Nitrate and Prussic Acid Concerns in Warm Season Forages
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Collapse ▲Nitrate and Prussic Acid Concerns in Warm Season Forages
Warm-season annuals are a great tool when tall fescue and other cool-season perennials become dormant in the summer. Whether it is a planted sorghum/sudan variety or volunteer johnsongrass (perennial), both can be palatable to livestock and have excellent nutrient values. However, at certain times, they can be a potential risk to livestock due to toxicities. Nitrates and prussic acid accumulation are the two biggest concerns when utilizing warm-season annuals.
Nitrate Poisoning
Warm-season annuals have the ability to accumulate nitrates. When the concentration of nitrates becomes too high, they can become deadly. In ruminant animals, the nitrates are converted to nitrite in the rumen. Once it turns to nitrite, it is absorbed into the bloodstream. This becomes an issue because nitrite interferes with blood’s ability to move oxygen. Symptoms can include trembling, staggers, heavy breathing, and frequent urination. A lot of the time, symptoms are missed because death can occur within a couple of hours.
Nitrate accumulation can be from heavy nitrogen fertilization followed by a drought. During a period of drought, nitrates are not converted into proteins and other plant nutrients. One thing to note with nitrates is that the levels do not dissipate when the forage is harvested as dry hay. When ensiled, the nitrate levels can be dropped 40-60%.
When feeding summer annual forages, it is a good practice to test for nitrates as well as a complete nutrient analysis. The NCDA&CS offers forage testing. Nitrate testing is free all year long and a complete analysis can be done as for $10. The NCDA&CS submission form can be found here. Many private labs can be utilized for forage testing as well.
Below is a chart from Penn State Extension to help determine nitrate levels from a forage analysis:
Prussic Acid Poisoning
Prussic acid is also a concern in warm season annuals. Prussic acid is formed when the plants are stressed or damaged. The most common threats are after a drought-ending rain and after a killing frost. Prussic acid is a cyanogenic compound that is in all species of sorghum as well as johnsongrass. Sorghum and johnsongrass have the highest levels of prussic acid. It has been found that sudangrass contains on average 40% less prussic acid than sorghum varieties. To help reduce the risk of prussic acid poisoning, pearl millet can be incorporated. Pearl millet does not contain toxic levels of prussic acid.
When these summer forages become stressed due to killing frost or drought-ending rain, wait 7-10 days to graze. With regrowth, it is best to wait until plants reach at least 2 feet tall or 2 weeks. The new regrowth may contain a high level of prussic acid.
Prussic acid poisoning symptoms are similar to nitrate poisoning and can result in death. Unlike nitrate accumulation, prussic acid does dissipate. Therefore, hay and ensiled forages that are harvested properly are safe for livestock to consume.