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"Producers need to take precautions as they attempt to locate
alternate forage sources or consider chopping drought-stressed
corn," said Dick Wallace. "Several crop species can accumulate
nitrates but especially grasses such as corn, oats, wheat,
barley and sorghum." Most plants extract nitrogen from the
soil in the form of nitrates, he explained. Under normal
conditions plant tissues contain relatively small amounts of
nitrates. Typically, plants rapidly reduce nitrates to nitrites,
which are then converted to ammonia and incorporated into amino
acids.
Nitrates accumulate in plants when the rate of nitrate uptake
increases or the reduction of nitrates to nitrites decreases.
Increased application of nitrogen fertilizers may increase
uptake.
"Other factors that affect nitrate uptake include the levels
of sulfur and phosphorus in the soil and plant maturity,"
Wallace said. "When grass crops are stressed, such as by drought
conditions, the concentration of nitrates in plant tissues
increases because the ability of the plant to reduce nitrates to
nitrites is impaired."
Rumen microorganisms rapidly reduce nitrate to nitrite, which
is further converted to ammonia and incorporated into new
microbial protein. Both nitrates and nitrites can be absorbed
readily across the rumen wall. Rumen microbial populations can
adapt to higher levels of nitrates, but rapid changes in dietary
nitrate levels cause problems.
"When unconditioned cattle consume plants containing high
levels of nitrates, the nitrate-to-nitrite conversion exceeds
the ability of the rumen microbes to convert nitrites to
ammonia," Wallace said. "Blood levels of nitrites increase and
alter the hemoglobin in red blood cells to produce methemoglobin.
Unlike hemoglobin, methemoglobin cannot carry oxygen to the
tissues of the body.
"When the oxygen-carrying capacity of the blood has been
reduced to 40 to 50 percent, affected cattle will display mild
clinical signs, including pale membranes, depression and
slightly increased breathing. Once greater than 60 percent of
the oxygen-carrying capacity is gone, most cattle will have very
rapid breathing, have a wobbly gait or be unable to rise. The
blood will have a characteristic brown color."
Emergency treatment with methylene blue solution
intravenously will rapidly alleviate clinical signs, he noted.
"Deaths are not uncommon without immediate treatment," he
said. "If cattle do not die, the body systems can gradually
return the oxygen-carrying capacity back to normal. Pregnant
cows may abort several days after clinical signs."
Wallace said that chopped drought-stressed grass crops are a
common source of nitrate toxicity in dairy operations.
[to top of second column in this article] |
"Fresh chopped corn, or green chop, can contribute toxic levels of
nitrates when used as a forage extender," he said. "Fermentation of
chopped corn into silage will remove roughly half of the nitrate in
plant tissues.
"Hay should not be overlooked as a source of nitrate toxicosis.
Pigweeds, dock and other weeds found in many hay fields can also
accumulate nitrates. Plant nitrate levels can vary from field to
field and even from area to area within a field."
Wallace recommended that dairy producers feeding freshly cut
drought-stressed corn and hay harvested as forage extenders (from a
field not routinely used for hay -- that is, weedy) may want to test
these forages for nitrate levels.
Most veterinary diagnostic laboratories as well as many forage
testing laboratories can analyze forages for nitrate levels. The U
of I Laboratories of Veterinary Diagnostic Medicine, (217) 333-1620,
and Centralia Diagnostic Lab, (618) 532-6701, are two Illinois state
labs available.
Interpretation of results is best accomplished by the lab
performing the analysis, he noted. When test results are converted
to nitrate levels (versus nitrate-nitrogen as reported by some
labs), 6,000 to 10,000 parts per million of nitrate in feeds can
create problems in unconditioned cattle.
"Nitrate toxicosis can be controlled by diluting the offending
forage with hays containing low nitrate levels," Wallace said.
"Cattle can be slowly conditioned to the higher nitrate levels.
Feeding excessive quantities of high-nitrate forage could
paradoxically reduce the amount of nitrates consumed; cattle tend to
preferentially consume the leafy plant material and leave the stems,
where the nitrate accumulates."
Setting the cutter bar higher on the corn chopper (six to eight
inches from the soil surface) will leave more stalk residue in the
field, leaving more nitrates behind.
"Allowing chopped corn to ferment in a silo for three weeks or
more will nearly eliminate any concern," Wallace said. "Providing
additional soluble carbohydrate -- starch -- in the form of grain
can assist the rumen microbial populations in adapting to the
excessive nitrate levels in the forages."
[News release from the
University of Illinois College
of Agricultural, Consumer and Environmental Sciences] |
