Extension Instructor Comments on Risks of Summer Annual Forages

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Summer annual forages like sorghum-sudangrass and millet have become popular additions to many feeding programs across our region. With robust growth in the hot summer months, they can help to fill some of the gap during the “summer slump” in many of our pastures, and provide a somewhat deer resistant option for fields that experience heavy feeding pressure. With a high forage quality and potential for multiple cuttings from some varieties, they can be an enormous asset to any animal feeding operation. However, Penn State Extension Educators want to remind farmers of some potential toxicity concerns that should be addressed for these forages, especially under stressful environmental conditions.

Nitrate poisoning can be a major concern with these forages during periods of drought stress or after excessive nitrogen fertilization. Under these conditions, nitrates and nitrites accumulate in the lower portion of the plant and can be toxic when consumed in large concentrations, or in combination with drinking water with a high nitrate content. The only way to determine if high nitrate levels are present in the forage is with a plant analysis.

There are several practices that can help to mitigate potential issues with nitrate toxicity. The best option is to delay harvest until stressful conditions have passed. Increasing the cutting height of the forage can also help to reduce overall nitrate concentration as most of the accumulation is in the base of the plant. Finally, fermenting the forage has been found to reduce nitrate concentration in the forage by about half over the period of fermentation. These forages are well suited to store as baleage or silage. If forage quality testing reveals a high level of nitrate, it may still be possible to feed these forages on a limited basis by diluting them with forages with lower nitrate levels. Consult a veterinarian or qualified nutritionist to discuss options.

Prussic Acid poisoning becomes a concern after a killing frost. Prussic Acid is a cyanogenic compound found in many of these summer-annual grasses. After a killing frost, the compound is altered to a form that can be toxic to livestock in extremely low doses. Sorghum and sorghum-sudangrass hybrids contain the greatest levels of prussic acid. Pearl Millet and Foxtail Millet typically do not contain toxic levels of prussic acid, so they can continue to be grazed at any time during the year.

For species with higher concentrations of prussic acid, proper timing of grazing and cutting is very important after a killing frost. The toxicity levels do not decline in the plant until after the leaves have died. To be safe, wait at least 7-10 days after a killing frost to harvest these grasses. Ensiling and fermentation can help to reduce the levels of toxic compounds slightly. Forages should be fermented for a minimum of 8 weeks and should be analyzed for cyanogenic compounds prior to feeding.

Northeastern and northcentral Pennsylvania producers are invited to contact Casey Guindon, Agronomy Extension Educator, with issues related to summer annual forages. She is based out of the Bradford County office. Casey can be reached at 570-265-2896 or cvb5538@psu.edu.

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This article, including its text, graphics, and images (“Content”), is for educational purposes only; it is not intended to be a substitute for veterinary medical advice, diagnosis, or treatment. Always seek the advice of a licensed doctor of veterinary medicine or other licensed or certified veterinary medical professional with any questions you may have regarding a veterinary medical condition or symptom.

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