The weather forecast is calling for temperatures to dip close to freezing tonight (10/12/16) in south central/southeast Iowa, which could mean that we may see a light frost. What does this mean for forage crops, particularly forage crops such as Sorghum-Sudan?
A frost event, even a light frost, with Sorghum-Sudan causes prussic acid to build up in the forage. Special steps should be taken to manage the forage to prevent prussic acid poisoning with livestock.
Dr. Steve Barnhart, retired Extension Forage Specialist, wrote the articles "Prussic Acid Poisoning Potential in Frosted Forages" and "Flurry of Forage Questions Come with First Fall Frost and Freeze." These articles include some great tips on how to manage frosted forages.
Some key points that Dr. Barnhart shared in these articles on managing frosted forages like Sorghum-Sudan include:
- Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost.
- Immediately after frost, remove the animals until the grass has dried thoroughly. Generally, the forage will be safe to feed after drying five to six days.
- Do not graze wilted plants or plants with young tillers or new regrowth. If new shoots develop after a frost they will have high poisoning potential, sudangrass should not be grazed until the new growth is at least 18 to 20 inches (24 to 30 inches for sorghum-sudangrass).
- Frosted/frozen forage should be safe once baled as dry hay. The forage can be mowed any time after a frost. It is very rare for dry hay to contain toxic levels of prussic acid. If the hay was not properly cured, it should be tested for prussic acid content before feeding.
- Waiting five to seven days after a frost to chop frosted forage for silage will limit prussic acid risks greatly.
- Delay feeding silage for 8 weeks after ensiling.