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Fescue
toxicosis is a term used to encompass three disorders of grazing animals
that are associated with tall fescue: fescue foot (a gangrenous
condition of the body extremities of cattle), bovine fat necrosis (a
condition in which masses of hard fat form in the abdomens of cattle
that can cause digestive or calving problems) and fescue toxicity.
Fescue foot and bovine fat necrosis can be a serious problem for an
individual producer, but occur relatively infrequently; fescue toxicity
is a widespread problem.
Fescue
toxicity symptoms include reduced feed intake, decreased weight gain,
lower milk production, higher respiration rate, elevated body
temperature, rough hair coat, more time spent in water and/or shade,
less time spent grazing, low blood serum prolactin level, excessive
salivation, and lower reproductive performance. Some or all of these
responses have been observed with various species of animals consuming
pasture, green chop, hay and/or seed of fescue. In addition, horses
consuming fescue forage often have reproductive problems including
abortions, prolonged pregnancy, foaling problems and agalactia (little
or no milk production).
About
30 years ago, an endophyte (internal fungus) was linked with fescue
toxicosis. The fungus is exclusively seed-transmitted and its presence
can only be detected via laboratory analysis. Ergot alkaloids it
produces are believed to be responsible for the disorders previously
mentioned. Unfortunately, most ‘Kentucky 31’ fescue (the variety in
most fescue pastures in the USA) has a high level of fungus infection.
Thus, animal production on most fescue pastures is substantially lower
than it could be. Several ways to reduce economic losses caused by the
fescue endophyte have been identified.
Avoidance
of the Endophyte:
In some situations a livestock producer can avoid some or all of the
problems that would otherwise result from animals grazing toxic
endophyte fescue. The classic example occurs with horses. The foaling
and other reproductive problems of horses result mainly from mares
grazing toxic fescue during late pregnancy. Thus, simply preventing a
pregnant mare from grazing toxic fescue during the last three months of
pregnancy avoids the problem.
Avoidance
has some application with other animal species. For example, since
consumption of even low levels of endophyte toxins sharply reduces milk
production of cattle, a dairyman might allow only non-lactating animals
to graze a toxic fescue pasture. Another example could be a beef
producer who pastures beef cows on toxic infected fescue but excludes
yearlings because of greater economic impact of ingesting endophyte
toxins. Also, the adverse reaction of grazing animals to the endophyte
is greatest during warm weather. Hence, in some situations the timing of
grazing of toxic fescue may be an important consideration.
Use
of Endophyte-free Tall Fescue: Since
the early 1980s, endophyte-free seed of a number of fescue varieties has
been commercially available. Endophyte-free fescue does not contain
endophyte toxins. Therefore, there are no endophyte-related livestock
disorders and animal gains and reproductive performance is thus
strikingly better than on toxic, endophyte-infected fescue.
It
is now known that endophyte-free fescue also does not contain certain
endophyte-produced compounds (believed to be non-ergot alkaloids) that
are important in stress tolerance and pest resistance in fescue plants.
Consequently, endophyte-free fescue stands tend to be much less
persistent than endophyte-infected stands, especially in climates such
as Alabama where heat and drought are stressful on tall fescue. As a
result, little endophyte-free fescue seed is planted in the South, even
though persistence can be favored by planting endophyte-free fescue on
soils with good moisture-holding capacity, use of rotational stocking,
and avoiding grazing during summer.
Dilution
of Toxins: The
quantity of endophyte toxins consumed by animals is directly correlated
with the amount of toxic fescue consumed. Therefore, any management
technique that reduces the quantity of toxic fescue in an animal’s
diet will reduce fescue toxicosis. Management of toxic fescue pastures
to favor other forage species can have the result of diluting the fescue
toxins in animal diets. For example, close grazing during spring will
reduce shade competition by fescue for lower-growing plants, and summer
application of nitrogen will encourage bermudagrass and crabgrass.
Planting
legumes such as clovers, alfalfa or annual lespedeza in toxic fescue
pastures is often a particularly feasible, effective, and relatively
inexpensive way to dilute fungus toxins in animal diets. For most
producers this is not a long term solution to the problem, but it can
significantly improve animal performance in the short term, especially
in beef cow-calf operations.
Close
Grazing or Clipping: Keeping
toxic endophyte-infected pastures grazed closely (3 to 4 inches) have
been shown to improve animal performance as compared to allowing forage
to accumulate. Furthermore, mowing toxic fescue pastures during spring
and early summer may reduce subsequent toxin intake by reducing the
ability of animals to selectively graze seed heads in which endophyte
growth (and toxin levels) tends to be greater than in other plant parts.
Nitrogen
Fertilization:
Increasing the level of nitrogen fertilization of fescue has been shown
to increase ergot alkaloid production. Therefore, a strategy for
reducing toxin intake is to avoid applying high rates of nitrogen
fertilizer to fescue pastures. Lower levels of nitrogen also make fescue
less competitive, thus making it more likely that volunteer species
(including legumes) may be present that help dilute toxin intake by
grazing animals.
Novel
Endophytes:
Research has led to identification of endophyte strains that do not
produce the toxins that cause animal disorders but do impart pest
resistance and stress tolerance to fescue plants. These fungus strains,
referred to by scientists as |
