FOLEY, Minn. — This last month’s temperatures created a great rollercoaster ride. Every time I thought that we were about to get our first hard frost forcing all forages into dormancy we danced above it and jumped right back into summertime temps. For those with winter forages this is great because we might just get some more growth out of them after all. This will also help with regrowth on the alfalfa and hopefully help stands overwinter. However, temperatures did drop low enough to freeze off the corn, soybeans, and sudangrass. Looking at my forage plots on Monday it was clear that the sudangrass was hit hard by the recent frosts and is currently dying back. This means it is time to review the potential prussic acid issues with the sudangrass/sorghum family of grasses.

Species of concern are in the sorghum family with sudangrass and sorghum being the more commonly used species within this family. Sudangrass and sorghums are warm season grasses that can be planted as an alternative forage crop source. The potential toxicity after frost varies by species. Sudangrass varieties and hybrids are in the intermediate potential toxicity, grain sorghum is in the high potential, and pearl and foxtail millets rarely cause toxicity. There are a variety of environmental and plant stages that will also influence the potential for toxicity. Plants that are grown in soils with high nitrogen levels or that are deficient in phosphorus and potassium will have a higher risk of prussic acid poisoning. Younger tissues will have a higher concentration of these toxins versus the older tissues.

Stay away from grazing on nights where the frost potential is high and wait until plants are dry before returning. The drying process will usually take around 7 days. Also, steer away from grazing new growth after a frost. After a light frost, allow 7-10 days before grazing or green chopping. Sorghum and sudangrass crops should be 18 inches tall before grazing or green chopping to prevent prussic acid toxicity. At this point in the season dry hay and silage will be the safer option. The forage can be mowed any time after frost if it is to be baled. As long as the hay is cured correctly it is rare for it to contain toxic levels of prussic acid. Silage is a little different as it may contain harmful levels of prussic acid when chopped, but due to the fermentation process is expected to dissipate. To be safe, do not feed the silage from these crops for at least 3 weeks after harvest.

For more information please feel free to contact me at (608) 515-4414 or through email at ndrewitz@umn.edu. To receive future events, educational programming, and agronomic updates by email, signup at z.umn.edu/tricountysignup. In addition, if you would like to provide input for future local extension programming in Stearns, Benton, and Morrison counties please fill out the survey at z.umn.edu/tricountycrops.

— Nathan Drewitz, University of Minnesota Extension

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