Cyanide poisoning and nitrate toxicity – Do you know the difference?

– Dr. Jimmy Henning, Extension Professor, Livestock Forage Specialist, University of Kentucky

Some aspects of forage management are just confusing enough that the same questions come up every year. Take the forage disorders, cyanide poisoning and nitrate toxicity, for example. Questions on these disorders come up anytime the forage sorghum species are grazed and especially in the fall as light frosts predicted. This article gives a quick reminder about these two forage disorders of cattle. (Cyanide toxicity is also called prussic acid toxicity or poisoning).

But first, you have to take a test. What follows is taken from an exam given to juniors, seniors and graduate students who took the UK Forage Management and Utilization class. Ready? Okay, here you go:

Please indicate whether the description below is true of cyanide or nitrate toxicity. In some cases either choice will be correct. (Answers below the ‘quiz’).
______ Dissipates in hay
______ A problem when leaves of freshly frosted johnsongrass or young tender regrowth of sorghums is grazed
______ Causes suffocation
______ Never a problem with pearl millet
______ Usually detoxified by the ensiling process
______ Can be avoided by waiting until sorghums are 24 inches tall before grazing
______ High rates of nitrogen and drought

Will this plant kill cattle? This tender regrowth of forage sorghum can be very toxic to cattle if grazed in quantity. Often confused with nitrate toxicity, cyanide toxicity is a potential problem with all sorghum species, including johnsongrass.

So, what do you think? Easy? Hard? My students had a bit of a problem with it the first time (just might have been the instructor, I am afraid). Here are the answers and some explanations.

Dissipates in hay: Cyanide. Cyanide is released as a gas as sorghums (sudangrass or sorghum sudangrass or johnsongrass) dry out during haymaking.

A problem when leaves of frosted johnsongrass or tender regrowth of sorghums is grazed: Cyanide. In both cases these forages will have high levels of cyanide-producing compounds in their leaves. When consumed by ruminants, cyanide is released in the rumen. Please note that cyanide risk can be several times greater in johnsongrass than the sorghums; some estimate it to be three to five times as toxic. Toxicity with johnsongrass is most frequent in freshly frosted forage, and especially in the new growth that may start after a non-killing frost, like the photo above.

Causes suffocation: Cyanide and nitrate. Both of these toxic chemicals react with the oxygen transport in the blood. Blood from ruminants exposed to high nitrates will be brown. Cyanide toxicity causes the blood to be bright red.

Never a problem with pearl millet: Cyanide. Pearl millet does not contain cyanide-generating compounds like the sorghums. For this reason, many prefer pearl millet over the sorghums for supplemental grazing.

Usually detoxified by the ensiling process: Both cyanide and nitrate. Significant amounts of cyanide and nitrate are either evolved as a gas (cyanide) or metabolized during ensiling (nitrates). Generally, the ensiling process will detoxify forage that would be harmful if consumed fresh. If nitrate toxicity is a concern, collect a sample after a month of ensiling and test for nitrate concentrations. Although nitrate toxicities are infrequent, it always pays to be prudent and test.

Can be avoided by grazing sorghums after they reach 24 inches: Cyanide. Young plants of the sorghums have high concentrations of the cyanide-generating compound dhurrin. Concentrations of this compound are diluted as sorghums grow to 24 inches.

High rates of nitrogen and drought: Nitrate. When heavily fertilized with nitrogen (usually above 80 lb N/A) and under drought stress, the sorghums AND pearl millet (and many other plants) can accumulate toxic levels of nitrate in their stems. The concentration of nitrate is higher near the soil and gets lower as you move up the stem. UK ag agents have access to test strips that can indicate if high levels of nitrate are present in stems. If this quick test is positive for nitrate, submit a sample for analysis to measure actual concentrations present.

How did you do? Pretty well, I hope. As you might imagine, there is much more information available on the production of summer annuals, and toxicities of cyanide and nitrate. To learn more, please see UK publications AGR 229 “Warm Season Annual Grasses in Kentucky”, ID 220 “Cyanide Poisoning in Ruminants” and ID 217 “Forage-related Cattle Disorders: Nitrate Poisoning.”

Happy Foraging.