Managing Forages to Minimize Nitrate Poisoning
David W. Koch, Extension Agronomy Specialist, University of WyomingEnvironmental conditions cause toxic levels of nitrates in some forages every year in
Wyoming. Nitrate effects on livestock can be minimized if conditions for nitrate accumulation
are understood and appropriate management steps taken.
Plant roots absorb mainly soil nitrates. Nearly all nitrates are reduced once inside the
plant to form amino acids and, subsequently, proteins. However, under some stress conditions,
nitrates can accumulate to toxic levels in plants. Actually, it is nitrites, which once the forage is
consumed, are produced from nitrates by rumen bacteria that are toxic to the animal. Toxicity
results when nitrites accumulate faster than they are reduced to ammonia. Nitrites in the blood
converts hemoglobin to methemoglobin, which cannot transport oxygen, and the animal dies
from asphyxiation.
Plant species. Nearly all plants contain some nitrates. Some of the highest accumulators are
weeds such as pigweed, kochia, lambsquarters and sunflower. Sorghums, including sudangrass,
sudangrass hybrids and grain sorghum have a reputation for accumulating nitrates. Among the
cereals, oats seems to be the most prone to high nitrates. Under some conditions, corn and other
forages can accumulate nitrates sufficient to be toxic to animals. Legumes and cool-season
perennial grasses rarely accumulate toxic levels of nitrates.
Plant parts. Nitrates are highest in the lower one-third of the stem. Nitrate reductase enzyme
levels are high in the leaves and prevent buildup of nitrates in leaf tissue. Very little nitrate is
found in the grain.
Plant maturity. Generally, nitrate concentration is highest in the boot stage (just before head
emergence) and dilutes out as the plant matures, if conditions are suitable for continued
maturation. Stress at any stage of plant development can induce accumulation of nitrates. New
regrowth can be high in nitrates.
Drought. Even under water stress, plants can continue to take up soil nitrates. High
temperatures, which are often associated with drought, can inhibit enzymatic conversion of
nitrates. With a drought-ending rain, surviving plants rapidly absorb nitrate and accumulate in
high amounts in the stems. It will require the plant 7 to 14 days following a significant rain to
reduce the accumulated nitrates.
Other environmental factors. Hail can increase nitrate hazard in two ways. Hail usually
reduces leaf area, leaving a higher proportion of stems, where nitrates are in higher concentration.
Also, the stress of reduced leaf area reduces nitrate conversion. Frost can act in the same way as
hail, particularly in plants that require a hard freeze to kill. Nitrates can be readily absorbed at
temperatures below 500F, but enzymatic conversion is slowed at these temperatures. For this
reason, spring and fall growth can pose more of a risk than summer growth. Forages harvested
or grazed after several days of cloudy weather have been found to contain higher nitrate levels
than after sunny weather.
Nitrogen fertilizer. Fertilizer and/or manure application increases soil residual nitrates and,
therefore, the tendency of accumulated nitrates in forages. Plant species, stress factors and plant
growing conditions seem to be more important, however, than amount of nitrogen fertilizer
applied. An imbalance of soil nutrients caused by low availability of phosphorus and potassium
can contribute to nitrate accumulation.
Other factors. Herbicides such as 2,4-D can temporarily increase nitrate content of forage and
weeds; however, since many weeds accumulate nitrates at high levels overall hazard can be
reduced if weeds are killed. Weeds damaged but not killed; however, will have high nitrate
levels because of decreased leaf area and reduced enzyme activity. Plant diseases that interfere
with normal growth have been reported to increase nitrate accumulation.
Forage utilization.
Ensiling forage will reduce nitrates through the fermentation process as much as 50-60%
within a month after ensiling. Silage can still be a hazard if nitrates were extremely high before
ensiling. Haying will not reduce nitrate content, except that high cutting (6 to 12 inches) will
leave the portion of the crop highest in nitrate in the field. Grazing of immature forage should be
avoided, unless shown safe by forage testing. Avoid grazing the lush new growth after a
drought-ending rain or a hail that destroys a significant amount of leaves. Allow 7 to 14 days of
plant growth before grazing. Animals can adapt somewhat to high-nitrate forages if they are first
fed dry roughage to limit intake and if stocking rate is not so high that they are initially forced to
eat stems. Forage harvested as green chop should be fed immediately. As forage heats nitrates
are converted to nitrites, which are even more dangerous.
If on the basis of lab analysis forage is high in nitrates, special precautions are needed.
Feeding hay in which amount fed at one time can be controlled and hay can be blended 1:1 with
other feeds low in nitrates. Over time animals normally become adjusted to higher nitrate
contents and amount of high-nitrate feed can be increased. Feeding a few pounds of grain dilutes
the amount of nitrate, as well as providing the energy to enhance rumen bacterial conversion of
nitrite to ammonia. Animals that are stressed from hunger, sickness, pregnancy or lactation have
a lower tolerance for nitrates than healthy animals. Plenty of clean water free of nitrates will help
dilute forage nitrates. Do not allow animals to drink runoff from feedlots, heavily fertilized
fields or manure disposal areas. Nitrates in forage and water are additive in their effect.
Checklist for Reducing Nitrate Toxicity
. Collect representative samples of species most likely to accumulate nitrates and, particularly if
one or more of the above stress factors have occurred. Contact your local University Extension
Educator for advice on sampling for nitrate analysis and interpretation of lab results.
. Avoid heavy nitrogen fertilizer or manure application on annual forages, particularly if not
irrigated or if water will be short.
. Delay harvesting or grazing stressed forage. Allow forage 7 to 14 days to grow out following
drought-ending rain, hail or light frost that does not totally kill the crop.
. Hay harvest at 6- to 12-inch stubble height to reduce nitrate in stored feed. Hay feeding will be
easier than grazing to control access and utilization, if forage is high in nitrates.
. Do not allow green chop forage to heat before feeding.
. Ensiling significantly reduces nitrate levels of forage.
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