Over the past several weeks I have had numerous questions regarding the safety of Johnson grass as a forage or for hay. The extremely, hot and dry summer left many pastures short and several producers feeding hay as we approach fall. Johnson grass is a plant that tends to grow and proliferate during periods of heat and drought when other grasses fail to grow. As a result many pastures have an abundance of Johnson grass this year in comparison to other forages and during periods when feedstuffs are in short supply, producers naturally consider grazing or haying the Johnson grass. However, under the right conditions Johnson grass can accumulate high levels of nitrate and/or prussic acid and become highly toxic to livestock.
While Johnson grass seems to be the most common concern, other plants can also be toxic during periods of drought stress. Most sorghum and sudan species are predisposed to accumulate toxins, however, certain weeds such as sunflower and pigweed can also accumulate high nitrate levels. The question of safety versus toxicity is often a difficult one to answer due to the many variables at play. Under most conditions Johnson grass and other similar grasses are safe but many factors may influence the plant and in select cases the forage may become highly toxic and quickly kill multiple animals.
Young, growing plants generally have higher levels of nitrate and prussic acid. Plant stress due to drought, frost, trampling or herbicide application can also increase toxicity. A drought-ending rain may lead to high levels of nitrate in the plant for seven to 14 days thereafter as the plant quickly absorbs nitrate from the soil. Plants on heavily fertilized ground are more likely to accumulate toxic levels of nitrate.
There are also some factors at play as it pertains to the livestock consuming potentially toxic plants. Animals accustomed to some level of nitrate intake are more resistant to toxicity.
Testing forage or hay prior to feeding is ideal. However, certain bales of hay or spots in a field may test normal when other spots are toxic. This makes testing helpful but not foolproof. Keeping in mind the various factors that may cause high levels of toxicity is essential and preventing animals from grazing during these times of highest risk is important. When turning into a new pasture be sure that animals are not hungry at turnout and consider using a few sentinel animals turned in first as a ‘test’ to prevent herd-wide toxicity should there be an unsuspected issue. Observe animals closely for the first several hours after turnout. Oral boluses are available to help cows tolerate higher levels of nitrate in the diet and can act as a preventative measure. Furthermore, ensiling harvested forages can reduce toxin levels by 40 to 60 percent.
Should a problem occur it is often hard to cure animals of the toxicity primarily due to the acute onset of disease. Acutely poisoned animals may go down and die in minutes to a few hours post ingestion. Antidotes are available and can be successful but may be hard to find and administer on short notice.
Dr. Darren Loula, DVM, is owner of Christian County Veterinary Service, LLC, a mobile large animal vet clinic.