Prussic acid is a form of hydrogen cyanide that is produced by the partial oxidation of methane and ammonia. Prussic acid often shows up in lethal concentrations in sorghums due to accelerated uptake of ammonia during the rapid growth phase that occurs prior to the plant reaching 18-24 inches in height. Once the sorghum plant reaches 18-24 inches in height, uptake decreases and concentrations dilute to the point where they are no longer toxic to livestock.
"Unfortunately, prussic acid problems in forage sorghums aren't limited to early summer as it tends to rear its ugly head again in the fall on fields that have been hayed or grazed and substantial regrowth has occurred," Mousel said. "Once again, these plants are growing quickly and when they are under 18-24 inches tall or so, prussic acid toxicity to livestock should be a concern for producers."
Mousel explains that this year could be a red-flag for grazing sorghum regrowth.
"In most years, August rains will allow forage sorghum regrowth to quickly get above the 18-24 inch threshold reducing prussic acid problems dramatically. This year though, August has been really dry in a lot of the state and regrowth has been slow," Mousel said. "In most fields in the state, unless you test your forage sorghum regrowth for prussic acid levels, I would not recommend grazing it. The risk for poisoning livestock is simply too great to take a chance grazing it."
Mousel suggests the alternative option of haying the regrowth.
"Although prussic acid in forage sorghum hay can still poison livestock, producers can grind and blend this hay in with other roughages when feeding this winter. When blended at 4:1, the risk of prussic acid poisoning becomes very small. Grinding, blending, and feeding is a real pain, but that is the only safe way to use forage sorghum hay that likely contains high concentrations of prussic acid," he said.
Millet regrowth on the other hand, may have problems this fall as well, Mousel encourages growers to have a nitrate test done before turning livestock out to graze millet regrowth.
"Although millets do not produce prussic acid, they do tend to accumulate nitrates in the regrowth when the soil is dry. Nitrate accumulation in millet regrowth is not as predictable as prussic acid accumulation in forage sorghums therefore; the best thing to do is have a nitrate test done before turning livestock out to graze any millet regrowth. Millet regrowth that tests less than 2ppm nitrates is ok to graze however, if the millet regrowth tests higher than 2ppm, grazing will result in dead livestock," Mousel said.
Like prussic acid, nitrates will remain in the forage and can be toxic to livestock even if the regrowth is cut for hay. Therefore, haying, grinding and blending at 4:1 for feeding this winter is the alternative strategy to manage millet with high nitrates.
When to test
Mousel says paying for a chemical test for prussic acid concentrations in forage sorghum regrowth is probably unnecessary as concentrations are very predictable based on plant height. Predicting nitrate concentrations in millet regrowth however, is a real crap shoot so it is definitely worth the money to have millet regrowth tested before turning livestock out.
To test regrowth for nitrates, simply clip a handful of regrowth in four or five spots around the field. Don't just clip around the edges, get into the middle of the field as well. Place your samples in a Ziploc freezer bag and put in the freezer for 24 hours. It can take several days from the time the sample is taken, shipped, processed, and tested by the lab, so if the wet sample isn't frozen solid, you will not get an accurate test. Be sure to overnight the sample in an insulated container and don't send it at the end of the week so it sits over the weekend.
A nitrate test plus shipping will cost about $50. On 100 pairs that's about $0.25/head.
Nitrate concentrations spike following frost
One final caveat is how nitrate concentrations respond to freezing.
"Nitrate concentrations in forages tend to spike significantly following frost. Producers have to be a little careful because if they test for nitrates in mid- to late-September and the test comes back a little below 200 ppm nitrates but then the field gets a fairly heavy frost a few days later; nitrate concentrations can spike well above 200 ppm for a couple days," Mousel said.
He warns that if this happens, producers need to pull livestock off of the field or confine them to a corner and feed them for a few days then turn them back out to graze. The high nitrate concentrations will diminish rapidly and after a few days will no longer be an issue for livestock.
Regrowth from forage sorghum and millet makes excellent fall forage, just be aware of prussic acid or nitrates before turning livestock out. For more information, contact Eric Mousel at Ericm@millbornseeds.com or 888-498-7333.