Recently, Keith sat down with grazing expert, Dale Strickler, to discuss prussic acid and nitrate poisoning. Here are some practical takeaways from that conversation.
Prussic Acid – The Science
One of the most commonly used cover crops for summer grazing is forage sorghum and sorghum-sudan hybrids. These plants are extremely productive but also contain the components needed to 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 is in low enough concentrations that is not harmful to cattle, but in some circumstances prussic acid can build up to toxic levels and the cyanide poisoning causes serious illness and 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 producers
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 mesophil, which has many functions such as nutrient transport. Think of that layer as everything else in between the bread layers in the sandwich. Sorghum plants contain two compounds that, when paired, create cyanide. Under normal conditions those two compounds are housed separately, one in the epidermis and one in the mesophil. 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 the formation is slow and the levels are low. Freezing temperatures or frost, however, expands the water in the cells and causes all the leaf cells to burst creating a perfect opportunity for a mass build up of hydrogen cyanide. Grazing livestock on forage in this state will likely cause a whole host of problems, even fatalities. Luckily prussic acid will volatilize or dissipate off 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.
Prussic acid, or hydrogen cyanide, is a greater concern 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 across types. Below is a list of sorghum products in descending order of prussic acid content.
Johnsongrass > Grain Sorghum > Forage Sorghum > Sorghum Sudan > Sudangrass
How to keep animals safe?
Sorghum products are great options for growing huge amounts of highly palatable biomass for livestock, but if you know you’re going to have livestock grazing in that particular area with sorghum when the frost comes, it’s very important to have a plan to keep animals safe while the sorghum dries out after a frost.
- Utilize a cover crop without sorghum – Perhaps it’s simply easier and safer to use a cover crop mix without sorghum. Millet serves as a great replacement for sorghum sudan though you may not see quite the tonnage as you would with the sorghum. You could also plant just a portion of the pasture or field in a mix without sorghum so you know you have a safe place to graze the livestock immediately following a frost.
- Bale graze – Partition off an area without the sorghum plants and set up a bale grazing system for a couple weeks until the sorghum has time to dry out.
- Swath a paddock – The prussic acid poisoning is only a problem when the cells burst and there is a huge flood of cyanide. By swathing a paddock one or two days before a freeze you allow the forage to dry down enough to where the cells won’t burst and release the cyanide. Then you are basically swath grazing a portion of the pasture and you can return to the standing forage after a couple weeks.
Nitrate Poisoning – The Science
High nitrate levels can affect nearly any plant, posing a risk of nitrate poisoning in livestock, which is similar to prussic acid poisoning as both cause cellular suffocation. Nitrate poisoning disrupts the blood’s ability to absorb oxygen from the lungs, leading to symptoms like labored breathing in affected animals. A blood sample from an animal with nitrate poisoning would appear brown due to oxygen depletion. In contrast, prussic acid poisoning prevents cells from using oxygen from the bloodstream, causing the blood to appear bright red, though animals still show similar respiratory distress. In both cases, oxygen-deprived cells malfunction and gradually die.
Since many plants can accumulate toxic nitrate levels, how can you identify the risk and take steps to mitigate it?
Understanding the Risk
Plants convert nitrogen into protein, which is essential for both the plant and the animals that consume it. Nitrogen is absorbed by plants primarily 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 is worsened when soils have excess nitrogen, such as from over-fertilization of a previous crop.
How to manage and mitigate risk
To maximize tonnage when growing forage crops, fertilization is key, but it must be managed carefully to prevent toxic nitrate accumulation in plants. Here are strategies to fertilize effectively while minimizing risks:
- Conduct a Haney Soil Test: Start by assessing available nitrogen in your soil with a Haney soil test. This helps determine if additional fertilization is necessary.
- Apply Nitrogen Wisely: If fertilization is needed, avoid spiking nitrate levels in plants. Synthetic nitrogen converts to nitrate and is quickly absorbed by plants. Instead, combine fertilizer with molasses (or another sugar source) and sulfur to encourage soil microbes to convert nitrogen into bacterial bodies. This creates an organic, slow-release form of nitrogen, available to plants as microbes die.
- Ensure Balanced Soil Fertility: Proper photosynthesis requires adequate levels of nutrients like phosphorus, sulfur, zinc, molybdenum, and cobalt. Conduct soil tests and amend deficiencies as needed. A vibrant soil microbial community also enhances nutrient availability to plants.
- Incorporate Legumes: Legume cover crops fix nitrogen, providing a natural fertilizer for subsequent forage crops as they decompose. While legumes in the current crop may not supply enough nitrogen directly, mycorrhizal fungi can improve nitrogen transfer between neighboring plants, especially in biologically active soils.
- Split Nitrogen Applications: Instead of applying all nitrogen at once, split applications—part at planting and part later in the season. If conditions are dry, you can skip the second application to avoid nitrate buildup.
- Use Polymer-Coated Urea: Apply slow-release nitrogen products, such as polymer-coated urea, in furrows to provide a steady supply of nitrogen without overwhelming the plant.
When planting forage crops after a heavily fertilized cash crop, residual nitrogen in the soil can lead to nitrate accumulation in the forage. Since the fertilizer is already present, test forage samples before grazing. These tests are quick, affordable, and straightforward. Consult your veterinarian to confirm nitrate levels are safe for livestock.
Haying vs. Grazing
Prussic acid dissipates in hay during drying, but nitrate levels remain a concern and can become more concentrated in dried forage. If standing forage already has high nitrate levels, the hay will have even higher concentrations, as about 90% of nitrates are stored in the lower stems, with minimal amounts in the leaves and upper plant parts. To reduce nitrate content in hay, cut forage as high as possible to avoid the nitrate-rich lower stems.
In grazing scenarios, particularly with rotational grazing, nitrate poisoning is less of a concern. Livestock typically eat the leaves and tops of plants first, which are lower in nitrates. As long as animals are not forced to graze plants too close to the ground, the risk of nitrate poisoning is significantly reduced. When in doubt, testing forage for nitrate levels is a cost-effective way to prevent livestock losses.
Final Thoughts
Animal health and well-being guide most management decisions for livestock producers. Understanding the risks of prussic acid and nitrates, combined with regular forage testing and careful observation, ensures your herd stays healthy and well-fed throughout the growing season.
Thanks to Dale Strickler for sharing all his wisdom on these topics.
