Liliane Silva, Forages Specialist
Nitrate is a naturally occurring component in plants, and its accumulation is usually associated with plant stress conditions (e.g., prolonged drought). When plants grow slowly, they continue to uptake nitrates, but these do not convert to proteins and amino acids, so there is a buildup of nitrates in the plant. For example, after a drought-ending rainfall event, it is recommended to wait around 7-10 days before allowing animals to graze an area. Nitrate concentrations may be high after a prolonged drought, and it takes several days for plants to convert those into amino acids and proteins. Generally, forage crops that receive high nitrogen fertilization are more prone to contain high nitrate concentrations, in addition to some species that naturally accumulate higher nitrate levels. Properly managing forage fields and livestock consumption when there is an occurrence of conditions that can elevate the nitrate buildup in plants is recommended to avoid nitrate toxicity in livestock.
High nitrate levels in the animal diet increase nitrite levels in the bloodstream, which binds with hemoglobin, preventing normal oxygen transference. Signs of acute nitrate toxicity may include rapid and difficult breathing, rapid pulse, tremors, staggering, collapse and death. A key diagnostic feature of acute toxicity is dark brown blood color, however, in most of these cases, treatment is not practical due to the rapid evolution of the symptoms. Therefore, understanding how to reduce the risk of nitrate toxicity occurrence and how to identify symptoms and test the forage is crucial. In terms of management practices for forage fields, utilizing split applications of the full rate of N inorganic fertilizer and properly replenishing the system with nutrients needed by plants will support proper growth rate and improve plant use efficiency. In addition, livestock should not graze areas where there may be nitrate buildup during a prolonged dry period or after a frost event. Managing the grazing event to avoid exposition and providing supplements (e.g., cottonseed) or low-nitrate roughage before turning animals into pasture is recommended.
When harvesting the forage to conserve it, the drying process does not decrease nitrates. In this situation, hay may need to be diluted in the diet with other low-nitrate forage. If harvested and done correctly, ensiling [silage or ‘wet hay’ (haylage)] is a great way to decrease the potential for nitrate toxicity in forage crops. If high-nitrate forage cannot be avoided, ensiling should be considered. To prevent nitrate toxicity, forages within fields prone to high nitrate concentrations should be tested. There are commercial nitrate test kits available that can be used in the field. Values greater than 2,500 ppm in forage for beef cattle are considered moderate or high in nitrate concentration, and samples should be tested in commercial laboratories. These results will help to determine when it is safe to have animals grazing a particular field after plants have had time to dilute their concentrations after a rainfall event, for example, or even to properly develop rations that can support nitrate dilution to feed animals. For testing nitrate concentrations in forage samples, consult your local extension agent, who will be able to direct you to available nitrate tests to use or to which commercial laboratories to send samples.
