by Ryan Lee

Frost is a concern for certain forage crops/pasture plants like forage sorghum, grain sorghum, johnsongrass, sorghum-sudangrass, and sudangrass. These grasses contain a compound called Dhurrin which gets converted to cyanide (also known as Prussic acid) during stress events like frost or drought. Cyanide is an acute toxin so we want to be very careful with these forages, particularly after stress events. The highest concentrations of the toxin are found in young leaves while stems/stalks tend to have the lowest concentrations of cyanide. 

1. Grazing the crop after a frost represents the biggest risk of acute cyanide poisoning as animals will likely primarily eat the young leaves (highest cyanide concentrations). If a non-killing frost occurs, we’d recommend waiting 10-14 days before allowing the animals to graze the frost-injured crop. Purdue calls this a “rolling” recommendation meaning that every new frost event that doesn’t terminate the crop re-starts the 10–14-day clock.

2. Green-chopping or fresh chopping will allow some of the cyanide to off-gas and will mix stems (less cyanide) with leaves (most cyanide) lowering the total concentration of cyanide in the forage. But you still want to be very careful with this approach. Either chop BEFORE the frost or wait 7 days after the last frost to chop. 

3. Ensiling (this is the best option) or wrapping green bales will greatly reduce cyanide concentrations. This is the safest way to manage this type of forage.    

You can sample for prussic acid concentration if there is a concern.  The labs typically provide an interpretative guide with the results. 

Interestingly, cyanide can be found in a variety of other crops and plants including corn, white clover, and wild black cherry but these tend to pose considerably less of a concern because the concentration of the poison is much lower. 

There is a summary of these tips on the “Managing Seasonal Disorders” section of the CISCO Summer Annual Product Guide.