Fall Freeze and Prussic Acid

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It’s no secret that there have been cool temperatures this week. Stokes County has seen freeze watches during the beginning of this week, and experienced a freeze on October 19th. While it’s not uncharacteristically early for the first hard freeze, there are implications on forage production to keep in mind.

Sorghum-sudangrass hybrids, sudangrass, forage sorghum, and johnsongrass are all in the sorghum plant family. As such, they are susceptible to accumulations of cyanide, more commonly referred to in forages as prussic acid. Prussic acid accumulations are especially common during periods of drought stress and immediately following frost.

As described by Alabama Cooperative Extension, nitrogen application history and plant factors can also impact prussic acid accumulation. If nitrogen has been applied at higher than 70lbs/acre, there is an increased probability of prussic acid accumulation following stress events. Concentrations of prussic acid will be higher in young plants rather than mature plants, and increased in plant leaves as compared to stems.

frost damaged forage sorghum

Frost-Damaged Forage Sorghum. Photo Courtesy of Michigan State University

Prussic acid can be deadly to ruminant livestock in relatively small doses. For example a 1,200 lb. cow would need to consume only 2 – 14 lb. of frost-damaged forage sorghum for it to be lethal. Despite this, prussic acid will dissipate from forages in about 5-7 days following a stress event. If you have one of the forage species listed above, it will be safe to graze them again in about one week, if there are no more freeze events during that time. Alternatively, these forages can be harvested for hay or silage immediately, but it is advised to wait 3-4 weeks before feeding the harvested forages to allow prussic acid levels to decrease.

If you have concerns about prussic acid levels in your forages, please contact your local Extension office.