Drought-stressed crops tend to accumulate high nitrate levels in the lower leaves and stalk of the plant. Crops such as forage and grain sorghum, sudangrass, hybrid sorghum-sudan, and pearl millet are notorious accumulators. Nitrates accumulate in the lower portion of these plants when stresses reduce crop yields to less than expected, based on the supplied nitrogen fertility level. Nitrate toxicity in livestock is caused by its absorption into the bloodstream and binding to hemoglobin, rendering it unable to carry oxygen throughout the body. The result is eventual asphyxiation and death.
Animals under stress (sick, hungry, lactating, or pregnant) are more susceptible to nitrate toxicity than healthy animals. Toxicity is related to the total amount of forage consumed and how quickly it is eaten. Generally, forages containing more than 6,000 ppm nitrate should be considered potentially toxic. Symptoms of toxicity may appear within a few hours after eating or not for several days. Signs include reduced appetite, weight loss, diarrhea, and runny eyes.
It is wise for producers to test their drought-stricken forage prior to harvest. Nitrate testing can be done through several labs, including the K-State Soil Testing Laboratory. Not all labs use the same testing procedure so be sure to use the scale provided by the testing lab. Harvesting the forage 6-to-12 inches above the ground to avoid the highest concentrations of nitrate in the plant is a good practice. Producers should collect a good representative forage sample above this cutting height to get an accurate determination of the nitrate concentration. It's important to consider factors like actual nitrate concentration, storing and feeding methods, and forage availability, as these can significantly impact the safety of your livestock. Toxicity is related to the total amount of nitrate in the diet and how quickly it is eaten. Generally, if forages contain more than 6,000 ppm nitrate, they should be considered potentially toxic.
Depending on the planned feeding method, a producer may wish to test different parts of the plant. If wrapping the forage into a bale and feeding it directly to livestock, a producer may want to test the lowest part of the stalk to determine the greatest risk of nitrate forage that the animal could ingest. If a producer was planning on grinding the bale, a whole plant sample above what will be left in the field may be a more accurate representation of what will be eaten. High-nitrate forages chopped for silage and properly ensiled are a safer option for livestock feeding. During the ensiling process, potentially 50 percent of the nitrates in the forage will be metabolized by the microbes and can vastly reduce the risk of poisoning.
Grazing high nitrate forages can be a dangerous practice. This option requires very careful management. Grazing pressure should be limited so that animals do not consume the parts of the plant forage testing has shown to be dangerous. Although animals tend to consume the leaves and the top portions of the plant, which contain less nitrates, the risk of consuming a high-nitrate portion of the plant still exists. In addition, the longer the animal is left on a field and the more that animal is forced to eat the remaining forage at the lower portions of the plant, the greater the risk of nitrate poisoning.
Alicia Boor is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact her by e-mail at [email protected] or calling 620-793-1910K-State Research and Extension is an equal opportunity employer and provider. Kansas State University, County Extension Councils, Extension Districts, and U.S. Department of Agriculture Cooperating.