Around mid-April one of our readers phoned me to ask a question about prussic acid. Before I discuss the conversation that he and I shared, let me write about information that I found on that subject after we both said goodbye.

Crop Comments: Reader asks about prussic acidFor a long time prior to this gentleman’s call, I had thought that term had a German origin – above and beyond whatever the subject might have to do with livestock and crops. With some online sleuthing, I learned that the term originated approximately 150 years ago, when some scientists from Prussia performed chemical experiments which resulted in the creation of a compound containing cyanide.

For those of us reacting quickly and poorly to that word, let’s be reminded of the word “cyano-cobalamin,” which is 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 new compound broke down chemically, yielding an acid, it wasn’t much of a stretch to call it Blausäure (blue acid) and ultimately prussic acid. Not surprisingly, this material did become quite popular as a pigment in paint, adopted by many European artists.

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

My suggestion was that as soon as his 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. And that follow-on crop could sensibly be sorghum, sudangrass or one of their hybrids. I told Jason those crops might experience prussic acid issues at the other end of the growing season.

I also let him know my intentions to advise readers regarding how to avoid prussic acid problems with those hot climate summer annuals. I call them that because, to a large extent, those crops 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. That’s its full name, and its chemical abbreviation is HCN – an extremely simple compound containing one atom each of hydrogen, carbon and nitrogen, whose common name is prussic acid.

Following is how prussic acid wreaks its havoc: When the chemical combines with hemoglobin in the 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. The 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.

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

There’s a magic number relative to 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 applies whether it pertains to the first growth in spring/early summer or attempted regrowth in autumn following non-killing frost.

Here’s the ranking of vulnerability of plants prone to HCN problems – the plants that have the most issues with prussic acid poisoning are, in order of susceptibility, grain sorghum (most), johnsongrass, corn, sorghum/sudan hybrids and finally pure sudangrasses. Those five plants just listed are in the sorghum genus, except corn. Millets (such as pearl millet) 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” – carry-overs 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 the hay dries and 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.) “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.