Cool weather and the first freeze occurred in many Northern states in the last few weeks. In addition to considering how it affects the corn and alfalfa crops on many operations, dairies that utilize crops such as sudangrass, sorghum, and sorghum-sudangrass need also be aware of the health concerns that accompany those crops when frosted.

Prussic acid is a major concern even after a light frost with these forage crops. Animals should not be on pastures featuring these hybrids on nights when frost is likely.

Retired Iowa State University forage specialist Steve Barnhart explained that animals should remain off pasture until the grass has dried thoroughly. Five to six days should be adequate for that drying.

Wilted plants and plants with young tillers carry high poisoning potential and should not be grazed. The young tillers must reach 18 to 20 inches (sudangrass) or 24 to 30 inches (sorghum-sudangrass) before grazing again.

Frosted plants have feeding potential
The feeding value of these plants exist when processed into dry hay and silage. Barnhart said in Iowa State extension articles that it is rare for baled dry hay to carry prussic acid poisoning. However, hay that is improperly cured still could carry toxic levels and should be tested before feeding.

As for silage, chop the forage five to seven days after a frost to limit prussic acid risk. Feeding should be delayed eight weeks after ensiling.