Crop Comments: Hydrogen Cyanide: Bad for Cows, Even Worse for Corn Rootworm

Around mid-April, a reader from western New York asked me a question about prussic acid. Before I discuss our conversation, I’ll share some background information.

For a long time prior to this phone call, I thought that term had a German origin. With some online sleuthing, I learned that the term originated about 150 years ago, when scientists from Prussia performed chemical experiments resulting in the creation of a compound containing cyanide.

Let’s forget the negative connotations and be reminded of the term “cyano-cobalamin” (more commonly referred to as vitamin B12). Something made with cyanide isn’t necessarily bad.

Because this new compound was blue and was first concocted in the German state of Prussia, they called it “Prussian Blue.” When the compound broke down chemically, yielding an acid, it wasn’t much of a stretch to call it “blue acid” (in German), and ultimately prussic acid.

What my telephoning reader wanted to know was the proper height to harvest autumn-planted triticale, so as to avoid any threat of prussic acid poisoning. He said I’d written about it in an earlier column. I didn’t recall writing about prussic acid as a health threat borne by triticale; there was little likelihood of that toxin causing health issues with cattle consuming this winter forage.

My suggestion was that as soon as triticale starts showing visible heads, he should start harvesting it for forage, as baleage or haylage. Harvesting at that stage would maximize the total feed nutrients harvested per acre. That would also give him plenty of time to get the next crop in the rotation started. That follow-on crop could sensibly be sorghum, sudangrass or one of their hybrids. Those hot climate summer annual (HCSA) crops might experience prussic acid issues fairly early in their growing season.

I let him know my intentions to advise readers regarding how to avoid prussic acid problems with those sorghum genus HCSAs. I call them that because those crops mostly originated in sub-Saharan Africa.

All these annuals have at least one component in common: dhurrin. Dhurrin is believed to play a role in defense against pathogens, insect pests and uninvited herbivores and helps in regulating critical metabolic processes. On the minus side, under certain circumstances dhurrin breaks down into hydrogen cyanide (HCN). It’s an extremely simple compound containing one atom each of hydrogen, carbon and nitrogen, with the common name prussic acid.

Following is how prussic acid wreaks havoc: When the chemical combines with hemoglobin in red blood cells, the cells will take up oxygen but will not be able to release it. This causes suffocation. The oxygen saturation in the red blood cells causes the blood to have a bright cherry red color. Symptoms of prussic acid poisoning are similar to those of nitrate poisoning, but in the latter health trauma the red blood cells cannot take up oxygen, causing blood to be a dark chocolate brown color.

Nitrate toxicity symptoms include anxiety, weakness, labored breathing and death. Many times, the first proof of nitrate poisoning is a dead animal. But prussic acid inhibits oxygen utilization by the animal at the cellular level resulting in suffocation. According to Warren Rusche, Ph.D. (South Dakota State University Extension beef specialist), ruminants are more susceptible because the rumen microbes have enzymes that release prussic acid in the digestive tract. Death often occurs within minutes of exposure.

One plus for prussic acid toxicity is that this chemical’s residue in sorghum roots does a great job of eliminating corn rootworms, assuming that the grower opts to plant corn the following year. Rootworms have zero tolerance for cyanide.

There’s a magic number for growers dodging prussic acid toxicity: 18. That’s how many inches sorghum, sudangrass and their hybrids should be allowed to grow before ruminants start grazing them. That number pertains to the first growth in spring/early summer or attempted regrowth in autumn following non-killing frost.

The plants that have the most issues with prussic acid poisoning are, in order of susceptibility, grain sorghum (most), johnsongrass, corn, sorghum/sudangrass hybrids and finally pure sudangrasses. The five plants just listed are in the sorghum genus, except corn. Millets (such as pearl and Japanese) do not produce prussic acid but are nitrate accumulators.

Prussic acid levels are higher in younger plants than older plants, higher in leaves than stems and higher in upper leaves than lower leaves. This means the “volunteers” – carryovers from last year’s crop – in the newly planted small grain fields are likely to have the highest HCN levels.

When forages in the sorghum family are cut for hay, prussic acid dissipates as this crop dries, so such hays are safe to feed once bales have reached the stable storage phase. This is also why frosted sorghum/sudangrass is safe to graze after it has field-dried to “standing hay.” (It does tend to dry quite rapidly – enough drying so that, left unharvested very long, it’s prone to field shattering.) “Green” sorghum hays that are still in the heating phase should not be fed.

Prussic acid is destroyed by ensiling and is gone by the time fermentation is complete. A good rule of thumb is to wait three weeks after harvest before feeding hay or silage made from sorghum family forages. Time would be on the patient grower’s side, as the organic acids in the fermenting feed tend to be chemically ruthless on the HCN.

A caution with these HCSAs is that they all want soil temperature to be at least 65º F prior to planting.

by Paris Reidhead