NC STATE UNIVERSITY COLLEGE OF AGRICULTURE & LIFE SCIENCES
NITRATE
MANAGEMENT
IN BEEF
CATTLE
NITRATE
MANAGEMENT
IN BEEF
CATTLE
2 Nitrate Management in Beef Cattle
itrate poisoning, which can kill beef
cattle within a matter of hours, is on the
rise in North Carolina. Beef producers
should learn how to prevent nitrate from
accumulating in forage plants. If producers
must feed high-nitrate forages, they can
take steps to limit the risk of the economically
devastating losses associated with
acute nitrate poisoning. Hay producers also
should learn how to manage nitrogen to
avoid liability resulting from the sale of potentially
toxic, high-nitrate hay.
Nitrate can reach toxic levels in forages any time
the nitrogen supply in the soil exceeds the nitrogen
needs of the plant. Plants absorb nitrogen from the
soil in the form of nitrate, which is later converted
to protein. When nitrate is available to the plant
but protein synthesis is slow, nitrate accumulates,
especially in the lower part of the plant, until protein
synthesis increases. Anything that slows plant
growth can lead to increased nitrate levels in wellfertilized
plants. This could include drought, long
periods of cloudy or cool weather, or herbicide
applications.
Some plants are called “nitrate accumulators”
(Table 1) because they often accumulate toxic levels
of nitrate during drought conditions even when
soil nitrogen levels are moderate. These nitrate accumulators
include summer annuals (millet and
N
Nitrate accumulation in
forage plants
Nitrate Management in Beef Cattle 3
sorghum x sudangrass hybrids), johnsongrass, and
annual weeds such as pigweed and lambsquarters.
Other forage plants like bermudagrass, fescue,
orchardgrass, small grains, and ryegrass accumulate
nitrate only when high levels of nitrogen fertilizer
are applied.
In the past, concern over nitrate poisoning
mostly focused on nitrate accumulators. However,
in recent years excessively high nitrate levels have
become common in bermudagrass and fescue due
to the large amounts of animal wastes (and in
some cases commercial fertilizers) being applied to
pasture and hayland.
Nitrate itself is not toxic to cattle. However, in
unadapted animals, nitrate is converted into nitrite
in the rumen and absorbed into the bloodstream.
Nitrite interacts with oxygen-carrying hemoglobin
in red blood cells, which leads to the formation of
methemoglobin, and that reduces the ability of the
blood to transport oxygen. Blood that is high in
methemoglobin is chocolate brown, a sight sometimes
observed in cattle suffering from acute nitrate
toxicity.
The most common sign of acute nitrate poisoning
is sudden death of one or more animals that
appeared healthy the previous time they were observed.
Early signs of acute nitrate toxicity can be
seen within 6 to 8 hours after ingestion of a large
amount of nitrate. They include labored breathing,
frothing at the mouth, frequent urination, diarrhea,
staggering, and a brown cast to the mucous
membranes. Collapse, convulsions, and death usually
follow within 30 minutes of the onset of symptoms.
Less noticeable signs may be a low calving
rate (due to abortions) and in some cases reduced
weight gain in growing cattle. Producers should
Legumes
TABLE 1. ACCUMULATOR AND
NON-ACCUMULATOR PLANTS
ACCUMULATORS NON-ACCUMULATORS
Johnsongrass Bermudagrass
Pearl millet Fescue
Lambsquarters Orchardgrass
Pigweed Small grains
Sorghum x sudangrass Ryegrass
hybrids
Nitrate toxicity in cattle
4 Nitrate Management in Beef Cattle
test forage and employ other management strategies
to prevent cattle deaths and minimize the
more subtle problems.
Significant cattle deaths most commonly occur
when a producer feeds high-nitrate forages to
hungry cattle that are not adapted to nitrate. Every
fall, reports of cattle deaths increase at the start of
the winter feeding period. In this scenario, cattle
have been allowed to graze a pasture until there is
little or no forage remaining and the animals are
hungry. They are then fed round bales and rapidly
eat a large amount from one section of a bale. If
the hay is high in nitrate, and especially if a portion
of the bale is exceptionally high, death of
some animals is likely. Most producers have several
cuttings of hay on hand, and knowing the nitrate
levels will allow them to use the hay with the lowest
nitrate level at the start of the feeding period.
Cattle deaths also commonly occur when the
animals are turned into summer annuals shortly after
a drought. When plants receive sufficient moisture,
they can take up nitrate very rapidly. After a
drought, however, the growth of the plant, and
therefore the protein synthesis that uses the nitrates,
resumes more slowly. After droughts, producers
want to get back to grazing as soon as
possible and often turn out hungry cattle when nitrate
levels are at their highest. Allowing 2 weeks
of forage gr