Crop Update
10/16/15
Prussic Acid Poisoning Frost on Soybean Late Planting Cool Season Grass Meetings
Prussic Acid Poisoning Potential in Frosted Forages
Prussic acid (hydrocyanic acid) is produced in certain forage species that contain cyanogenic
glucoside compounds. These species include the sorghum family of grasses: johnsongrass,
shattercane, milo, sorghum-sudan, and sudangrass. When plant cell walls are ruptured by
mowing, chewing, or frost these compounds are released and quickly convert to prussic acid.
The compounds are at their highest concentration in new growth less than 18 inches tall. This
is why livestock should not be turned in to graze sorghum fields after sufficient growth of
greater than 18 inches has occurred (any time of year). Cyanide is a gas which dissipates during
the hay drying or silage ensiling processes.
Prussic acid interferes with oxygen transfer in the blood stream which causes death by
asphyxiation. Ruminants are more susceptible to prussic acid poisoning than horses or swine
because cud chewing and rumen bacteria help release cyanide from plant tissue.
Frost/freezing rapidly ruptures plant cell walls releasing cyanide gas and increasing the risk of
prussic acid poisoning in plants of any age or size. High levels of prussic acid can accumulate
hours after a ‘burning’ frost or ‘killing’ frost, therefore, remove livestock from sorghum fields if
there is a risk of either type of frost.
In the event of a ‘killing’ frost, the cyanide gas will remain in the plant for 5 to 7 days as
it dries down. Do not graze until plants dry down.
In the event of a ‘burning’ frost, the concentration in combination with green tissue
increases the risk, therefore, avoid grazing for 14 days. If new growth occurs at the base
of the plant during this time, wait for a killing frost plus 7 to 10 days for dry down. In
the event we do not receive a killing frost then new growth must reach sufficient height
as mentioned above before grazing.
Supplement with feed in order to avoid turning hungry livestock into a field to graze
young growth of species with prussic acid potential.
Wait 5 to 7 days after a frost before chopping silage. Delay feeding silage for 8 weeks
after ensiling sorghum species. If forage contained high levels at chopping, consider
analyzing silage before feeding.
Other species of plants known to produces prussic acid include black cherry, elderberry and
indiangrass.
One to two days after a hard frost, legumes have an increased risk of causing bloat, especially
when grazing pure legume stands. Wait until the legume begins to dry from frost damage.
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Frost Risk to Soybean
The potential for frost in some parts of the region this weekend, has raised some concerns on
soybean yield. Generally, temperatures dropping below 30 degrees F for any length of time will
damage all plant parts. Frost from temperatures above 30 degrees will typically damage upper
canopy leaves. Frost damage begins as water soaked areas on leaves that later turn brown.
Soybeans at the R7 stage of development (one yellow pod over 50% of plant population) are at
the least level of risk from a frost or freeze and loss is generally minor. Soybeans in the R6
stage of development (swollen green beans in green pods) can sustain damage depending on
duration of freezing temperature, usually more loss associated with quality than quantity. R6
soybeans that sustain frost or freeze damage can have a shriveled seed coat and can retain the
green color, which potentially can become lighter over time if stored. Soybeans in early stages
than R6 can sustain significant losses, especially if temperatures fall enough to result in
complete foliage loss.
If a soybean field sustains severe frost/freeze damage soybeans should be harvested between
16% and 18% moisture and then dried to below 13% for safe storage. Shatter loss can increase
in frost/freeze damaged soybeans when allowed to field dry below 16%. Frost damaged
soybeans will probably be more difficult to harvest; therefore, combine threshing adjustments
may need to be made.
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Establishing Cool Season Grass – How Late?
With the dry weather, there have been some questions about late planting cool season grass.
The recommended time frame for cool season grass, such as novel endophyte fescue,
establishment is September through mid-October. The concern with planting outside this
window is not so much germination but root development. Cool season grass, much like cereal
grains, will germinate at soil temps of 50 degrees F or higher. Germination can occur at a
slightly lower temperature, but is not practical due to prolonged emergence issues.
The concern with late planting is providing enough time for shoot emergence, growth and
carbohydrate supply for root development. Cool season grass maximum root growth is
between 50 and 65 degrees F, however, roots will continue to grow slowly until the soil freezes.
Shoot growth will slow and eventually stop as temperatures consistently remain below 50
degrees F. Therefore, there must be a balance of enough shoot growth to support enough root
growth to avoid winter heaving.
For Southeast Missouri, this planting window may be extended if November is favorable for
emergence and shoot growth to supply needed energy for root development which is all
dependent on good soil moisture going into the winter. Keep in mind that under ideal
conditions it takes cool season perennial grass such as fescue 7 to 14 days to emerge. If this fall
continues to be dry and planting is delayed, an alternative is to consider an annual cereal grain
which can have more success of establishment in late fall or early spring followed by planting a
warm season annual grass for summer.
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University of Missouri Fisher Delta Center Certified Crop Advisors Meeting
A two-day Certified Crop Advisor program will be conducted at the University of Missouri Fisher
Delta Research Center in Portageville, Missouri on November 23 and 24, 2015. Separate
sessions in Pest Management and Crop Production will be offered on Monday, November
23rd. Separate sessions in Soil Fertility and Soil and Water will be offered on Tuesday,
November 24th. Each session will offer 4 CEU’s. Classes will begin at 8:00 a.m. each day.
Registration will be $50 per session or $180 for all four which will include lunch. For more
details or specific needs, contact David Dunn (573-379-5431, dunnd@missouri.edu) or Anthony
Ohmes (573-243-3581, ohmesg@missouri.edu).
Upcoming University of Missouri Production Meetings
Livestock and Forage Meeting – November 12th, Marble Hill, MO
6:00 pm to 9:00 pm
Contact Susan Fisher at the Bollinger County Extension office (fishersl@missouri.edu or
(573) 238-2420
See attached flyer:
Corn Meeting – December 9th at the Miner Convention Center, Miner, MO
8:00 am to 1:00 pm
Guest speakers include Dr. Chad Lee, University of Kentucky
Contact myself or David Reinbott to register.
Soybean Meeting – January 12th at the Miner Convention Center, Miner, MO
8:00 am to 1:00 pm
Contact myself or David Reinbott to register.
Anthony Ohmes
University of Missouri Extension
Southeast Region
573-243-3582
http://extension.missouri.edu/capegirardeau