How to Prevent Prussic Acid and Nitrate Poisoning in Livestock

Recently, Green Cover founder Keith Berns sat down with grazing expert Dale Strickler to discuss two critical livestock health risks: prussic acid and nitrate poisoning. Understanding these toxins can mean the difference between a healthy herd and devastating losses. Here are the essential takeaways from their conversation.

Part 1: Prussic Acid Poisoning

The Science Behind Prussic Acid

One of the most commonly used cover crops for summer grazing is forage sorghum and sorghum-sudangrass hybrids. These plants are extremely productive but also contain components that can form hydrogen cyanide, more commonly known as prussic acid. It is produced from the breakdown of cyanogenic glycosides, primarily dhurrin, when plant tissues are damaged by chewing, frost, or wilting. Under most conditions this compound exists in low enough concentrations that it is not harmful to cattle, but in some circumstances prussic acid can build up to toxic levels, causing serious illness or even death in livestock. Understanding how prussic acid forms and how to manage around it is important and should be taken seriously by every livestock producer.

Consider the structure of a leaf. No matter how thin a leaf may seem, it contains many layers of tissue—sort of like a sandwich. On the outside, you have the epidermis, which functions similarly to our own skin. Think of that as the bread layers in the sandwich. In the middle, you find the mesophyll, which has many functions, including nutrient transport. Think of that layer as everything else in between the bread layers in the sandwich.

Sorghum plants contain two compounds that create cyanide when combined. Under normal conditions those two compounds are housed separately, one in the epidermis and one in the mesophyll. However, when the two compounds come in contact with each other, hydrogen cyanide (prussic acid) is produced.

Livestock chewing on plants can cause some leaf tissue cells to burst, which produces hydrogen cyanide. This usually isn't an issue as formation is slow and levels remain low. Freezing temperatures or frost, however, expand the water in the cells and cause all the leaf cells to burst, creating a perfect opportunity for massive buildup of hydrogen cyanide. Grazing livestock on forage in this state can cause serious problems, including fatalities.

Luckily, prussic acid will volatilize and dissipate from the plant as it begins to dry down. Once the plant turns dry and brown, it is safe to graze again, usually after 7-14 days, depending on the severity of the freeze.

Risk Factors for Prussic Acid

The risk of prussic acid poisoning is greatest in sorghum plants with new regrowth under 18 inches tall, especially in grain sorghum, which typically has higher levels of this toxic compound. Implementing proper rotational grazing can reduce risks by allowing sufficient rest for sorghum to regrow adequately before grazing resumes.

All sorghum and sorghum hybrid plants contain some prussic acid, but levels vary significantly by type. From highest to lowest prussic acid content:

1. Johnsongrass — Highest risk; avoid grazing when possible
2. Grain Sorghum — High risk; careful management essential
3. Forage Sorghum — Moderate risk; good with proper timing
4. Sorghum-Sudan — Lower risk; popular for grazing
5. Sudangrass — Lowest risk among sorghum species

Keeping Animals Safe from Prussic Acid

Sorghum products are excellent for growing large amounts of highly palatable biomass for livestock, but if you plan to have livestock grazing sorghum-containing forages when frost arrives, it's crucial to have a safety plan. When planning to graze sorghum-containing forages during frost season, choose one of these strategies:

Strategy 1: Plant Alternative Species

Perhaps the simplest approach is using a cover crop mix without sorghum. Millet serves as an excellent replacement for sorghum-sudangrass, though you may see slightly lower tonnage. Alternatively, plant just a portion of the field in a sorghum-free mix, ensuring a safe grazing area immediately following a frost.

Strategy 2: Bale Grazing

Partition off an area without sorghum plants and set up a bale grazing system for 7-14 days until the sorghum has dried sufficiently.

Strategy 3: Pre-Frost Swathing

Swath a paddock one to two days before an expected freeze. This allows the forage to begin drying, preventing the massive cell rupture that occurs when standing plants freeze. The prussic acid poisoning only becomes critical when cells burst simultaneously, causing a flood of cyanide. By swathing before the freeze, you allow the forage to dry enough that cells won't burst and release cyanide. You can then graze this swathed area while waiting 7-14 days for standing forage to dry completely.

Part 2: Nitrate Poisoning

The Science Behind Nitrate Poisoning

While prussic acid is specific to sorghum species, nitrate poisoning presents a broader risk across many forage types. High nitrate levels can affect nearly any plant, posing a risk of nitrate poisoning in livestock. Like prussic acid poisoning, nitrate poisoning causes cellular suffocation, though through different mechanisms.

Nitrate poisoning disrupts the blood's ability to carry oxygen from the lungs, leading to symptoms like labored breathing in affected animals. Prussic acid poisoning prevents cells from using oxygen from the bloodstream. In both cases, oxygen-deprived cells malfunction and gradually die.

