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Prussic acid advisory on sorghum, sudangrasses

Oct. 24, 2013 | 0 comments

MADISON

With most of Wisconsin having been hit with a killing freeze early this week, any remaining stands of sorghum or sorghum-sudangrass mixes were among the crop species whose growing season ended.

Because of the danger of prussic acid poisoning for livestock from the frosted foliage of those species, University of Wisconsin Extension Service forage specialist Dan Undersander issued an advisory on what to do and not to do with those crops.

Undersander points out that the greatest danger of prussic acid exists with the pure forage sorghum varieties, less so for sorghum-sudangrass mixes, and minimally for sudangrasses.

The highest degree of danger exists with dark green foliage at standing at heights of less than 18-24 inches, he notes.

A frost will add to the prussic acid in the initial growth or regrowth at those heights, Undersander states.

If there is no substantial regrowth after a freeze, that foliage can be grazed safely within a few weeks after the freeze, he advises.

Soil fertility also affects the amount of prussic acid that is likely to form, Undersander adds.

He indicates that a high availability of nitrogen and/or a low availability of phosphorus can lead to a high build-up of prussic acid.

Natural processes

It is natural for sorghum and sorghum-sudangrasses to produce a combination of glucosides and bonded cyanide which, by themselves, are not poisonous, Undersander explains.

But the glucosides break down into glucose sugars when water and certain enzymes are present, thereby freeing the cyanide from its previous bonding and turning it into the toxic prussic acid, he points out.

It's the rupturing of plant cells that leads to the conversion of the glucosides into prussic acid, Undersander continues.

That rupture will occur as a result of drought, freezing, cutting, chopping, maturity, or chewing by livestock but the highest potential for this happens when livestock consume plant regrowth after a frost or drought, he indicates.

Prussic acid is fast-acting and can easily result in the death of affected livestock, Undersander warns. The symptoms in livestock start with gasping, staggering, muscle trembling, and convulsions before death is caused by respiratory failure, he notes.

In a condition known as cyanosis, livestock might also have a blue coloration in the eyes and mucous membranes of the mouth, Undersander observes. If the livestock recover, no permanent effects are apparent, he adds.

Harvesting advice

Undersander lists recommendations for making silage or dry hay, greenchopping, and grazing if the sorghum or sorghum-sudangrass has been frozen or if there is some regrowth after a frost.

In all cases, the danger is reduced by mixing the crop with a potential for prussic acid with other feedstuffs, he observes.

Making silage from a freeze-affected crop is generally a safe choice because much of the poison escapes as a gas during fermentation and when being fed, Undersander explains.

As an added precaution, however, be sure to allow the silage at least three weeks to ferment before it is fed, he advises.

Although the waiting period for feeding it is somewhat longer, making hay from a freeze-affected crop is also a fairly safe choice, Undersander notes.

He points out about 75 percent of the prussic acid disappears during the curing of the hay and most of the cyanide potential is gone after two or more months of storage.

Dangerous scenarios

What is dangerous is to graze sorghum or a sorghum-sudangrass hybrid soon after a series of light frosts because the potential for a prussic acid accumulation increases for a while, Undersander cautions.

He recommends allowing at least 7-10 days after a light frost before either greenchopping or grazing.

Following a killing freeze, give the foliage enough time to dry, Undersander advises. In normal weather, that should take about seven days, he suggests.

For grazing, do not put livestock into a frost or freeze-affected sorghum or sorghum-sudangrass hybrid stand if they are hungry, Undersander remarks. He points out that the potential for poisoning rises with the amount of high-risk forage that is consumed.

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