UT Extension News: “Poisonous Plants in the Pasture”

July 28, 2020

by Chris Hicks, County Director – UT Extension Smith County

It doesn’t take many of these 90+ degree days without rain for pastures to really suffer from drought stress. As pastures turn toasty and brown not only do they lose much of their nutritional value, but they may actually lead to an increased risk of poisoning for livestock. 

During drought and other times of pasture shortage, animals will eat plants they might otherwise not, and the possibility of consuming toxic plants increases. Plants may be toxic to livestock because they are nitrate accumulators, cyanide (prussic acid) producers, or because of certain chemicals that are found naturally in the plant. Being able to identify what plants can be potential problems is a critical step in keeping livestock healthy during periods of dry weather. 

In Tennessee soils, application of nitrogen fertilizer is critical to establishing and maintaining a healthy, productive stand of grass. Unfortunately, in certain situations, these nitrates can accumulate in the stems of plants and cause major problems in livestock. Nitrate toxicity is caused by cattle consuming forages that contain higher than normal levels of nitrates or nitrites.

Certain plants have a higher tendency to accumulate nitrates than others. Summer annuals such as sorghum, Sudan grass, and millet normally receive high levels of nitrogen fertilization that contribute to nitrate accumulation. Weeds such as pigweed and ragweed are also prone to accumulate high levels of nitrates. Fortunately, the UT Extension office has a free quick screen to test for the presence of these nitrates. 

Some plants have especially high levels of cyanide early in their growth phase or have the ability to concentrate high levels during times of stress such as a drought. A drought while the plant is in an active growing stage can cause release of cyanide acid (prussic acid) from its bound state within the plant. Potentially toxic levels can develop in most varieties of Sorghum, Sudan grass, and Johnsongrass. 

Finally, many plants contain chemicals which make them toxic to livestock. Weeds such as perilla mint, jimsonweed, and the various nightshades contain toxins that can cause respiratory disease, decreased production, and even death. Animals will normally stay away from these plants if other grasses are available. However, during periods of drought when other options are limited, they may consume amounts that will be harmful. 

Note that many of these toxic weeds become more palatable to livestock as they die down following an herbicide application. It is always a good idea to turn cattle off of a pasture where poisonous weeds have been sprayed until they completely die down. 

The hot, dry weather experienced recently can be hard on livestock and pastures, just as it can be to humans. Take some time to check your pastures and see what plants are there that might be potential problems. If you need help in weed identification, or would like your forages tested for nitrates, contact the UT Extension office at 615-735-2900.