Download Forages Frost and

Forages and Frost
With the onset of colder weather, the potential
for animal death due to grazing frost-damaged
forages increases. Careful attention to local
pasture and environmental conditions will
greatly reduce this risk.
it is at least 18 inches tall; sorghum, sorghumsudangrass, and sudangrass hybrids should not be
grazed until they are at least 24 inches tall.
When sorghum or sudangrass is damaged by frost,
large amounts of prussic acid are rapidly released.
It is important to not graze these pastures at least
five days in order to allow time for the plant to dry
and prussic acid to dissipate from plant tissues.
Ensiling the forage will also reduce potential
problems, since prussic acid will be dissipated
during the ensiling process.
Sorghum and Sudangrass Forages
The major concern is the risk of prussic acid
(hydrogen cyanide) poisoning of animals that are
grazing sorghum, sudangrass, or sorghum-sudan
forages. These plant species have a cyanidecontaining compound called dhurrin in the
surface tissue cells. By itself, dhurrin is harmless.
Unfortunately, the plant also contains enzymes
deeper in its tissues that, when mixed with
dhurrin, allow prussic acid to be released. The
prussic acid combines with enzymes in the
animal’s bloodstream, preventing hemoglobin
from transferring oxygen from the blood to
individual cells. Symptoms of prussic acid
poisoning include excessive salivation, difficult
breathing, staggering, convulsions, and collapse.
The animal dies rapidly, often within minutes,
due to asphyxiation.
In normal, healthy plants, the dhurrin compound
and the enzymes that release prussic acid are
kept separated by plant cell walls. If the plant’s
cell walls are damaged by mowing, chewing, or
frost, the enzymes and dhurrin come in contact
with each other and prussic acid is released.
Dhurrin content is much higher in young leaf
tissue than in stems and older leaf tissue.
Therefore, sudangrass should not be grazed until
Prussic acid poisoning may also occur if sorghums
are lightly frosted but not killed, and a following
warm period promotes the growth of new tillers at
the base of the plant. These new tillers have very
high dhurrin concentrations, and animals will often
preferentially graze this new growth. If new tillers
are being formed, animals should be removed from
the pasture until the new growth reaches the
minimum heights listed above, or else the plant is
totally killed by a later frost.
Millet and Sorghum Type
Cyanide Potential
Pearl and Foxtail millet
very low
Sudangrass varieties
low to intermediate
Sudangrass hybrids
intermediate
Sorghum-sudangrass hybrids
intermediate to high
Forage sorghums
intermediate to high
Shattercane
high
Johnsongrass
high to very high
Grain sorghums
high to very high
-over-
Management steps to avoid prussic acid
poisoning include:
§
Do not allow hungry cattle to graze where
prussic acid may be a problem. Feeding
grain or hay before turning animals to
pasture will reduce rapid intake and dilute
the amount of prussic acid consumed.
Animals do not become immune to prussic
acid, but they can detoxify low levels.
§
Do not allow animals to graze prussic acid
containing forages after a frost until the
plants have dried for at least five days.
§
Ensile plants with high prussic acid
concentrations.
§
Have suspect forages analyzed before
feeding to determine prussic acid content.
The table below lists prussic acid
concentrations in dry forages and their
potential effect.
Alfalfa and Other Legumes
There is also some concern with grazing
legume forages such as alfalfa and red clover
after a frost until it is well wilted or dried.
Although there is no known toxic compound
produced in these forages, cattle are known to
have a tendency for diarrhea and bloat
responses when grazing freshly frosted
legumes. This is because the frost ruptures
plant cells through ice crystal formation.
Upon thawing, the broken cell contents are
more readily available for digestion. This
increases the soluble protein and soluble
carbohydrate concentration of the forage,
which increases bloat potential.
Problems with legumes can be avoided by
holding cattle off of the pasture for a few hours
until the forage dries and wilts. Feeding dry
hay to the animals before turning them onto
the pasture will also reduce the rapid intake
of easily digested forage, and reduce
bloat potential.
Volunteer Corn
There is little data available regarding the safety
of grazing frosted, volunteer corn. While prussic
acid is not an issue, there may be a greater
tendency for bloat when grazing freshly frosted
tissue. Removing the cattle from the forage for a
few hours until the tissues wilt and dry will
minimize potential problems.
Dry Forage Prussic Acid
Concentration (ppm)
Effect on Animals
0-500
Generally safe – should not cause toxicity
500-1,000
Potentially toxic – should not be the only feed source
1,000 and above
Highly toxic and will usually cause cattle death
Prepared by
Robert Bellm
Extension Educator, Crop Systems
Revised October 2007
University of Illinois • U.S. Department of Agriculture • Local Extension Councils Cooperating
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