Oklahoma Extension
educators and producers know that drought and heat stressed summer annuals
(forage sorghums, millets, sudan hybrids, and johnsongrass) can be accumulators
of nitrate. Many samples are being tested by the OSU Soil, water,
and forage testing laboratory. Occassionally, however, producers
will forget that some of the highest nitrate concentrations will occur
following a drought-ending rain. They waited and watched while the
plants in the fields have been under stress. Now that much anticipated
rain comes along, and there is a temptation to quickly bale the hay or
turn out cattle on the field. The first days of recovery after a
drought-ending rain will often result in dangerous levels of nitrate in
these forages. Therefore, producers need to wait another 7
to 8 days after the good rain to allow the plant to return to more normal
metabolism that will convert much of the excess nitrate to plant proteins.
Read OSU Fact
Sheet F-2903 for more about nitrate toxicity.
It was discovered
in the early 1900s that under certain conditions sorghums are capable of
releasing hydrocyanic acid or commonly called prussic acid. Prussic
acid when ingested by cattle, is quickly absorbed into the blood stream,
and blocks the animal's cells from utilizing oxygen. Thus the animal
dies from asphyxiation at the cellular level. Animals affected by
prussic acid poisoning exhibit a characteristic bright red blood just prior
to and during death.
Lush young regrowth of sorghum
plants are prone to accumulate prussic acid especially when the plants
are stressed such as drought or freeze damage. Light frosts, that
stress the plant but do not kill it, are often associated with prussic
acid poisonings. Producers should avoid grazing fields with sorghum
type plants following a light frost. The risk of prussic acid poisoning
will be reduced, if grazing is delayed until at least one week after a
"killing freeze". As the plants die and the cell walls rupture, the
hydrocyanic acid is released as a gas, and the amount is greatly reduced
in the plants. One can never be absolutely certain that a field of
sorghum is 100% safe to graze. Pearl millets are generally considered
to be unlikely to accumulate prussic acid.
Cattle that must be grazed on
sorghum pastures during this time of year should be fed another type of
hay before turning in on the field, and should be watched closely for the
first few hours after turn in. If signs of labored breathing, such
as would be found in asphyxiation, are noted, cattle should be removed
immediately. Call your local veterinarian for immediate help for
those animals that are affected. Be certain to read OSU
Fact Sheet F-2904 before turning cattle to potentially dangerous fields.
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