Maurice L. Eastridge
William P. Weiss
Inadequate water, other plant stressors, and high application of nitrogen fertilizer can cause nitrates (NO3) to accumulate in the vegetative portions of various plants. Plants require less soil moisture to absorb nitrates than is necessary to attain maximum plant growth. During a drought, plants can uptake nitrates but cannot convert them into plant proteins as rapidly as during normal growing conditions. This causes nitrates to accumulate in certain plants. After an extended dry period, rainfall may cause an immediate increase in nitrates for 2 to 3 days until the plant's growth begins to respond by convertiong nitrares to protein. The highest concentration of nitrates will be in the lower part of the plant. Therefore, a sample for nitrate analysis must be representative of the whole plant (Table 1). Various plants are prone to accumulation of nitrates, including corn, wheat, oats, barley, sorghum species, sweet clover, and most weeds. Some plant species, such as sorghum, planted for Fall forage may also accumulate nitrates late in the growing season. The cool nights and short days will reduce plant growth, but ample moisture may be present for rapid uptake of nitrates.
Corn plants existing in drought conditions.
Nitrate toxicity can be a problem for ruminants because of the extensive feeding of forages. Nitrates from feeds are converted very rapidly to nitrite (NO2) by the rumen microorganisms, and then nitrite is converted to ammonia which can be used to synthesize bacterial protein. However, if the conversion of nitrates to nitrite is more rapid than conversion of nitrite to ammonia, absorption of nitrite will occur. Once in the bloodstream, nitrite will compete with oxygen for binding sites on hemoglobin. Hemoglobin with bound nitrite (called methemoglobin) loses its ability to carry oxygen to and carbon dioxide from tissues. Clinical signs of nitrate toxicity include pale mucous membranes, chocolate-brown-colored blood, and labored breathing. Sudden death may even occur. If toxicity is suspected, a veterinarian should be contacted immediately because an antidote is available.
A qualitative test is available for nitrates:
a) Reagent A: Dissolve 500 mg of diphenylamine in 20 ml of water and bring to a final volume of 100 ml with concentrated sulfuric acid. (Use an amber bottle, and place the solution in a dark place for storage).
b) Reagent B: Cautiously add 80 ml of concentrated sulfuric acid to 20 ml of water.
c) Field test solution: Mix equal parts of Reagent A and Reagent B.
Remove a section of tissue from the lower section of the plant and apply a small amount of test material to the cut side of the tissue. A blue coloration using the reagent kit indicates presence of nitrates. If nitrates are found present with a qualitative test, a quantitative test is recommended.
Many laboratories will conduct quantitative analyses for nitrates. When analytical
reports are returned, be sure to note units used to express nitrate level:
|= nitrate (NO3) x 0.23,|
|= potassium nitrate (KNO3) x 0.14,|
|= sodium nitrate (NaNO3) x 0.16;|
|NO3||= NO3-N x 4.4,|
|= KNO3 x 0.61,|
|= NaNO3 x 0.73.|
Samples for nitrate analysis should be representative of the feed, and wet samples should not be mailed in plastic bags because fermentation will reduce nitrate level.
Forages high in nitrates generally should not be grazed or fed as green chop or hay. However, ensiling forages will reduce nitrate levels by 35 to 80 percent. Regardless of feeding method, a quantitative test for nitrates is advised if high levels are suspected. Silage and hay should not be sampled for analysis until two weeks after harvesting. Table 2 provides guidelines for using feeds with known nitrate levels. Since nitrate toxicity is dependent upon nitrate level in the total diet, water quality must also be considered (Table 3).
Feeds marginal-to-high in nitrates should be slowly introduced to animals, and the allowable amount of such feed should be fed over the course of the day instead of feeding it all at once. Also, hungry animals should not be allowed access to feeds with moderate-to-high nitrate levels but should be fed low nitrate feeds first to curb their appetite. Adequate energy, particularly rapidly fermentable carbohydrates, must be in the diet to facilitate the utilization of nitrate nitrogen by rumen bacteria, and adequate minerals and vitamins, especially Vitamin A, must be supplied.
Feeding urea or other sources of non-protein nitrogen (NPN) with nitrate containing feeds can be practiced without harmful effects. These supplements should be fed within guidelines because excess NPN can result in ammonia (NH3) toxicity. Ammonia toxicity is different than nitrate toxicity.
Gases produced from fermentation during the first few days after filling a
silo can be harmful. Therefore, caution should always be practiced when working
around freshly filled silos, especially when the silo is filled with forage
containing high levels of nitrates. During fermentation, nitrates will be used
to form yellowish-orange gases called nitric oxides that are lethal. If it is
necessary to enter a silo shortly after filling, adequate ventilation must be
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