Nitrate Toxicity in Drought-Stressed Forages

W. P. Weiss, Department of Dairy Science, Ohio Agricultural Research and Development Center, Wooster, and W. L. Shockey , Center for Agriculture, Natural Resources, and Community  Development, WVU Extension Service, Preston County

4/2000

I. Accumulation in plants

  1. Nitrate (NO3) is taken up by plants and is converted to protein by the plant cells under normal conditions.
  2. In drought, plants do not grow, protein synthesis stops, and NO3 accumulates.
  3. Factors affecting NO3 accumulation:
  1. Such species as corn, oats, and sudangrass can tend to accumulate high levels.
  2. Lower portions of the plant tend to contain higher levels of nitrate than the top parts.
  3. High rates of nitrogen fertilization increase nitrate accumulation.
  4. Rapidly growing plants tend to have low levels of nitrate.
  5. Drought stunts plant growth, causing nitrate to accumulate. Immediately after a rain, plants take up even more nitrate. Once plants begin growing rapidly (2 to 5 days), nitrate levels drop.

II. Metabolism of NO3 by ruminants

  1. Nitrate is not highly toxic, but nitrite (NO2) can be lethal.
  2. In the rumen, microorganisms convert to nitrite, which is converted to ammonia and, finally, to protein.
  3. If the rate of conversion of nitrate to nitrite is greater than conversion of nitrite to ammonia, toxicity occurs.
  4. Nitrite decreases oxygen-carrying capacity of the blood in two ways: (1) it binds with hemoglobin to form methemoglobin and blocks binding of oxygen; and (2) it constricts blood vessels.

III. Toxicity in ruminants

  1. Provided that the diet is well balanced and contains components other than forage, such as grain, nonpregnant ruminants can tolerate up to 0.3 % nitrate-nitrogen in the diet dry matter. For all-forage diets, the upper limit of nitrate-nitrogen is 0.2%.
  2. Feeding management influences the tolerance of animals to nitrate nitrogen.
  1. Feeding grain increases tolerance.
  2. Feeding several times per day increases tolerance.
  3. Adaptation by slowly increasing nitrate-nitrogen levels, increases tolerance.
  1. Clinical signs
  1. Brown blood and brownish or grayish color to normally pink tissue of the nose, mouth, or vulva.
  2. Excessive salivation, grinding teeth, unsteady gait, rapid labored breathing.
  3. In advanced stages, animal collapses, followed by coma with or without convulsions, and finally death.
  4. Abortions.
  1. Treatment
  1. If you notice brownish or grayish coloration, call veterinarian immediately!
  2. Treatment is methylene blue given via IV (dose is 2 g methylene blue per 500 pounds body weight).

IV. Sampling-Analysis

  1. Corn. Cut 6 to 10 stalks at same height as chopper. Send samples to analytical laboratory for quantitative results. See your local Extension agent for laboratory information.
  2. Silage. Sample several days after ensiling but before feeding. Send to lab for analysis. Because ensiling reduces NO3 concentration, it is more important to test before feeding than before ensiling.
  3. Hay. Take a core sample from several bales prior to feeding. Send to lab. NO3 levels may decrease some during curing, so waiting several days after baling to sample the hay might give better information.

V. Interpreting Results

  1. Nitrates can be reported as nitrate-nitrogen (NO3-N), nitrate (NO3), or potassium nitrate (KNO3). You must know what units are on your report.
  2. Conversion factors:
NO3-N = NO3 x .23
= KNO3 x .14
NO3 = NO3-N x 4.4
= KNO3 x .6
  1. Guidelines (values are for NO3-N as % of dry matter)
<0.1% Safe; no precautions necessary.
0.1 - 0.2% Generally safe. Limit to half of diet dry matter for pregnant animals. If diet is composed entirely of forage, do not allow hungry animals unlimited access to the NO3 forage.
0.2 - 0.34% If possible, do not feed to pregnant livestock. If absolutely necessary to feed them, limit to 25% of diet dry matter. For other livestock, limit to 50% of diet dry matter. Measure water nitrate level and include in total dietary nitrate risk.
0.34 - 0.4% Do not feed to pregnant livestock. Limit to 25% of diet of other livestock. Feed with grain as part of a balanced diet. Insure that water source is low in nitrate.
>0.4% Potentially toxic; do not feed.

VI. General guidelines for handling nitrates

  1. Be concerned, but do not panic. It is manageable.
  2. Ensiling reduces nitrate content by 30% to 70%. This usually makes feed that is high in nitrate safe to feed.
  3. Ensiling high-nitrate forage also produces large amounts of silo gas. Be especially careful around silos during the first week after ensiling.
  4. Blend high-nitrate forages with low-nitrate feeds, such as alfalfa and grain.
  5. Feed adequate energy, vitamins, and minerals. Have all feeds analyzed, and balance rations accordingly.
  6. Do not feed hungry animals feeds that are high in nitrates. Offer low-nitrate feeds first and then high-nitrate feeds.
  7. Adapting animals to increasing levels of nitrates slowly over several days or weeks increases the amount of nitrate livestock can consume safely.

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