This time of year, occasional questions arise about the risk of nitrate and prussic acid poisoning in livestock. This is typically a concern of livestock producers utilizing annual forages that experience a freeze. However, this matter is much more nuanced and can be difficult to understand.
During times of plant stress, decreased light (cloudiness and day length) or as a result of excessive nitrogen fertilization, nitrates can accumulate and become toxic to livestock. Warm season forage crops, including sorghum, sudangrass, sorghum/sudangrass hybrids and millets are some of the common forages in which this can occur. Yellow sweet clover and Brassicas (mustard family) that are commonly used in cover crop mixes can also accumulate nitrates. What has been called “oat hay poisoning” historically is nitrate poisoning. The mechanism of nitrate poisoning starves livestock tissues of oxygen and can occur rapidly. The only way to determine if excessive nitrate levels are present is with a nitrate analysis. Local extension offices have a simple presence or absence test, but it cannot tell actual levels. The lab test is inexpensive, especially when compared to the potential cost of poisoning livestock.
Prussic acid poisoning
Prussic acid is naturally found in sorghum, sudangrass, millet and others. Sorghum and sudangrass, and their hybrids, typically have the highest concentrations. Generally, the greatest levels of prussic acid can be found in the leafier areas of the plant. Prussic acid does not begin to decline until after the leaves have died. New vegetative growth is highest in prussic acid and poisoning most commonly occurs on regrowth following drought or a non-killing frost. Testing for prussic acid levels is not done locally and tests are expensive. Prussic acid poisoning has similar symptoms to nitrate poisoning, but it causes asphyxiation by inhibiting the enzyme that links oxygen to red blood cells.
Simple management points
If forages high in nitrates are cut, those nitrates stay in the cut forage. The only way to use them safely is through dilution with other feed or ensiling. Prussic acid on the other hand will dissipate following cutting.
Do not graze annual forages until one to two weeks after a killing frost. Plants should be dried. Verify that no new tillers are emerging.
Stressed annual forages are more dangerous. This can be from drought, frost, trampling, nutrient deficiency, etc.
Higher nitrogen soils cause greater risk of increased levels of both nitrate and prussic acid. Toxicity could occur well before a frost under heavy fertilization.
Nitrate levels of at-risk forages should be tested regardless of how long ago a killing frost occurred.
Nitrate poisoning: aces.nmsu.edu/pubs/_b/B807/welcome.html
Prussic acid poisoning: aces.nmsu.edu/pubs/_b/B808/welcome.html