Nitrate Toxicity & Prussic Acid Poisoning

While we have had some nice showers this summer we have still had some drought situations in parts of the state. Producers should still be concerned about toxicity in forage crops such as sorghum-sudan, sudangrass and millet. Understanding the toxins and how they function is important to utilizing potentially troublesome forages successfully.  The most common toxins are prussic acid and high nitrate concentrations.

Prussic acid is also known as hydrogen cyanide (HCN). It becomes toxic when the precursor dhurrin, which is located naturally in plant tissue cells, converts to cyanide in the rumen of the livestock consuming the forage. In essence the cyanide complex prevents blood hemoglobin from transferring oxygen to individual body cells, and the animal dies from asphyxiation.

Nitrate toxicity is actually a misnomer since nitrite (NO2) not nitrate (NO3) is poisonous to animals. Once the plant is eaten, rumen bacteria rapidly reduce nitrates in the forage to nitrites. The nitrites are generally converted to ammonia and used by rumen micro-organisms as a nitrogen source.

If the nitrite intake is faster than its breakdown to ammonia, they will begin to accumulate in the rumen. Absorption of nitrites in the blood system is rapid, the red blood cells now contain methemoglobin and are unable to transport oxygen and the animal dies from asphyxiation.

Both issues are dependent on the amount of forage consumed, the rate of consumption and the general condition of the animal. Prussic acid acts rapidly, often killing animals within minutes, while nitrate toxicity may not show signs for a few hours after eating or even a few days.

Cyanide is normally more concentrated in the growing point and young leaves of plants while the highest nitrate levels occur in the lower one-third of the plant stock.

Prussic acid poisoning is most commonly associated with regrowth following a drought-ending rain or the first autumn frost. Any stress condition that retards normal plant growth can increase prussic acid content.

How does prussic acid poisoning happen? Hydrogen cyanide is released when plant leaves are physically damaged by trampling, cutting, crushing, freezing, wilting or chewing. The plant cells rupture and the HCN is released into the leaf tissue.

A good rule of thumb is to wait at least five days after a killing frost or until the frozen leaf tissue has completely dried out before grazing. Concentrations are greater in fresh forage than in silage or hay because the HCN is volatile and dissipates as the forage dries or ensiles.

For nitrate toxicity the rules are different. Ensiling stressed forages will convert about 50 percent of the nitrates to a nontoxic form. If forages are harvested as hay, nitrate concentrations remain virtually unchanged over time.

High-nitrate forages may be grazed, but dry roughage should be fed first to limit intake. Light grazing rates should be used because overgrazing forces cattle to eat the stems, which contain the highest nitrate levels. Grazing should not occur for at least seven to 14 days following a drought ending rain.

The best rule of thumb to follow for both conditions is to have samples of suspect forage analyzed before feeding. With the price of cows and calves today $10 is not much to spend to prevent death.

If you have any questions or need additional information contact me at the Harper County Extension Office 620-842-5445 or jlcarr@ksu.edu

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