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Description
Johnsongrass is a vigorous, course, perennial grass
with scaly root stalks capable of reproducing by underground
rhizomes and seed. This plant has broad leaves and grows to
a height of three to six feet. The numerous seeds developed
in the fall are yellow to purplish in color, and occur in
a large spreading open panicle type inflorescence.
Distribution and Habitat
Johnsongrass was introduced into South Carolina from
Turkey in the early 1800's. Since that time it has spread
across most of the southern United States. Johnsongrass is
considered a weed in cultivated fields, waste places and along
irrigation ditches and stream bottoms, although at times it
is valued as a forage and erosion control plant. It is found
in all regions of Texas.
Regions: 1, 2,
3, 4,
5, 6,
7, 8,
9, 10.
Toxic Agent
Most losses from johnsongrass are due to hydrocyanic
acid (HCN) poisoning, although this plant is also capable
of accumulating dangerous levels of nitrates under certain
conditions such as after fertilization and during drought.
Most mortality occurs in cattle, with sheep poisoned to a
lesser extent. The plant is considered to be a fair forage
in most instances but becomes toxic under conditions that
favor HCN or nitrate poisoning.
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Livestock Symptoms
Hydrocyanic acid is one of the most rapidly acting
poisons. Signs of illness may begin within five minutes of
the time the animal starts eating the plant. Death may occur
within fifteen minutes, or the animal may live for several
hours. Symptoms in general order of occurrence are as follows:
- salivation and labored breathing
- muscle tremors
- incoordination
- bloating
- sustained contraction of voluntary muscles
- bright red venous blood
- convulsions
- death due to respiratory failure
High levels of nitrate in the plant can
complicate the issue and produce symptoms of nitrate poisoning
in sheep and cattle. Typical nitrate poisoning symptoms
may be evident (see Descriptions of Animal Conditions).
Sorghum cystitis may also be a problem in horses. This disease
is characterized by loss of control of the rear legs and
bladder.
Integrated Management Strategies
Even when plants contain considerable quantities
of potential HCN, they may not poison livestock. Since poisoning
depends upon the presence of actual HCN, anything that prevents
its development in the stomach lessens or entirely removes
the danger of poisoning. Certain feeds, such as alfalfa
hay and linseed cake, retard the production of HCN and may
prevent poisoning.
If symptoms are typical of HCN poisoning,
treatment with sodium thiosulphate will be effective. If
nitrate poisoning is the problem, treatment with methylene
blue is effective. Management to defer infested pastures
during danger periods is the best preventative measure.
HCN poisoning is most likely to occur when soils are high
in nitrogen and low in phosphorous, when plants have been
exposed to drought or disease which results in slow or stunted
growth, and when plants are making rapid regrowth or have
been frosted. Nitrate poisoning is most likely to occur
with soils high in nitrogen, during drought or after a period
of extended cool and cloudy weather. Rations high in carbohydrates
will also help reduce losses from nitrate poisoning. Only
a small percentage of horses consuming sorghum will develop
cystitis. However, there is no treatment and the animals
have to be destroyed. Therefore, sorghum forage or hay should
never be fed as a staple diet to horses.
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