When frost hits, danger rises

It looks like we’ll be into November this year before we get a serious frost. 

But don’t let the delay in cold weather put you off guard. When one does arrive, a frost event will cause prussic acid to be released from growing sudan grass, johnsongrass or sorghums. The prussic acid is poisonous to livestock. 

I often get the question, “How much just-frozen sudan/sorghum/johnsongrass can I feed?” This question is similar to “How many chambers do I load when playing Russian roulette?” The correct answer is, “Don’t play Russian roulette.” If pressed for an answer, there is a simple one: zero. Zero is the amount of just-frozen sudan/sorghum/johnson grass you can feed. Wait for at least a week. You will be glad you did. 

Most of the prussic acid in plants exists as a bound, nonpoisonous chemical called dhurrin. Dhurrin is a glucoside and when dhurrin reacts with the enzyme emulsin, it forms extremely toxic hydrocyanic acid, prussic acid, cyanide, or HCN. The production of prussic acid is drastically increased when plant tissue is damaged. Freezing is particularly conducive to prussic acid formation, although chopping, stomping or chewing plant tissue will increase prussic acid release.  When prussic acid is absorbed the animal, it prevents the cells from getting oxygen. Actually, the blood can transport oxygen, but the body tissue can’t get it. As a result, animals smother. Affected animals will have bright red, oxygen-saturated blood. It’s a distinctly different color than the chocolate brown blood of animals with nitrate problems. 

Straight sorghums will have the highest levels of dhurrin. As a rule, the shorter the sorghum, the higher the level, i.e. grain sorghums have more than do forage sorghums. Sudan grass will have the least, and Johnson grass is between the sorghums and Sudan grass. Millets seem to not have the problematic concentrations. Wild cherry is known to produce it, and animal problems have been reported when the wilted leaves are eaten, particularly if the leaves had been either storm damaged or pruned. 

HCN-dhurrin concentration will decline with age of the plant. In forage sorghums, this can be particularly frustrating because forage quality drops off rapidly with maturity. HCN concentration will be higher in leaves than in the stalks. The highest levels will be found in short, young, sorghum regrowth, particularly in the suckers, since those are all leaf tissue.

A common recommendation is to let plants reach at 24 inches of height prior to using. Plant stress and increased nitrogen fertility tend to increase concentration, consistent with what is seen with nitrate concentrations in sorghum species.  Maintaining adequate potassium and phosphorous fertility helps lower incidence. 

Challenges with nitrates and prussic acid are often confounded. The plant species involved, the animals affected, the visible symptoms, and the growing conditions are similar. However, the compounds collect in different parts of the plant, prussic acid is highest in the leaves. Nitrates are highest in the lower stalk. And prussic acid is very labile—it dissipates from the forage quickly after harvest-insult. Nitrates do not. Thus, we generate recommendations of manageable nitrate levels in rations, say 0.25% for milk cows. With prussic acid, 0.1% or 1,000 ppm is toxic, but we will have drastically lower prussic acid levels in the forage by the time we get the lab results back. Instead of developing manageable prussic acid levels for rations, we wait for the levels to decline.  When munching on standing sorghums, cattle select for the leaves and against the lower stems.  Grazing behavior biases against the animals eating more nitrates; grazing behavior biases toward them eating more prussic acid.  

Ruminants are more susceptible to prussic acid poisoning than horses or swine. Toxicity symptoms: include anxiety, progressive weakness and labored breathing, followed by death. Animals may show increased rate of respiration, increased pulse rate, gasping, muscular twitching and convulsions. Some mortalities may have no visible symptoms. If a relatively large amount is eaten quickly, the animals will die suddenly. What is more likely to happen is for cattle to eat smaller quantities of the forage over a longer time. The first observations of trouble will be salivation, then a gradual increase in respiratory rate followed by staggering, falling, severe convulsions and death. Veterinarian colleagues have told me that animals that survive 2-3 hours after onset of symptoms will most likely recover. In other words, when such symptoms are observed, immediate veterinarian intervention is the best course of action. 

Steps to avoid/reduce prussic acid issues:

• Do not to graze sorghum, sudan grass or heavily infested johnsongrass fields with young, short growth. Plants two feet tall should be low risk. Grain sorghum regrowth can be especially troublesome. Saying that we should have some sheep come in to clean up the sorghum regrowth on the frost is similar to saying that we need distemper to come in and clean up the barn cats—expect high mortality. 
• Delay grazing sorghum species pastures for a week following a frost; it will frost first on low ground fields. The frost kill may be uneven, the sure visual trigger is that the forage is dried out and brown in color, brown as in paper-bag brown. 
• Do not allow such pastures to be grazed when initial frosts are likely.

Visit with your MFA solutions provider for more information on grazing management this fall.