Drought-Stressed Forage: Nitrate Considerations

(Credit: SDSU College of Agriculture, Food and Environmental Sciences)

It is shaping up to be another dry year across much of the state, and many producers are evaluating forage supplies and considering alternative feed options for their cow herds. While drought-stressed forages can provide valuable feed, they may also carry an increased risk of nitrate accumulation. Producers should be aware of before grazing, harvesting, and feeding these forages.

Nitrates are found in many forage species, including millet, oats, wheat, corn, sorghum, sorghum sudangrass, kochia, pigweed, lambsquarter, bromegrass, and orchargrass. Plants absorb nitrates from the soil through their roots, and these nitrates are normally converted into amino acids, proteins, and other nitrogen-containing compounds. While all forages contain some level of nitrates, certain management and environmental conditions can lead to accumulation at toxic levels. Fields that received significant nitrogen fertilization or manure applications prior to drought, as well as plants under drought stress, are at the greatest risk. Nitrate concentration also varies within the plant, with the highest level typically found in the lower third of the stem.

Managing Nitrate Risks

When livestock consume forages with excessive nitrate levels, rumen microbes convert nitrate to nitrite. Nitrite interferes with the blood’s ability to carry oxygen, leading to symptoms such as rapid or labored breathing, weakness, muscle tremors, staggering, abortions, and, in severe cases, sudden death. Fortunately, nitrate risk can often be managed successfully with proper planning.

Forage testing is one of the most effective and economical tools for managing nitrate risk. SDSU Extension offers nitrate testing services to help producers make informed decisions. If elevated nitrate levels are detected, forage can often still be used safely by diluting it with low-nitrate feedstuffs, limiting intake, or gradually adapting cattle to the feed. When adapting cattle to higher-nitrate forages, proper supplementation is critical to prevent overconsumption and allow rumen microbes time to adjust. Ensiling can reduce nitrate concentrations but does not eliminate the risk; therefore, ensiled forages should still be tested before feeding. Producers should also avoid turning hungry cattle directly into pastures that contain high-nitrate forages.

With breeding season being in full swing and abortion being a key symptom of nitrate toxicity, careful management of pregnant females is especially important. One strategy to reduce nitrate exposure is to avoid overgrazing drought-stressed forages, where nitrate levels may be elevated. Because the highest nitrate concentrations are found in the lower stem, allowing cattle to selectively graze leaves and upper portions of the plant can help reduce intake. However, this selectivity is reduced when pastures are overstocked, increasing the likelihood of excessive nitrate consumption.

Because nitrate reduces the blood’s ability to transport oxygen, there can be a decrease in oxygen delivery to the fetus that leads to abortion. Abortions occurring in the second or third trimester may be associated with nitrate toxicity. A telltale sign is dark or chocolate-brown blood, indicating oxygen deprivation. Additional signs may include bluish discoloration of the mucous membranes or around the eyes.

Producers should also remember that rainfall following drought conditions does not immediately eliminate nitrate concerns. In fact, nitrate levels may significantly increase for several days after a rain event, as plants resume nitrogen uptake faster than they can convert it into protein. Other environmental stressors, such as frost, hail damage, or extended periods of cloudy weather, can further increase nitrate accumulation.

Nitrate Testing Guidelines

Below is a chart showing nitrate concentrations based on the nitrate strip quick-test procedure, along with general interpretation guidelines. However, if elevated nitrate concentrations are detected, nitrate strip test results should be confirmed through laboratory analysis to ensure accuracy. These tests are intended to provide rapid information for preliminary management decisions, but they do not offer the same level of precision as laboratory testing.