Understanding Nitrate Risk

Plants convert nitrogen into protein, which is essential for both the plant and the animals that consume it. Plants primarily absorb nitrogen as nitrate from the soil. Photosynthesis provides the energy needed to transform nitrate into protein. However, nitrates can accumulate in plants when nitrogen is abundant in the soil or when photosynthetic activity is reduced.

Factors such as drought, insufficient sunlight, diseases, or pests can limit photosynthesis, leading to higher nitrate levels in the plant. This issue worsens when soils have excess nitrogen, such as from over-fertilizing a previous crop.

Plants particularly susceptible to nitrate accumulation include cereal rye, oats, and other small grains; corn stalks and stover; sorghum and sudangrass species; and certain weeds such as pigweed, lambsquarters, and kochia.

Managing Nitrate Risk Through Fertilization

To maximize forage tonnage while preventing toxic nitrate accumulation, follow these strategies:

• Before Planting: Test your soil using a Haney soil health test to assess available nitrogen. Ensure adequate phosphorus, sulfur, zinc, molybdenum, and cobalt for proper photosynthesis by conducting soil tests and amending deficiencies. Plan to split nitrogen applications rather than applying all at once.
• At Planting: Apply slow-release products like polymer-coated urea in furrows for steady nitrogen supply. If using synthetic nitrogen, combine fertilizer with molasses and sulfur to encourage soil microbes to convert nitrogen into microbial biomass, creating an organic, slow-release form. Include legume cover crops for natural nitrogen fixation; they provide nitrogen as they decompose for subsequent crops.
• During Growth: If conditions remain dry or cloudy (limiting photosynthesis), skip late-season nitrogen applications. If following a heavily fertilized cash crop, test forage nitrate levels before grazing, as residual fertilizer makes testing essential.

Testing and Safe Levels

Forage testing for nitrates is quick, affordable (typically $15-30 per sample), and straightforward. Understanding test results helps you make safe feeding decisions. Nitrate levels below 0.5% of dry matter are safe for all livestock. Levels between 0.5-1.0% are generally safe but require monitoring of pregnant animals. Levels between 1.0-1.5% are potentially toxic and should be limited to half the diet. Levels above 1.5% are toxic and should not be fed. Always consult your veterinarian for interpretation of test results and feeding recommendations specific to your herd.

Special Considerations: Haying vs. Grazing

Prussic acid dissipates during the hay drying process, but nitrate levels remain a concern; they can become more concentrated in dried forage. If standing forage already has high nitrate levels, the hay will have even higher concentrations, since approximately 90% of nitrates concentrate in the lower stems, with minimal amounts in leaves and upper portions.

To reduce nitrate content in hay, cut forage as high as possible (8-12 inches) to avoid the nitrate-rich lower stems.

In grazing situations, particularly with rotational grazing, nitrate poisoning is less of a concern. Livestock naturally consume leaves and plant tops first, which are lower in nitrates. As long as animals aren't forced to graze too close to ground level, the risk of nitrate poisoning is significantly reduced. When uncertain, forage testing for nitrate levels is a cost-effective prevention strategy.

Part 3: Recognition and Emergency Response

Both prussic acid and nitrate toxins cause similar symptoms including labored rapid breathing, weakness and staggering, muscle tremors, excessive salivation, and convulsions in severe cases. Death can occur within minutes to hours depending on exposure level. The key difference is timing: prussic acid poisoning progresses very rapidly (minutes), while nitrate poisoning develops more slowly (hours). If you suspect poisoning, immediately remove animals from the forage and contact your veterinarian without delay. Be prepared to describe the symptoms and what the animals were eating. Veterinary treatments include sodium thiosulfate for prussic acid poisoning and methylene blue for nitrate poisoning, but both must be administered quickly. Time is critical—early veterinary intervention significantly improves survival chances.

Animal health and well-being should guide all management decisions for livestock producers. Understanding the risks of prussic acid and nitrates forms the foundation, but protection requires three ongoing practices. First, test regularly—forage testing is inexpensive insurance against devastating losses. Second, observe carefully by watching for changes in plant appearance and animal behavior. Third, plan ahead by knowing your frost dates, monitoring weather forecasts, and having alternative grazing areas ready. Remember: When in doubt, test. A $20-30 forage test is far cheaper than losing even one animal. Understanding these risks, combined with regular forage testing and careful observation, ensures your herd remains healthy and well-fed throughout the grazing season.

Many thanks to Dale Strickler for sharing his extensive expertise on these critical livestock health topics.

Quick Reference: Prussic Acid vs. Nitrate Poisoning

• Prussic Acid: Bright red blood (cells can't use oxygen); symptoms develop rapidly (minutes)
• Nitrate: Brown blood (blood can't carry oxygen); symptoms develop more slowly (hours)
• Both: Labored breathing, weakness, muscle tremors, cellular suffocation