Tips for Preventing Prussic Acid and Nitrate Poisoning in Cattle

— Written By and last updated by JoAnne Gryder

prussic acidPrussic acid and nitrate poisoning may be two of your biggest challenges when it comes to managing and feeding your summer annuals, but there are a few simple steps you can take to steer clear of livestock health threats.

Sudangrass, forage sorghums and sorghum-sudangrass crosses (all in the genus Sorghum) are often planted for summer pasture and sometimes fed as green chop, silage or hay. Under certain environmental conditions, livestock may develop symptoms of prussic acid poisoning when these forages are pastured or fed as green chop.

Most of the prussic acid in plants exists as a bound, non-poisonous chemical called dhurrin. It is present in most sorghums, but some species and varieties contain less than others.

Also present in the sorghums is a material called emulsion, which under certain conditions can react with dhurrin to form prussic acid (also referred to as hydrocyanic acid). If plants are damaged, such as by freezing, chewing or trampling, the emulsion-dhurrin reaction is enhanced, freeing sufficiently larger quantities of poison (cyanide) to cause a potentially hazardous condition.

Death can result from prussic acid poisoning, most commonly when livestock have fed on plants that are either very young, stunted by drought or frosted. Cattle and sheep are more susceptible than swine, since they are more likely to consume large quantities of the poison.

The signs of prussic acid poisoning appear suddenly, within 15-20 minutes after animals consume the “tainted” forage. These visual symptoms include staggering, labored breathing, spasms and foaming at the mouth. Affected animals then often lie down and thrash about. Treatment must be administered quickly to prevent death.

Generally, animals that survive 2 hours after the onset of symptoms will recover.

  • Never turn hungry animals out onto questionable forage. If feed is questionable, feed good quality hay or silage first.
  • Wait until plants are at least 18 inches to graze.
  • After frost, wait at least 7-14 days to graze or cut, or until leaves are dead or dried out.
  • Make sure hay is properly cured before baling, since cyanide escapes from the plant tissue. Curing decreases prussic acid content by up to 75%.
  • Wait at least 3 weeks after harvesting and storing new silage.
  • If high N is applied to soil that is low in phosphorus and potassium, plants may be at greater risk.

Plants usually break down nitrates into ammonium ions and assimilate them into amino acids or protein, but under various forms of stress, this process stops and the plant continues accumulating nitrates to toxic levels. If ingested by the animal, nitrates in the feed are converted to nitrite in the rumen, which interferes with the blood’s ability to carry oxygen.

Plants commonly accumulate nitrates if they’re stressed by heat and drought, especially if the drought stress was preceded by heavy N fertilization. Most summer annual grasses (and many winter annuals) are susceptible to nitrate toxicity.

  • Split your N applications over the course of the growing season, so the plant has less opportunity to take up excess.
  • Delay harvest 7 days after a drought-ending rain. Nitrate levels will be highest just after the plant resumes growth, and it will take several days of active growth for levels to go down again. Nitrates are non-volatile, and will remain in non-ensiled plants after cutting and baling.
  • Raise cutting height. This can mean 6-8 inches or higher. Nitrate concentration is higher at the plant base, and lower in the leaves.
  • Test all suspect forages at a qualified lab. Take representative samples from the field or core samples from the bale.
  • Segregate all forages high in nitrates. Depending on the nitrate level, these can be fed if diluted with a certain proportion of normal feed, especially by feeding hay to hungry animals before giving them access to the contaminated feed. Generally horses and other monogastrics tend to be more tolerant than ruminants.

Purdue Cooperative Extension

King’s Agriseeds