Nitrate toxicity can be a concern for farmers and could be rearing its ugly head in 2020 given some of the challenging growing conditions. Nitrate livestock poisoning can occur when nitrates, that accumulate in most plants, are changed within ruminant animals respectively to nitrites, to ammonia and to protein. The nitrite form is the toxic component. When absorbed into the bloodstream, nitrate changes the oxygen-carrying molecule hemoglobin into the non-oxygen-carrying molecule methemoglobin. Without available oxygen, the animal’s system begins to shut down with symptoms that include chocolate-colored blood, breathing and coordination difficulties, muscle tremors, diarrhea, frequent urination, becoming lethargic, and bluish discoloration of unpigmented skin and mucous membranes.1
Nitrate vs. Nitrate Nitrogen
If there is a concern about potential nitrate feed toxicity, it’s best to get the sample tested at a lab. Most labs provide nitrate concentration in the nitrate form (NO3), which specifically looks at the nitrate ion alone. However, some labs provide a nitrate-nitrogen result (NO3—N), which looks specifically at the amount of nitrogen in the nitrate ion, which is a smaller amount. Both tests essentially tell the same story but make sure to understand the metric being used to measure the sample.
1. Plant Species
a. Several weed and crop species are known for accumulating nitrate and should be considered when turning out livestock to graze or planning a cover crop mix.
i. Common crop species: barley, corn, flax, millet, oats, radishes, rape, rye, soybeans, sorghum, sudangrass, sugarbeet, sweet clover, turnip and wheat.
ii. Common weed species: Canada thistle, curly dock, jimsonweed, johnsongrass, kochia, lambsquarter, nightshade, pigweed, russian thistle, smartweed and wild sunflower.
2. Stage of Plant Growth
a. Nitrate decreases as plants mature; however, if soil and environmental conditions are favorable some mature plants can have large amounts of nitrate. Therefore, it’s crucial to test a sample of forage if there is any doubt about nitrate levels.
a. Drought conditions correlate directly to high nitrate levels, but there must be some moisture present to absorb and accumulate the nitrate. Plants that survive drought conditions are likely to have a high nitrate concentration after a rain event due to regrowth.
b. Low temperature, frost, hail or any event that reduces the amount of leaf area decreases photosynthesis. Therefore, nitrates accumulated by the roots cannot be turned into plant proteins and are therefore accumulated in the stalk or stem. Temperatures below 55o F inhibit plant growth. Nitrates can quickly accumulate during low temperatures because they are not converted into amino acids and proteins.
4. Plant Physiology
a. Plant parts closest to the ground, i.e., stalk, typically contain the highest nitrate concentration. The lower third of the stalk contains the highest level of nitrates. The leaves contain less nitrate than the stalk, and the flower and grain contain little to no nitrate concentration.
a. Plants that are shaded do not have the ability to photosynthesize correctly, which is crucial to turn nitrates into amino acids and plant proteins. This causes nitrates to accumulate in the plant. It has also been shown that nitrate accumulations are higher in plants in the morning than midafternoon and can be higher if the forage is harvested on a cloudy day versus a sunny day.
Management of High-Nitrate Concentration Feed (Figure 1)
Green Chop - Harvested at an immature stage and fed immediately. Never allow a high-nitrate, green chop feed to heat before feeding to livestock as allowing this to heat leads to the conversion of nitrate to nitrite, which is significantly more dangerous to livestock.
Silage - Ensiling a high-nitrate crop allows it to lose 40%-60% of its nitrate accumulation during the fermentation process. Another method is to slightly raise the cutter head to avoid the bottom third of the corn plant, where the highest nitrate accumulation will be. If there are still concerns about high nitrates, send a sample of the silage to be analyzed to prevent animal losses.
Physiological Maturity - Crops have lower nitrate levels at maturity; therefore, the closer harvest occurs relative to maturity, there is a better potential for lowering the amount of nitrate harvested. This is especially critical with crops that have been grown during a drought.
Figure 1. Feeding recommendations based on amount of nitrate ion and nitrate-n in parts per million (ppm).1 Figure courtesy of Drewnoski, M. E., Anderson, B. E., Kononoff, P. J., and Reynolds, M. B. 2019. Nitrates in livestock feeding. NebGuide G1779. Nebraska Extension. University of Nebraska.
1Drewnoski, M. E., Anderson, B. E., Kononoff, P. J., and Reynolds, M. B. 2019. Nitrates in livestock feeding. NebGuide G1779. Nebraska Extension. University of Nebraska. https://animalscience.unl.edu/Our_People/g1779.pdf
Web sources verified 9/21/2020
Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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