Drought-stressed crops such as corn and sorghum tend to accumulate high levels of nitrate in the lower leaves and stalk of the plant. The accumulation is because the plant assimilation of these nitrates into amino acids is slowed because of the lack of water, a crucial component to numerous plant processes. Nitrate toxicity in livestock is because of 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.
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. Harvesting the forage 8-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 what the nitrate level could be.
Depending on the planned feeding method, a producer may wish to harvest 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 could be ingested by the animal. 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. If a harvested forage is high in nitrate, blending the feed with another forage such as prairie hay or brome will dilute the total nitrates in the animal’s diet and could potentially reduce the risk of poisoning.
High-nitrate forages chopped for silage and properly ensiled are a safer option for livestock feeding. During the ensile process, potentially 50 percent of the nitrates in the forage will be metabolized by the microbes and can vastly reduce the risk of poisoning. It is still not a bad idea to leave 6 inches of stubble in the field. That is the portion of the stem with the highest concentration of nitrates.
Grazing high nitrate forages is a dangerous practice. 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 risk of nitrate poisoning.
For more information, see K-State Research and Extension publication MF3029, “Nitrate Toxicity”, at your local county Extension office, or at https://bookstore.ksre.ksu.edu/pubs/MF3029.pdf
Dorivar Ruiz Diaz, Nutrient Management Specialist