Nitrate toxicity is a problem that many producers have to deal with in Montana. Particularly this year, with a large amount of cereal grains being harvested for hay, it is something that should be kept in the back of your mind at harvest and feedout.
Nitrate toxicity is an accumulation of nitrates in the plant. Typically, the lowest third of the plant stem will have the highest level of nitrates. Why are these high levels of nitrates toxic to the animal? When the animal consumes a large amount of nitrates, it is unable to be completely converted to microbial protein in the rumen. Instead, an intermediate in the conversion of nitrate to protein, nitrite, will be transported to the small intestines and absorbed into the blood system. This is where the real problem starts. Nitrite will bind to hemoglobin, the oxygen carrying component of blood, to form methemoglobin. Methemoglobin is unable to supply oxygen to the rest of the body, and the animal will start to become stressed.
Acute toxicity signs include things such as labored breathing, muscle tremors, weakness, and ultimately death. Animals that have chronic toxicity, or have been consuming high levels over a long period of time, will have decreased production, rough hair coat, and reproductive issues, among others.
What is happening in the plant to cause increased nitrate levels? Nitrate accumulation can occur for many reasons: environmental stress such as drought or hail, excessive application of Nitrogen fertilizer, deficiency of other nutrients such as K and S, shade, prolonged cool temperatures, the list goes on. Another important factor is plant species. Cereal grains tend to accumulate nitrates more-so than cool-season forages. Oats have been found to be the most prone to accumulating toxic nitrate levels, with wheat and barley a little less likely. Warm-season grasses such as sorghums and sudangrasses also have potential to accumulate nitrates.
If any of these situations occurs, or the plant becomes stressed, the likelihood of accumulating toxic levels is increased. In most of these situations, photosynthesis is inhibited or decreased, which supplies the Carbon, or energy, that is required to convert the nitrate to plant protein. Without the required Carbon, similar to in the rumen, the nitrate will not be converted to the final, desirable product, protein.
How can we decrease the chances of nitrate toxicity? There are a few things that we can watch out for. Always make sure that you are monitoring soil nutrient levels, and try to apply fertilizer according to soil and plant needs. Make sure to not overlook other soil nutrients, like Potassium and Sulfur, as they can have a significant impact on Nitrate uptake and accumulation in plants. Take care when applying and handling any N fertilizer, so that large amounts don’t get spilled or over-applied in one particular area. When harvesting, wait for several days after a drought-ending rain before harvesting, or after a hail event.
If you suspect your forages may be “hot”, I would advise to test them before harvesting or feed out. Your local county agent has an in-field test called the Nitrate QuikTest, which allows them to qualitatively estimate whether your forages may have toxic levels or not. This test does not provide a level of nitrate, but rather advises you as to whether there may be toxic levels or not based on what color the testing solution turns, and how rapidly that change occurs. We are also looking to test some in-field quantitative nitrate testing kits, to evaluate whether these commercially available products are accurate and reliable ways to determine nitrate levels in standing forage.
If your forage tests hot, or may be marginal, I always recommend sending it in to a lab for a quantitative nitrate analysis. This will help you determine if you need to dilute the hay in the ration by feeding it with a low-nitrate forage, or if it is safe to feed. Toxic levels also depend on production stage of the animal, with young and pregnant animals having low tolerance to nitrates, as depicted in the table below. You can contact your county agent, or myself, to help develop a ration that is suitable to feed to your animals.
If you have any questions or comments, please contact Dr. Emily Glunk at 406.994.5688 or at firstname.lastname@example.org. For more detailed information on nitrate toxicity, you can also reference the Montana Extension MontGuide “Nitrate Toxicity of Montana Forages” (MT200205Ag).