of “toxic,” we should call it “medicinal.” Kathy Voth, author of Cows Eat Weeds and founder of the website On Pasture, teaches owners of livestock how to train their animals to eat, among other things — you guessed it —- horse nettle. Our own cows seek out and consume the yellow berries of horse nettle in late fall, and although they can’t tell us what the specific benefits are, their robust health on winter pasture assures us they know what they are doing.

People Issues

We’re not saying there is no danger involved with toxic plants. Every year livestock get sick and die from grazing toxic plant species, despite the excellent botany instincts of most herbivores. But often it’s management issues — which is to say, people issues — that set the stage for toxic forage tragedy. 

Lack of proper food can override an animal’s instinctive avoidance of non-nutritive species. Grazing animals are generally hesitant about sampling novel plants, but when they have been relocated to an unfamiliar ecosystem, hunger may drive away caution. Likewise, when one or a few animals are incorporated into a larger herd, hazing behavior can leave the new animals no option but to graze unfamiliar, and potentially toxic, plant species. 

This is where human management can invite — or avert — disaster. Care must always be taken to ensure all animals have access to good food, especially during hazing behavior, when younger, smaller animals may be harassed by larger stock, or polled animals intimidated by those with horns. If the herd is on pasture, paddocks should be sized to allow all the animals plenty of forage, with adequate residual; if they are on hay, there should be enough room at the feeding stations so all of the animals can get full without being tempted to oversample hedgerow plants.

In fact, the dangers of toxic forages highlight the many benefits of well-managed, holistic grazing. That’s because animals expecting regular paddock moves, especially daily or even twice-daily, have little incentive to sample non-food plant species. Paddocks sized for short (24-hour or less) grazing periods will seldom include large enough proportions of toxic species for animals to overindulge. Even should a large proportion of a toxic forage be inadvertently included in a paddock, livestock that are accustomed to receiving plenty of good forage at regular intervals are incentivized to wait for a fresh paddock, rather than to experiment with unfamiliar and potentially harmful plant species. Good management, it turns out, is the best security.

Steps You Can Take

So, how do we play it safe with medicinal and/or toxic plant species?

Do your own research. Journalists make mistakes. Popular articles on technical subjects should reference scientific studies and journals. Look these up. Read the source material. You want feeding studies, with information about which parts of the plant are toxic, to which animal species, when, and in what amounts. Apply your critical thinking skills here; often, when you assemble the facts, risk diminishes.

Reference what you know. Scientific studies happen under lab conditions, but your farm, ranch or homestead is where things happen in real life. A plant species may be toxic, and your livestock may be eating it; but if the results seem positive, it’s probably not time to reach for an herbicide.

Practice holistic grazing. Natural grazing patterns have played a vital role in animal health for millennia; they are still the best management tool we have. Move animals frequently; expose them to your native forages; encourage them to develop pasture wisdom. And then trust in the results.

Native plants are the foundation of food and fertility today, just as they have been for as far back as we can see. Some of those labeled “toxic” turn out to be therapeutic or medicinal. Harvesting grass with ruminants is still the most regenerative way — maybe the only way — we have in the long term to grow meat, milk, and soil — and toxic forages are an important part of the picture.