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Rethinking Nitrate and Prussic Acid "Quick" Tests

Posted Sep. 1, 2005

One of the articles archived on the Agricultural Division's Web site addresses nitrate and prussic acid poisoning. It was written during the dry years in the late 1990s. At the end of the original article were links to "quick" tests for nitrate and prussic acid accumulation for use on fresh plant material in the field. The facts about nitrate and prussic acid poisoning are still pertinent. However, we have pulled the links and supporting documents for the quick tests.

The quick tests are not meant to be quantitative. At best, a quick test only indicates whether or not potentially dangerous levels of accumulation might exist in a sample. It does not determine a specific level and may or may not be representative of a whole pasture or crop. Reactions in both tests are determined by a subjective, visual color change. For instance, the quick test for nitrate involves splitting a stalk and applying several drops of a chemical mixture on the exposed inner surface. The speed and degree the clear mixture turns blue is an ambiguous indication of nitrate content the darker the color, the greater the accumulation. The quick test for prussic acid involves suspending a chemically treated strip of filter paper from a stopper in a test tube above chopped up leaves of a suspect plant. Again, the speed and degree the strip turns brick red is an indication of possible prussic acid content. That's all these tests show.

The quick tests are not conclusive. Say you collect 20 plants at random across a pasture or crop and most or all react strongly and rapidly to the quick test. You can assume that a potential problem might exist, but you still don't know what levels those reactions represent. The real dilemma is when most or all of those plants test negative. Due to differences in soil type, stage of growth, etc., there still could be pockets of toxicity across the field or pasture.

Finally, the mixtures used to conduct these tests contain chemicals that are potentially dangerous. The nitrate test requires a mixture of highly concentrated sulfuric acid. The prussic acid test involves picric acid, which becomes highly explosive as it dries out.

The quick tests are very limited in usefulness. It is difficult to get uniform, representative samples, and the results are not quantitative. It's risky, at best, to develop a plan of action for a pasture or crop based on either a positive or negative quick test. If you suspect a field or pasture contains high levels of nitrate or prussic acid, a broader, more representative composite sample should be submitted to a lab for quantitative analysis. A listing of certified labs can be found at www.foragetesting.org. Collecting, handling and shipping a sample are critical. Many factors can affect the quality of a sample: heat, time in transit, humidity, etc. The quality of the results depends on the quality of the sample. Call the lab prior to sampling for their specific instructions. Additionally, each lab will offer guidelines for use of feedstuffs containing various levels of nitrate or prussic acid, from "safe to feed" to "do not feed."

Remember, too, that although a composite sample is more dependable than a quick test, it still isn't a guaranteed representation of the entire field or pasture. Also, actual conditions in your pasture or crop could change before your laboratory test results are back.

If you have questions, comments or experiences concerning this issue, please contact Clay Wright at (580)224-6500.