When it comes to the what, when and how of nitrates and prussic acid in your forages, there are differences and similarities. This basic information can help you distinguish these sources of poisoning in livestock.
A significant risk of grazing forages that are damaged by frost or growing after a drought-ending rain is hydrocyanic acid poisoning, more commonly known as prussic acid or cyanide poisoning. Plants in the sorghum family are susceptible to prussic acid formation. These include johnsongrass, sudangrass, sorghum and sorghum-sudan hybrids. Pearl millet does not produce prussic acid (but does accumulate nitrates) and can be safely grazed following a frost. Under normal growing conditions, these plants produce a nontoxic substance called dhurrin. When plants are injured by frost or wilting, enzymes come into contact with dhurrin and release toxic prussic acid or cyanide. Prussic acid is lethal to animals because the cyanide prevents oxygen transfer from the blood, and animals suffocate at the cellular level.
Prussic acid poisoning often occurs very rapidly. The time from ingestion of toxic forages to death can be as brief as 10 to 15 minutes. Typical animal symptoms include excessive salivation, rapid breathing and muscle spasms. Animals are occasionally observed staggering through the pasture before collapse and death. The only reliable method to avoid animal losses is preventative management. Successful treatment is almost impossible because of the rapid progression, so animals must be removed from toxic pastures immediately.
Managing for prussic acid
- High levels of prussic acid form when sorghums are injured by frost, resume growth following drought, have high soil nitrogen or low phosphorus levels and after 2,4-D applications. Having a good soil fertility program to ensure the conversion of nitrogen is most effective and will help minimize the risk of both nitrate and prussic acid poisoning. Excessive nitrogen applications will increase the risk for both types of poisoning.
- Don’t graze the crop until it is 18-24 inches high. Young plants and regrowth have higher prussic acid levels.
- Do not graze or green chop after a light frost. Wait 10-14 days before grazing or green chopping. After a killing frost, wait until the plant has completely dried for at least 10 days.
- Prussic acid tends to accumulate more in shorter sorghums, less in taller plants. Leaves accumulate more prussic acid and fewer nitrates. Stalks are higher in nitrates and lower in prussic acid.
- Testing is usually not useful. By the time lab results get back, the prussic acid will have dissipated.
- Most prussic acid is lost during the curing process. Crops cut and allowed to wilt before chopping or ensiling and field drying will allow for the acid to volatilize from the forage.
Nearly all forages contain nitrates. Ruminants (via rumen microflora) are able to convert the nitrates to nitrites and then eventually to ammonia, which is usable by the animal. However, nitrites are also able to enter the bloodstream where they convert hemoglobin (an oxygen-carrying molecule) to methemoglobin (unable to carry oxygen). Thus, animals consuming forages with too many nitrates, especially over a short period of time, may produce high levels of methemoglobin, which can lead to reproductive loss and/or death.
Physical signs of excessive nitrates are difficult and rapid breathing, muscle tremors, low tolerance to exercise, incoordination, diarrhea, frequent urination, collapse and death. Abortions can happen following nitrate poisoning. Nitrates dilate blood vessels that lead to peripheral circulation loss and loss of oxygen to the fetus.
Nitrates are taken up by the plant and converted to plant products in the leaves. However, when normal plant processes are disrupted, nitrates concentrate in the stalk. Plant stressors include growing under shaded or low-light conditions, herbicide applications, diseases and detrimental weather such as drought, hail, frost and low temperatures. The amount of nitrates accumulated will also depend on plant species, stage of maturity and nitrogen fertilization.
If you have questions or concerns about prussic acid or nitrates, contact your MFA livestock specialist or AGChoice location.
Managing for Nitrates
- Nitrates can accumulate when plants are stressed and in soils where ample nitrogen is available.
- Best practice is ensiling to reduce nitrate levels. Allow three weeks for full ensiling to occur. Dilute high-nitrate silage with normal forage and/or grain.
- Dry baling concentrates nitrates more. Nitrate doesn’t dissipate like prussic acid.
- Do not green-chop or feed direct-cut forage if high nitrate levels are present.
- Delay harvest several days after a drought-ending rain and right after herbicide application.
- Cut high-nitrate fields at a high stubble height. Nitrates tend to accumulate in the lower third of the stalk.
- Younger plants will be higher in nitrates than mature plants, unless mature plants are under stress and/or in high-nitrogen soil.
- Species such as sorghums/sudangrasses store high levels of nitrate, and brome and orchardgrass have very little under normal growing conditions. Small-grain annuals and millet that are hayed will have more nitrates if harvested immature. Legumes generally do not contain high levels.
- It’s best to test for nitrates to ensure they are at safe levels.