With much of the country experiencing drought, consider factors that may be a concern for livestock during dry conditions, especially nitrates and prussic acid.

In non-drought years, these compounds do not accumulate in plants to levels that could become toxic to livestock. But when plant growth is inhibited, the risk increases due to the plant not developing at a normal rate.

Prussic acid is easier to manage and not found in as many plants as nitrates, but caution is especially necessary after a killing frost.

Symptoms of poisoning

Most frequently, dead animals will be found before symptoms are observed. Within the rumen, the hydrogen cyanide (HCN) is rapidly converted to cyanide and absorbed into the bloodstream. Symptoms include nervousness, increased respiration, muscle twitching and trembling, foaming at the mouth, blue coloration of mucous membranes, convulsions and death.

Prussic acid does not prevent hemoglobin from transporting oxygen, like nitrates, but instead prevents body cells from receiving and absorbing oxygen, and the animal asphyxiates at a cellular level. Prussic acid poisoning is characterized by cherry-red blood, as it is over-oxygenated.

Grazing considerations

Only a small number of plants can contain prussic acid, and they are in the sorghum family. Common forages include sudangrass, sorghum, sorghum sudangrass hybrids and Johnsongrass. The prussic acid is concentrated in the leaves of these plants; therefore, grazing immature plants, or immediately following a frost, can be detrimental.

The HCN is highly concentrated in young, immature and leafy plant growth, so beware of regrowth in fields that have been hayed or in drought-stressed fields where growth has been stunted.

A general recommendation is that sudangrass should not be grazed until it reaches a minimum height of 18 to 20 inches tall, and hybrids should not be grazed until they reach 24 to 30 inches tall. Sorghums are generally safe for grazing after plants are harvested or fully mature (dormant). Nitrogen fertilizer can also increase HCN levels, similar to nitrates.

If you plan to graze any of the plants in the sorghum family and the plants have suffered a killing frost, wait 10 days to two weeks before grazing. This will allow for the prussic acid to dissipate. If it was only lightly frosted, ensure that the plant height exceeds the recommendations previously discussed and beware of regrowth.

Testing for toxicity

If plants are drought-stressed or have not reached the minimum recommended height, send a representative sample of the field to a commercial lab for testing before grazing to prevent livestock losses.

Once test results are received, management practices can be implemented to utilize the field. One option would be to swath-graze if levels of prussic acid are elevated. This would allow the prussic acid to dissipate before animals eat the forage and preserve the hay at a slightly higher nutritional value compared to leaving it standing and grazing it after a killing frost. Under most circumstances, silage and hay can safely be fed to livestock. About two weeks after a killing frost, standing crops would be safe to graze.

There are some important differences between nitrates and prussic acid, but it is important to take precautions when utilizing feeds that could contain one or both compounds.

Find more information about nitrate toxicity from SDSU’s frequently asked questions.

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