According to Steve Barnhart, Extension agronomist at Iowa State University, the first few frosts of autumn bring the potential for prussic acid poisoning when feeding forages. Some forage species, primarily sorghums and closely related species, contain cyanogenic glucosides. These are converted quickly to prussic acid in freeze-damaged plant tissue. Although in Iowa there are very few documented cases of prussic acid poisoning, Barnhart stressed that the risk is still present, and good management practices are necessary to minimize those risks.
Prussic acid (hydrocyanic acid) is a cyanide compound that can kill animals within minutes of ingestion under the right circumstances. HCN is a very simple compound, consisting of one hydrogen, one carbon and one nitrogen (HCN).
Quoting Barnhart, “HCN interferes with the oxygen-carrying function in the blood, causing animals to die of asphyxiation. Symptoms include difficult breathing, excess salivation, staggering, convulsions and collapse. Affected animals have bright cherry red mucous membranes caused by this compound. Ruminants are more susceptible than horses or swine because they consume large amounts of forage quickly, and the rumen bacteria contribute to the release of the cyanide from ingested plant tissue.”
Sudangrass varieties are low to intermediate in cyanide poisoning potential, sorghum-sudangrass hybrids and forage sorghums are intermediate to high and grain sorghum has high to very high poisoning potential. (All these hot climate summer annuals are in the Sorghum genus.)
Pearl millet and foxtail millet have very low levels of cyanogenic glucosides (and thus pose very little cyanide threat). A few other plants also can produce prussic acid, including cherry trees. Prussic acid does not form in sorghum and sudangrass plants until the leaf tissue is ruptured, as with grazing or chopping.
Young, rapidly growing plants have the highest levels of prussic acid. What takes place in the creation of this tiny toxic compound is a type of metabolic derailment – an unsuccessful attempt to synthesize protein – which I liken to a baby trying to take their first clumsy steps. The cyanide-producing compounds are more concentrated in young leaves.
Barnhart recommended minimum plant heights of 18 inches in order to avoid formation of risky (potentially toxic) young leaf tissue. Cornell animal scientists prefer a minimum of 20 inches. Scientists from both colleges say that plants growing under high N levels are more likely to pose elevated cyanide threats.
Frost and freezing can cause a rapid change in prussic acid risk in plants of any age or size. With frost, forage tissues rupture and cyanide gases form. The cyanide gas can be present in dangerously high concentrations within a short time, remaining in the frosted leaves for several days. Because cyanide is a gas, it gradually dissipates as the frosted/frozen tissues dry. Thus, risks are highest when grazing frosted sorghums and sudangrasses that are still green.
New growth of sorghum species, following frost, can be dangerously high in cyanide due to its young stage of growth (hence my term “clumsy”). Prussic acid content decreases dramatically during the hay drying process and during ensiling. Frosted foliage contains very little prussic acid after it is completely dry. Sorghum and sudangrass forage that has undergone silage fermentation is generally safe to feed.
On Oct. 26, Hank from Lewis County called me with questions relating to a possible sorghum toxicity threat. He had just finished filling a small (by today’s standards) silo. It was 12- or 14-foot diameter (I forgot to ask him which). His silo unloader had eight or 10 tines. He did not treat the chopped forage with a silage inoculant and wanted to know how long he should wait before feeding this fermenting forage to his small organic dairy herd.
I told him that two to three weeks of fermentation time should be adequate for the chemical destruction of any HCN in the silage. (The use of a probiotic forage inoculant would accelerate the fermentation process, thus shortening that wait time.) The beneficial chemicals here are the organic (carbon-based) compounds lactic acid and acetic acid.
I also told him that lactic acid is what you smell in properly prepared sauerkraut (something he was familiar with). And acetic acid is the main component in vinegar. These two acids react chemically with the HCN, detoxifying it by rendering it into non-protein nitrogen (NPN). Rumen microbes take this NPN and convert it into amino acids, ultimately incorporating them into their own microscopic bodies. These can now be utilized by the host organism – in this case, Hank’s cows – to make milk and meat.
Sorghum species’ natural prussic acid wipes out corn rootworms so corn can be planted the following year without damage (that’s the “plus” I alluded to in my title). It is direct harvested with one cut and with no grain, so growers don’t have to pay for extra fuel for processing.
Why is this simple chemical called prussic acid? It is the common name for hydrogen cyanide because it was first isolated from the pigment “Prussian blue” by a Swedish chemist in 1782; the name essentially translates to “blue acid” in German (“Blausäure”). That blue pigment originally developed in the German state of Prussia, hence its name.
Returning to my conversation with Hank, I told him that if his sense of smell is good, he will be able pick up the pleasant odor of a blend of lactic acid and acetic acid. This almost guarantees that prussic acid concerns are eliminated. For those whose sniffers are less than perfect, forage tests will prove to be money well spent.
Even folks with imperfect olfactory systems should be able to detect the presence of butyric acid (smells like rotten eggs). This acid also helps neutralize HCN hazards. However, fed to dry cows, butyric acid can predispose to the formation of ketone bodies – and we know where that might end up. Well, there’s another plug for forage testing, for which I apologize … sort of.
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