The Dose Makes the Poison

Using sleuthing skills when toxic weeds are suspect
Round bales in a hayfield

It’s like trying to uncover the culprit in a British mystery novel; determining if horses or livestock have been poisoned by toxic plants hidden in your hay or pasture. Difficult, since so many variables can influence the severity of an affliction. Numerous suspects, misleading clues? The symptoms we observe will differ depending on the plants involved and the animal species. For instance, your horse may be killed by eating enough Horsetail Rush, while your cattle merely exhibit a general loss of condition. Sorghum/Sudan grass can be a nutritious emergency forage to replace late planted corn. However, if the plants are stressed by weather or environmental conditions, then feeding them may result in nitrate and/or cyanide poisoning. Toxicity variables include the amounts and parts of the plant consumed, age, size, condition of an animal, the list goes on.

Any reader of mystery novels will tell you that the time between ingesting a poison and the onset of illness will also vary. Perhaps the toxic effect is sudden or occurs slowly, almost imperceptibly... I suspect mild cases that cause reduced animal performance or unknown health ailments sometimes go unrecognized for the plant poisonings they may actually be.

New England livestock, compared to other regions of the country, are said to rarely have problems with poisonous plants. No doubt our excellent grass-growing climate that provides us with an abundance of quality forage has something to do with it. Even so, I’m not convinced that we can ignore the hazard poison plants present. Which of your animals would you wish to decline in health, do without?

Keep good feed records, field data and weed references on your bookshelf alongside your copy of Merck’s Vet Manual; you’ll be better prepared when it’s necessary to call your DVM because your animal is “acting out-of-sorts.” Here are a few brief examples of cases I’ve experienced:

One call involved health problems with horses. We did a long walk around the pastures, which looked good, plenty of forage. Some Alsike Clover (not recommended for horses) but it didn’t fit the symptoms. Then into the shade where they liked to hang out. Hmm...Wild cherry saplings chewed down, bark cribbed on larger trees. Hydrocyanic poisoning a possibility?...check with your vet, fence it out.

Another time involved sheep, one or two dead if I remember right. Another walk-about. No, not cherries, he knew about them. Generally decent, plenty of pasturage at the height of summer. What’s that patch of weeds in the low, wet area, the ones the sheep are happily munching on? Dogbane (a milkweed relative) “Incidence of poisonings are rare because the plant is distasteful to animals” so said my reference. Nobody asked the sheep. Maybe this the problem? Weed control recommended.

Yet another occasion - before my time locally, but often in my thoughts during a wet year or when asked about growing grains (flour corn an easier choice) in the home garden was an unexpected dairy cow problem. Cattle poisoning this time with some quite nasty results. This by Ergot, not a plant but a fungus that infects cereal grains and other grasses, in this case it was in the grain. A good reason to look closely at late cut, over-mature grasses, cover crop rye gone to seed. Two Weed/Poisonous Plant references I have ready to hand and even keep a small stockpile for re-sale are:

Weeds of the Northeast by Cornell Press. Having color photos of common meadow weeds in various life-stages makes it quite useful.

Poisonous Plants of Pennsylvania by their Dept of Agriculture. Detailed line drawings, animal species affected, symptoms make it valuable.