Beware of plant toxins in hay and silage

Columnist Doug Warnock discusses how to detect plants that may be poisonous in hay and forage.


For the Capital Press

Published on June 9, 2014 1:41PM

Doug Warnock

Doug Warnock

This is the season for putting up hay and silage for later use as livestock feed. Contamination of feeds can occur when poisonous plants invade pastures and hay fields and are harvested along with the desired forage plants. Since the population of the toxic plants is not normally uniform throughout a field, the contamination is not evenly distributed in the hay or silage. This makes it harder to identify the toxic plants and to manage the toxicity within the feed.

An excellent guide to poisonous plants and plan toxins was developed by B.L. Stegelmeier and K.E. Panter, researchers with the USDA Poisonous Plant Research Laboratory at Logan, Utah. It was published by the Society for Range Management in Rangelands.

The best way to avoid these toxicity problems is to inspect the field prior to harvesting and identify and remove any plants that could cause problems. Livestock producers need to familiarize themselves with known poisonous plants in their area and be on the lookout for them. Local extension educators and veterinary practitioners can help with poisonous plant identification.

Livestock producers and their employees should inspect the hay or silage when it’s fed and collect any suspicious or unknown plants.

There are toxins in many plants, but they may not always cause illness in an animal. Sometimes weather and moisture can affect a plant so as to cause it to be more toxic than it would be under different conditions. Also, the general health of the animal, as well as its age and condition, can be a factor in whether a plant causes toxicity. Because of this, toxicity in plants varies from year to year and species to species.

Several plant species that are commonly used as livestock forage contain a chemical that can be toxic under certain conditions. Johnson grass, Sudan grass, forage sorghums and arrowgrass can be poisonous. Toxicity occurs when these plants are damaged by crushing, freezing or wilting. These physical changes cause a chemical reaction, which changes the cyanide to prussic acid, which is toxic. The concentration of cyanide is usually higher in young, rapidly growing plants.

Poison hemlock grows along many rural roads, ditches and fences. It may invade adjacent fields and pastures and is often fatal to livestock. Its leaves and stems are toxic and may contaminate hay and silage.

Certain weeds can accumulate nitrates and become toxic when mixed into hay and silage. Nitrate accumulation is provoked by nitrogen fertilization, drought or frost stress and some herbicide treatments. Plants in this group include Kochia, Lambsquarter, Canada thistle, Jimson weed, Wild sunflower, Smartweed, Dock, Nightshades and Pigweed.

Knowing which plants pose the most toxic threat and being very observant before and after forage harvest can prevent animal losses from plant poisoning.

Doug Warnock, retired from Washington State University Extension, lives on a ranch in the Touchet River Valley where he writes about and teaches grazing management. He can be contacted at


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