First Frost/Freeze of Fall Raises Forage Questions

By Stephen K. Barnhart, Extension Forage Agronomist, Iowa State University, Ames, Iowa

05 Oct 2009 -

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;


     ‑ suspected toxic of frosted alfalfa 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


period.


 


     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


crops.


 


     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.


  


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.   To improve plant crown insulation over the


winter, consider leaving a 5 to 6 inch stubble at this late-season


harvest.


 


     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.