By Nathan Drewitz – U of M Extension
It is hard to believe that we are already in the middle of November with Thanksgiving coming up on the weekend. While this season’s weather conditions and crop prices seemed to put a damper on this growing season; it is always good to be thankful for the harvest and everything that was accomplished through hard work and perseverance throughout this past year. I would like to extend my thanks to all of the farmers who worked hard producing the food that will be on tables this Thanksgiving and every day of the year.
With that being said, it just wouldn’t be an article from Extension without some information on crops. With the recent cold weather and frost, most forage species have gone into dormancy for the year. Frost can also have a secondary effect that may lead to a buildup of prussic acid in freeze damage tissues of specific annual grass forage species. 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 seven 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 three weeks after.