Toxic Plants, Nitrate Toxicity and Prussic Acid Poisoning

This season has been or has become a drought year for many of us in Texas. With drought conditions come fears in regards to toxic plants, nitrate toxicity and prussic acid poisoning.


Carolina Horsenettle

Carolina Horsenettle

There are numerous plants in Texas that can be toxic to livestock (cattle, horses, goats, etc.). Toxic Plants of Texas  is a great website with a list of toxic plants along with images, livestock affected and livestock signs. Always make sure your livestock have sufficient forage/feed that will meet their nutritional needs.


When livestock consume forages, nitrate is normally converted in the rumen from:

nitrate to nitrite to ammonia to amino acid to protein

When forages are unusually high concentration of nitrate, the animal cannot complete the conversion and nitrite accumulates. Nitrite is absorbed into the bloodstream directly through the rumen wall and converts hemoglobin in the blood to methemoglobin which cannot carry oxygen. An animal dying from nitrate (nitrite) poisoning actually dies from asphyxiation, or a lack of oxygen.

Many plants can accumulate nitrate. Plants in the sorghum family — Johnsongrass, sudangrass, forage sorghum and sorghum hybrids. Corn, small grains, pigweed, sunflower and leafy vegetables that are highly fertilized can accumulate toxic levels. Turning cattle into holding pens or corrals full of manure with careless weeds or grasses can result in immediate poisoning.

Nitrates do not accumulate when there is normal rainfall or irrigation. Under those conditions, nitrate nitrogen absorbed by roots and moved into the plant is rapidly transformed into plant proteins. However, under dry conditions, plant roots continue to absorb small amounts of nitrogen, but the plant has too little water to keep growing. Nitrate accumulates and is stored in the lower leaves and stems, ready for the plant to mobilize and use when rapid growth resumes. Following rainfall and some time for active growth, once high nitrate plants will eventually have safe levels (as long as they are not harvested at high nitrate levels).

Preventing Losses

  • Never turn hungry animals into possibly high nitrate forages.
  • Have hay tested before feeding if you suspect that it is high in nitrate. Nitrate levels remain constant in hay.
  • Ensile forages high in nitrate. When hay is properly fermented, nitrate levels are reduced by 40 to 60 percent.
  • Test drought-stressed warm season annual hay crops for nitrate prior to harvesting since the nitrate levels will not diminish after harvesting.


Prussic acid poisoning is also called hydrocyanic acid or cyanide poisoning. Cyanogenic compounds can develop in plants that are stressed; in the rumen the compounds are converted to cyanide, which can kill livestock.

Livestock can show symptoms of intoxication within 5 minutes of eating plants with the poison, and may die within 15 minutes. Salivation and labored breathing occur first, followed by muscular tremors, uncoordinated movements, bloating, convulsions and death from respiratory failure.

Prussic acid can accumulate in plants in the sorghum family, such as johnsongrass, sudangrass, forage sorghums and grain sorghum. It appears to occur when plants are injured by frost. Severe drought stress can also cause prussic acid to form.

High concentrations may be associated with rapid growth, such as shortly after a rain irrigation on previously drought-stressed fields, or warm weather after a cool period. Under good conditions, toxic concentrations can also form in young, rapidly growing plants. Prussic acid dissipates from plant properly cured for hay.

To prevent prussic acid poisoning:

  • If plants have been damaged by frost, defer grazing until they either are well recovered from injury or cut for hay, or after a killing freeze and the plants have been allowed to dry.
  • Do not graze plants in the sorghum family until they are 2 to 3 feet tall.
  • Remove all livestock from the feed source when an animal is found to have died suddenly after grazing forages under poor growing conditions.
  • After plants have grown rapidly, such as shortly after a rain irrigation on previously drought-stressed fields or warm weather after a cool period, wait at least 2 weeks after the plants begin to grow before grazing.

Contact the Texas A&M Veterinary Medical Diagnostic Laboratory for for more information on Nitrate or Prussic Acid analysis.


Vanessa Corriher-Olson, Forage Extension Specialist, Soil & Crop Sciences, Overton, TX

Larry Redmon, State Forage Extension Specialist, Soil & Crop Sciences, College Station, TX

Texas A&M AgriLife Extension Service, Texas A&M University System

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