Fall Frost and Forages

By Stephen K. Barnhart, Extension Forage Agronomist

18 Sep 2006 - The first frost of the autumn generally brings a flurry of
forage related questions. These questions usually center on three
general topics:

toxic prussic acid potential and management of
frosted sudangrass and sorghum sudangrass hybrids;
is frosted alfalfa toxic to grazing animals; and
now that we've had frost, should I harvest the
last alfalfa cutting?

Managing frosted sorghum sudangrass and sudangrass

The potential for prussic acid poisoning and management
suggestions are related both to the size of the plant when frosted
and the extent of frost damage. Producers should be aware that the
risk of damaging levels of prussic acid is very unlikely.

Prussic acid, more correctly called hydrocyannic acid (a cyanide
based compound) is formed in sudangrass or sorghum sudangrass
hybrids which are severely stressed or frost damaged. The
hydrocyannic acid develops within a few hours after the frost and usually
dissipates within a few days. The safest management is to remove
cattle and sheep from frosted fields for several days. Livestock can
be returned to frost injured sudangrass that is 18" or taller and sorghum
sudangrass 30" or taller after about 3 or 4 days. If the grass was
shorter than these heights when frost injured, withhold cattle and sheep
for 10 days to 2 weeks following the frost to avoid problems. Then
watch for new shoot regrowth, (tillers or “suckers�) on partially frost
killed plants! Direct grazing of these fresh new shoots can be toxic too.
Where new shoots appear following frost, avoid grazing until 2 weeks
after the "killing" frost that kills the new shoots.

Prussic acid poisoning is not a common occurrence. Very few verified
cases are reported by veterinarians. Maybe Iowa producers are just
using good management. Consider the recommendations above to
be at the ‘low risk’ or ‘conservative’ level.

If in doubt, move the livestock to another type of forage.
Livestock can be returned to the sudangrass or sorghum sudangrass
fields following a "killing" frost and appropriate post frost delay

Frost damaged sudangrass or sorghum sudangrass hybrids can be
cut and stored as silage. Hydrocyannic acid is dissipated during wilting
and partially during the ensiling process. Observe proper ensiling
technique, particularly moisture content, when ensiling these

Sudangrass and sorghum-sudangrass hybrids are difficult to dry
thoroughly enough for safe storage as dry hay. As with wilting and
ensiling, most if not all of the hydrocyannic acid is dissipated in the
drying process.

Producers who want to get frosted sudangrass or sorghums
tested for hydrocyannic acid content should first contact a forage or
plant tissue analysis laboratory near you and ask first whether
they can do the test for you and what they recommend as the proper
procedure for collecting, handling and shipping of the sample to
the lab. (See list of laboratories on Pm 1098A (Rev.) 'Forage
Testing Laboratories'. Sudangrass or Sorghum-Sudangrass should
never be used for horse pasture.

Is frosted alfalfa toxic?

Frost injured alfalfa, clovers, and the commonly used perennial
cool-season forage grasses Do NOT have the potential to form
hydrodynamic acid, are NOT considered toxic and can be safely grazed
or harvested for hay or silage following a frost. There is probably
a slightly higher bloat risk for grazed alfalfa and white clover the first
few days after a frost. Follow normal bloat preventing grazing
management when grazing alfalfa and clover.

The literature notes that Indiangrass (a perennial, warm-season
prairie grass) and birdsfoot trefoil have a low potential to form
Hydrocyannic acid. Actual problem cases using these forages
should be considered extremely rare and of minimal concern.

Now that we've had frost, should I harvest the last alfalfa cutting?

There is not a simple answer. In general, it will depend
whether the frost was a "killing frost" or not. A "killing frost"
is not the first light frost of the season; rather, it is a 23 or
24 F degree freeze that lasts for 4 to 6 hours or so.

If the producer does not need the forage, it is best for the
alfalfa plants to leave them uncut and standing through the winter.

If it was the hard, killing freeze, and the producer needs the
forage, harvest as soon as possible after the freeze to salvage as
much of the nutritive value as possible. The longer the delay, the
greater will be the weathering damage and leaf loss from the
standing frosted plants.

If the frost were a light, non killing freeze, the tops of the
alfalfa plants will be visibly damaged but will not likely stop the
plants' growth for the season. The damaged tops will deteriorate
in nutritive quality for the remainder of the autumn, but the plant
will still be attempting to regrow from crown buds and will be
using stored sugars. The best management for the plant is to allow
it to continue to grow using whatever green leaf area it still has
until the hard, killing freeze. Then if the producer needs the
forage, it can be cut and harvested for hay or silage; or grazed.

Alfalfa plants cut immediately after a partial freeze (non-killing
frost) and which experience further normal growing temperatures
will start new regrowth from crown buds, using accumulated proteins
and carbohydrates that would otherwise be used for over wintering
and regrowth the following spring. When these late-recovering plants
experience a killing freeze a few days or weeks later, they will be
physiologically weaker and more susceptible to winter injury.