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Nitrate Poisoning in Forages

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MT 9301
                O N T G U I D E
                                                                                                        Agriculture




                          Nitrate Poisoning in Forages

                                by Dennis Cash, Howard Bowman and Roger Brownson*


    Forage crops can accumulate toxic amounts of                   Nitrate Toxicity
nitrate (NO3). High quantities of nitrates can ac-                    Nitrate in itself is not toxic to animals, but at
cumulate in cereal grains (oats, rye, wheat, barley,               elevated levels it causes a noninfectious disease
triticale, etc.), sorghum, sudangrass, millet, corn                called nitrate poisoning. Nitrates normally found
and, on very rare occasions, alfalfa. Some weeds,                  in forage consumed by ruminant animals are bro-
like kochia and Russian thistle are sometimes                      ken down to nitrite (NO2) and then to ammonia
used for forage and have high nitrate levels espe-                 (NH3). The ammonia is then converted to protein
cially when growing under adverse conditions.                      by microbes in the rumen (first stomach of a ru-
Abnormal growing conditions such as drought,                       minant). Ruminant animals with high nitrate lev-
frost, unseasonable or prolonged cool tempera-                     els in their diet accumulate nitrite. Nitrite is ab-
tures, hail, shade, disease and herbicide damage                   sorbed into the blood and combines with hemo-
can cause high nitrate accumulation in forage.                     globin forming methemoglobin which causes a
The roots of growing plants will continue to take                  reduction in the ability of blood to carry oxygen
in nitrate nitrogen; however, normal plant me-                     from the lungs to the body tissue. When the
tabolism which converts nitrate to protein is dis-                 blood can no longer supply oxygen to the body,
rupted and high nitrate levels accumulate. All                     the animal suffocates.
plants contain nitrate, but nitrate levels toxic to
livestock are mostly associated with forages (hay,                 Symptoms of nitrate poisoning
fodder, silage, pasture or weeds) grown on soils                     Signs of early or chronic toxicity:
that have received high applications of manure or                    •Watery eyes
nitrogen fertilizer.                                                 •Reduced appetite
    As early as 1895, nitrate poisoning of livestock                 •Reduced milk production
was reported. Livestock losses occurred for many
                                                                     •Rough hair, unthrifty appearance
years before elevated nitrate levels in forage were
determined to be the cause of death. The term                        •Weight loss or no weight gain
“oat hay poisoning” was the common explana-                          •Signs of Vitamin A deficiency
tion for livestock losses in the 1930s because large                 •Abortion
acreages of oats were harvested for forage during                    Signs of acute toxicity:
the drought years.                                                   •Accelerated pulse rate
*Extension Crop Specialist, Extension Agronomist (Plant and Soil     •Labored breathing, shortness of breath              C-8
Science Department) and Extension Livestock Specialist (Animal       •Muscle tremors
and Range Sciences Department), respectively, Montana State
University.                                                          •Weakness
   •Staggering gait
   •Cyanosis (some membranes, such as the              Sampling plants or feeds for nitrate
     tongue, mouth, vulva and the whites of eyes,         Precautionary measures should be taken if
     turn blue)                                        high nitrate concentrations are suspected prior
   •Death                                              to harvesting or feeding the forage. Even under
   Nitrogen from the soil is taken up by plant         ideal conditions, nitrate accumulation is un-
roots in the nitrate form. Plants convert nitrate to   predictable. Nitrate concentration can vary in
nitrite which in turn is converted to ammonia          areas of a single field, haystack or silo. Therefore,
and then to amino acids, the building blocks that      nitrate testing is advised in many situations. Most
form protein. Higher nitrate levels are usually        county MSU Extension Service Offices can pro-
present in immature plants and decrease as             vide a rapid qualitative test for green forages to
plants mature. Nitrate concentration is highest in     determine the presence of nitrates.
the stems, especially in the lower third of the           Standing crops such as oats or barley should
stem, and at the nodes. An intermediate level          be sampled by collecting 20 stems randomly by
usually exists in leaves and very little is found in   traversing in a zigzag pattern across an entire
grain.                                                 field. The plants should be clipped at ground
   The effect of sub-lethal nitrate levels on live-    level, and tested by an Extension Agent trained in
stock health and performance is not well-defined;      nitrate testing. If nitrate is detected using the
however, safe and unsafe levels of nitrate in live-    qualitative test, and growing conditions are nor-
stock feed have been established. Despite the          mal, a harvest delay of several days will usually
guidelines, the effects of nitrate vary with each      reduce nitrate levels rapidly.
animal, condition of livestock, other feeds in the        Periodic testing may be necessary to assure
diet and weather.                                      that the nitrate level has declined. The qualitative
   The following nitrate levels are used as a          test should only be used as a preliminary screen-
