Grazing milo stalks in Barton County is a very useful way to feed cattle after pastures have gone dormant for the season. Prussic acid can be a concern when grazing milo and it is a good idea to know how to lessen the risk of poisoning when grazing. I wanted to share this article from Sandy Johnson, North West Livestock specialist on a few things to consider when grazing stalks in this unusual fall.
Many of you have been working on getting cows turned out onto corn residue but when it comes to sorghum stubble we need to exercise some caution before turning out this year. Many fields are showing new tiller growth which presents challenges for grazing due to concerns of prussic acid poisoning.
Most sorghums and sudangrasses contain a prussic acid precursor (dhurrin) in their epidermal cells. Grain sorghum generally has higher concentrations of dhurrin than forage sorghums or sudangrass. Under normal growing conditions, the dhurrin concentration is low enough that animals can detoxify it before it causes toxicity. Under certain conditions, an enzyme in the plant causes the conversion of Dhurrin to hydrogen cyanide or prussic acid. It is the prussic acid that is toxic to livestock.
Dhurrin concentrations are highest in young plants, new regrowth, and following rapid regrowth after a period of stunted growth, such as following a rain in drought-stressed plants, or regrowth following a frost or freeze. Under these conditions dhurrin concentrations can be high enough to poison livestock. The green growth evident in many of the sorghum fields fits this dangerous category.
Generally, any stress condition that retards normal plant growth can increase prussic acid content. Hydrogen cyanide is released when plant leaves are physically damaged by trampling, cutting, crushing, freezing, wilting, or chewing.
Freezing ruptures the plant cells and releases cyanide into the leaf tissue. After a killing frost, wait at least five days or until the frozen leaf tissue has completely dried out before grazing to allow the released HCN gas to dissipate. Prussic acid poisoning is most commonly associated with regrowth following a drought-ending rain or the first autumn frost. New growth from frosted or drought-stressed plants is palatable but can be dangerously high in cyanide.
When you do turn out on sorghum residue, feed cattle before turnout to slow intake and reduce poisoning risk. Remember both the amount and concentration of prussic acid in the diet and how rapidly it is consumed will influence the likelihood of toxicity. Tillers that are 4-5” tall are more likely to be consumed than shorter ones and thus increase the possibility of poisoning.
Monitor conditions in sorghum residue closely before turnout. Wait at least 5 days after a killing frost to graze and make sure animals are full going out.
For more questions about prussic acid poisoning contact your local county extension office.