Sorghum-Sudangrass and Frost

Forage Flash
Another in a series of Supporting Farmers During Challenging Times

Kevin Jarek, UW-Madison Division of Extension Outagamie County Crops, Soils and Horticulture Agent
October 16, 2019

Sorghum plants in standing water

Figure 1

Widespread alfalfa winterkill combined with cold and wet conditions during the spring of 2019 resulted in many farmers identifying alternative forage crops for their feed needs. Delays in planting combined with a late first crop alfalfa harvest resulted in many producers planting sorghum-sudangrass as a part of a mix or as a standalone crop to help meet forage needs. Much of this sorghum-sudangrass has been under stress all season long due to cold temperatures, flooding/standing water, etc (Figure 1). Sunday night (10-13)/Monday morning (10-14) we experienced widespread frost across east central and northeast WI (Figure 2). What does this mean for the sorghum-sudangrass still standing in the field?


Sorghum leaf with frost

Figure 2

A product known as cyanogentic glucosides is produced by sorghum/sorghum-sudangrass mixes while the plant is growing. If the bonded cyanide and glucosides remain intact, they are not poisonous. However, when plant cells rupture, the cyanide can be freed from its chemical bond. While the gluconosides may simply convert to glucose sugars, the cyanide can now convert to prussic acid. It is important to note water and certain enzymes need to be present for this process to occur.

While the plant may already have the enzymes necessary to go through the hydrolysis and chemical decomposition needed to produce prussic acid, once the feed is consumed by the animal, digestive juices can also lead to the hydrolysis necessary for conversion. Frost/freeze events are obvious condition that can lead to plant cell rupture, cutting, chopping, chewing, drought, and maturity are additional sources. While straight sorghum has the highest potential for poisoning, and straight sudangrass, the least, sorghum-sudangrass has the potential to lean either way.

When livestock ingest plant regrowth following a frost or drought, they are most susceptible as this represents the highest risk for poisoning to occur. Death can occur quickly as prussic acid is very fast acting. Animals’ respiratory systems are affected as convulsions, gasping, trembling muscles, and staggering will all be physical symptoms for which to watch. Cyanosis may be observed. The presence of a blue coloring around the mucous membranes around the eyes and mouth may be evidence of this condition.

Plant cell rupture is especially problematic in the fall due to the repeated light frosts which can result in prussic acid being released. So, what can you do?

1) Sorghum-Sudangrass should never be the sole source of forage. Mixing with other feedstuffs will help reduce potential problems from arising.

Sorghum plants with a ruler to judge height

Figure 3

2) Young regrowth, less than 18” up to about 24” represents the greatest risk of poisoning (Figure 3).

3) Fields containing high levels of soil available N and/or low phosphorus increase the risk of prussic acid. Therefore, know your soil tests.

4) Much of the prussic acid present, even if ensiled while at toxic levels, can escape during fermentation as a gas. Sorghum-sudangrass that is left to ferment for three weeks reduces the risk of poisoning.

5) Sorghum-sudangrass harvested as hay usually results in a 75% reduction of prussic acid during the curing process. As hay is stored, the cyanide potential gradually decreases if not fed for two or more months.

6) Sorghum-sudangrasses grazed or fed as greenchop should be left for 7-10 days following a light frost. After a killing freeze, DO NOT greenchop or graze sorghum-sudangrass until the plant has dried (approximately 7-10 days, but with saturated conditions this can be longer).

7) Lastly, prussic acid poisoning occurs when animals consume a large quantity of high-risk forage. Avoid feeding high-risk sorghum-sudangrass to hungry animals. Mix, dilute, or reduce the amounts fed to limit possible poisoning.

Lastly, I know we have many more issues than worrying about Sorghum-sudangrass right now, but, because of the large increase in acreage we have seen this year, it is important for you have a strategy as to how (when Mother Nature decides) you plan to harvest and feed that forage. More suggestions about harvesting and storing forages can be found at recent Supporting Farmers During Challenging Times Series meetings at

This article is a part of the Supporting Farmers During Challenging Times Series facilitated by Scott Gunderson, Tina Kohlman, Stephanie Plaster, Amber O’Brien, and Kevin Jarek. Information was selected in part from Sorghum & Sorghum-Sudangrasses and Frost, Dan Undersander, Forage Agronomist, UW-Madison.

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