guideline for most Montana conditions:                 ing measure, and forages suspected of having el-
                                                                   evated levels of nitrates should be sub-
                                                                   mitted for laboratory analysis. Nitrate
   Concentration                                                   levels can be quantitatively deter-
      of Nitrate                                                   mined at the Chemistry Station Ana-
  (% on Dry Matter                                                 lytical Laboratory, McCall Hall, MSU,
        basis)                    Comment                          Bozeman, MT 59717, or at private
    Less than 0.3 Generally safe to feed                           commercial laboratories.
      0.3 to 0.4      Can be fed if limited to 75% DM in ration       Sampling hay or haylage (low
      0.4 to 0.6      Can be fed if limited to 50% DM in ration    moisture silage) for nitrate requires
                                                                   that appropriate samples be collected
      0.6 to 1.2      Can be fed if limited to 25% DM in ration and tested. An accurate measurement
                      DO NOT USE FOR PREGNANT                      of forage nitrate is not possible unless
                      ANIMALS                                      the sample analyzed in the laboratory
    More than 1.2 DO NOT FEED                                      is representative of the forage lot in
                                                                   question. Poor sampling techniques
   Nitrate toxicity is most likely to occur when                   and an inadequate number of
livestock are pastured or fed green-chop, fol-         subsamples are the main sources of error in
lowed by hay. The least hazardous feed is silage.      analysis. For hay, at least 20 random bales from a
Ensiling forage usually lowers the nitrate level 10    lot should be sampled with a hollow core probe
to 60 percent. The nitrate level in hay will usually   and composited. A hay lot is defined as the hay
remain constant or decline slightly in storage.        from a single field that is uniform in maturity and
Producers should never assume their forage lev-        harvested within a 48-hour period. At least one
els are safe if they knows a crop was exposed to       pound of forage is needed for an adequate
any of the growing conditions mentioned earlier        sample. Representative silage samples should be
which can increase nitrate accumulation.               kept frozen until analysis to prevent nitrogen
losses from volatilization or chemical changes.         tons of hay tested at 0.1% (1000 ppm) to produce
Hay and silage samples should be sealed in plas-        25 tons of feed with 0.3% (3000 ppm) nitrate. The
tic bags and shipped to the laboratory for testing.     two hay lots should be processed and mixed thor-
                                                        oughly in a tub grinder to provide the proper di-
Feeding forages with elevated levels of                 lution. Levels of non-protein nitrogen (urea, etc.)
nitrate                                                 and nitrates in drinking water should be consid-
    Forages with sub-lethal nitrate levels can be       ered in rations blended to reduce nitrate prob-
fed to livestock with appropriate precautions. If       lem. Livestock should not be hungry when fed
there is a potential nitrate problem, growers           forages suspected of being high in nitrate. Rations
should first have an accurate laboratory analysis       should be rich in carbohydrates to encourage
of nitrate concentration. No single level of nitrate    rapid conversion of nitrate to ammonia. Animals
is toxic under all conditions. Cattle can safely con-   should be fed hay known to be safe prior to graz-
vert nitrates present at up to two percent of the       ing a pasture of forage suspected of having high
total dry matter of the ration, or about 20 grams       nitrate.
of nitrate per 100 pounds of body weight. If for-          Please consult with your local county Exten-
ages constitute 40 to 60 percent of the total diet,     sion agent and veterinarian for specific questions
then the nitrate concentration could be three to        regarding nitrate poisoning or sampling of forage
four percent of the ration dry weight to be consid-     suspected of containing dangerous levels of ni-
ered hazardous under most conditions.                   trate.
    High-nitrate feeds can be diluted with low-ni-
trate feeds to reduce the nitrate hazard by using       References
the following equation:                                 Fjell, D., D. Blasi, and G. Towne. 1991. Nitrate and
    WL = (WH) (%H - %B) / (%B - %L), where                   prussic acid toxicity in forage: causes,
    WL = weight of low-nitrate hay required,                 prevention, and feeding management. Coop.
                                                             Ext. MF-1018. Kansas St. Univ., Manhattan. 4 pp.
    WH = weight of high-nitrate hay,
    %H = nitrate concentration of high-nitrate hay,     Hanway, J.J., J.B. Herrick, T.L. Willrich, P.C. Bennett,
                                                           and J.T. McCall. 1963. The nitrate problem.
    %B = nitrate concentration desired in final
                                                           Coop. Ext. Spec. Rept. 34. Iowa St. Univ., Ames.
          blend,                                           20 pp.
    %L = nitrate concentration of low-nitrate hay
          to be used for blending.                      Wright, M.J. and K.L. Davison (eds.). 1963.
                                                           Proceedings of a Conference on Nitrate
    For example, a producer with 10 tons of hay            Accumulation and Toxicity. Agronomy Mimeo
tested at 0.6% (6000 ppm) nitrate, could blend 15          No. 64-6. Cornell Univ., Ithaca. 85 pp.
File under: Field Crops
 C-8 (Forages)
 Issued February 1993 2562000293 MS

				
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Description: Nitrate Poisoning in Forages