The facts about nitrates and prussic acid
Some producers throughout the Ozarks could be storing hay or silage that contains dangerous levels of nitrates or prussic acid. Due to the recent drought, conditions are particularly conducive for problems.
“I have seen more Johnsongrass this year than I have seen in 20 years,” Craig Roberts, Ph.D., state forage specialist with the University of Missouri Extension, said. “At the same time, people are putting it up for hay because we have a hay shortage. So, you take those two things together and you have a recipe for a disaster.”
The ingredients in this recipe for disaster are dangerous levels of nitrates and prussic acid found in forages like Johnsongrass. Elevated levels of nitrates or prussic acid can be fatal to animals. However, there are steps producers can take to make sure their forages of safe for their livestock.
Nitrate Accumulation: Toxic levels of nitrates can occur in warm season grasses such as Johnsongrass, sorghum and sorghum-Sudan grasses. After nitrogen gets applied to pastures, the grasses take up nitrogen from the soil in the form of nitrates. During normal growing conditions, grass will convert nitrates into plant protein. However, if the plant growth is stunted due to drought or other factors, the plant continues to pull in the nitrates but cannot convert them to protein. Thus, creating nitrate accumulation in the plant. When the nitrates pile up to an elevated level, they are toxic.
Prussic Acid: Prussic acid is created when a plant takes up nitrates and converts the nitrates into a potentially deadly compound.
“The nitrates are pulled up and they are assimilated,” Roberts explained. “Then they are incorporated into a compound that is part sugar and part cyanide.” At this stage the prussic acid is not toxic.
The danger arises when an enzyme is released in the plant that breaks apart the compound comprised of sugar and cyanide. The enzyme gets released when an animal chews the grass stem, or during a frost or when it is broken in another way. The cell containing the sugar and cyanide bursts under stressful conditions, releasing a fatal dose of cyanide into the animal.
Plants most commonly convert nitrates into prussic acid during times of quick new growth. In particular, after they endure a drought or sustain damage due to frost, hail or herbicide application. The weather pattern most producers have experienced this year has created conditions for the formation of nitrate accumulation and prussic acid in forages such as Johnsongrass, sorghums and sorghum-Sudan grass forages.
Levels in Hay: There is good news in regard to prussic acid levels in stored hay. Prussic acid does not remain in dry baled hay. Over the course of several weeks, the prussic acid dissipates in the form of a gas.
On the other hand, nitrate accumulation does remain in hay. County extension offices have nitrate test kits available to determine if the hay contains low or high levels of nitrates. If the level is high, producers should reach out to an expert to further test their forage for possible toxic levels of nitrates.
Levels in Silage: If the grasses containing elevated levels of nitrates are properly chopped, rolled, baled and wrapped for silage, then as many as half of the nitrates will dissipate. During the ensiling process, a chemical reaction takes place causing some of the nitrates to disintegrate. “Eventually, half of those nitrates will disappear. Unfortunately, the other half is still there,” Roberts stated. Therefore, producers should test their silage to make sure it is safe for consumption.
Managing Regrowth: Many areas in the Ozarks have now received several rounds of rain, spurring regrowth of forages including those susceptible to nitrate accumulation and prussic acid formation. The nitrates are highly concentrated at the base of a plant. Therefore, forage specialists recommend only allowing animals to graze after those forages have reached 2 feet.
“We just like to see that because you can test the tips and the tops and there is not a lot of nitrates,” Roberts explained. “You test the crown and the bottom, and it can be super toxic.”
Though nitrate accumulation can be problematic for producers, if managed properly the forages can be safe for livestock to consume.