Doc’s Memoirs: Cows never eat the stuff (or, the trouble with tansy)

D E Larsen DVM

Here I was again, standing over a dead cow, in the middle of a pasture filled with tansy plants, listening to a rancher explain to me that cows never touch the stuff. As I stood there sharpening my necropsy knife, I thought about my long history with tansy.

My first recollection of awareness was at a family picnic at Tom and Kathryn Lawson’s ranch, on the top of Catching Creek Mountain, out of Myrtle Point. The year was 1950, and the plant was just then starting to show up on the high ridges of Coos County. We knew it killed cows and horses, but I do not recall seeing a loss. My oldest brother had a summer job the following year, working for Coos County spraying tansy, mainly on the high ridges in the county. It did not take long for it to spread to the valleys. By the time I was 10, pulling tansy was a standard summer chore for almost any farm kid in the county.

At least this cow was dead. In the 1970s, the diagnosis was challenging in a live animal before it was near death. Blood work could show liver failure, but that was not specific to tansy toxicity.

I was always amazed at how these guys could be in such a state of denial. They wanted an answer to the death, but one that fits their opinion that cows would never eat the stuff.

It took me only a few minutes to open this cow up. I slit the skin down the ventral midline and reflected the hide up to her back. Elevating the legs and freeing them of their muscle attachment, I flipped both legs and the skin to lay out over her back. Then I opened the belly and ribs, reflecting them back, so I now had the cow opened for view.

“So, I want you to look at this, Tom,” I said as I pointed out the visible signs of liver failure. “This belly shows all the signs of liver failure. The yellowish discoloration to the tissues, the severe accumulation of fluid in the belly, the chest is normal, and the liver is swollen and pale yellow in color.”

“OK, I can see liver failure,” Tom said. “But there has to be a lot of things that cause liver failure. How can you be so sure it is tansy?”

“Well,” I replied, “an old veterinarian, Dr. Pierson, who I respect very much, always said, ‘When you are in a barn and hear hoofsteps, you look for a horse, not a zebra.’”

“I guess I don’t know what that means,” Tom said.

“That means you rule out the obvious diagnosis before you go off in some unrelated direction, trying to prove a once-in-a-lifetime diagnosis,” I explained. “In my mind, when I stand in a field filled with tansy, looking at a cow with liver failure, the diagnosis is tansy toxicity. That diagnosis stands until I prove that it is something else. Now let me get a piece of this liver and show you the insides.”

I sliced off a large section. The very sharp knife almost vibrated as it went through the dense liver tissue. I placed this piece of liver on the cow’s hip for a makeshift table.

“I can send a piece of this into the lab, and the pathologist will give us a confirmed diagnosis,” I said. “Tansy toxicity has a very characteristic appearance under the microscope. First, I want you to look at the cut surface of this liver. Think about the liver you see in the store and compare it to this liver. This liver is swollen with rounded edges, not dense with sharp edges, pale yellow in color rather than deep red, and this cut surface has the appearance of nutmeg, not a consistent deep red appearance.”

I handed Tom the knife.

“I want you to drag this knife through this liver,” I said. “You watched how sharp this knife is when I opened this cow. I want you to feel how it almost vibrates as it cuts through this chunk of liver.”

Tom took the knife and made a slice.

“It almost feels like it is cutting steel wool,” he said.

“That is maybe a good analogy,” I replied.

“OK, Doc, you have presented me a pretty good case,” Tom said. “And I guess we are going to send a piece of this to the lab, just to be sure. Why is it, then, that we don’t have a bunch of cows lying here dead?’

“You are a little bit correct, Tom, when you say the cows never eat the stuff,” I said. “Most cows will avoid it most of the time. It is most dangerous in the hay and also after it’s sprayed. The plant takes up a lot of sugars as it wilts after being sprayed. Cows will find it acceptable for a week or two as it dies. Also, some cows, and some horses, will develop a liking for the stuff, and they will seek it out.”

“So you are saying we are both right,” Tom said.

“Only sort of,” I explained. “This cow didn’t eat a bunch of tansy yesterday and then died. She could have eaten a toxic dose months ago. You are just lucky that you found her dead and not sick. Making a diagnosis in a cow getting ready to die from tansy is difficult and expensive. Sometimes I will make several visits before ending up doing a liver biopsy. There is a lot of frustration in treating a cow with tansy toxicity. But time always tells us, all of these cows die.

“I looked at a dead cow once and her 3-day-old calf, who was also dead. Both of them died of tansy toxicity. The cow is easy to understand. The calf is a little more of a question. It is doubtful it could have eaten enough tansy to be a problem. There is some evidence that the toxic alkaloids are passed in the milk, but probably not in a dangerous concentration. That leaves the placenta; this calf probably received a toxic dose from its mother through the placenta.”

“What do I have to do now to get control of this stuff?” Tom asked.

“It is tough, and it is not going to happen overnight,” I said. “Maybe not even in a year. This stuff is too high to benefit from spraying. I would mow it down and either compost it or burn it if you can. Then next spring, you need to spray the pasture. Keep the cows off the pasture after spraying for two to three weeks. Then next summer, pull any plants that make it through all of that. Probably most important, get all your neighbors to do the same. And talk to the county extension agent. They promise that a caterpillar is coming that will eat the stuff. I haven’t seen any yet.”

“What do I need to do with this carcass?” Tom asked. “Is it toxic? I mean, if my dog gets out here and eats on this, is it going to kill him?”

“Now, that is an interesting question,” I said. “And I don’t have an answer for that one. I will definitely check the books, but I am not sure anything is written about the toxicity of the tissues. I doubt if there is a problem, but I don’t know. I would call the rendering company, or I would bury it.”

– David Larsen is a retired veterinarian who practiced 40 years in Sweet Home. More of his stories are available on his blog at