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Angus crossbred stocker cattle in a lush rye grass pasture at sunset.

Cristie Preston, Ph.D.

Following drought stress and even early frost, forages can be susceptible to toxic levels of nitrate accumulation. Levels of toxicity vary based on species, but farmers need to be aware of the potential issues if they plan on grazing this fall. Specifically, accumulation of nitrates can occur in corn, oats, sudangrass, sorghum sudangrass, Johnson grass and alfalfa, to name a few.

In general, when plant growth is normal due to adequate moisture and normal temperatures, nitrates taken up from soil are converted to amino acids and proteins. When growing conditions are not ideal, the uptake of nitrate is faster than the conversion within the plant, and therefore, accumulation occurs.

When it comes to cattle, sheep and goats grazing forages with high nitrates, animals convert nitrate to nitrite faster than nitrite can be converted to ammonia. When nitrates in forages are high, there is an excess converted to nitrite, leading to the accumulation of nitrite in the rumen due to the lack of conversion to ammonia.

Being able to predict nitrate toxicity in forages can be difficult, but some analytical labs do offer nitrate testing of plant material. A good rule of thumb is for farmers to watch grazing crops that had less than normal growth or excessive nitrogen applications in regards to growth/yield.