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Effects of Forage Freeze on Cattle
Revised: October 05, 2020
Published: November 12, 2019

Fall is the time of year ranchers have to be concerned with prussic acid poisoning in their beef herds.  With a frost or freeze, high levels of prussic acid can be produced within hours especially in pastures of sorghum hybrids, Sudan grass and Johnson grass.  Prussic acid is a form of cyanide poisoning and treatment is difficult and usually unsuccessful because most cattle die so quickly that treatment doesn’t have a chance to work. 

Therefore, preventing poisoning is important and Dr. Steve Barnhart from Iowa State Extension indicates in Beef Magazine that there are several things you can do to decrease the chance of poisoning.  First of all, do not let them graze on nights when frost is likely as high levels of prussic acid accumulate in just a few hours after a frost.  Immediately after a frost, remove the cows until the grass has dried thoroughly and usually the grass will be safe to graze after about 6 days.  Also, do not graze young tillers or new regrowth as these plants produce new shoots after a frost and can be high in prussic acid so Sudan grass should not be grazed until it is two feet tall.  Frosted or frozen forage will be safe once baled as dry hay as the prussic acid decreases over time as long as the hay was baled correctly when dry. 

To be sure, the best method to determine prussic acid content is to take samples from the grass and send them to the lab for testing.  There are some specific methods of handling the grass for a prussic acid test and your vet can help make sure the samples are sent in correctly.  Of course, prussic acid levels change rapidly so testing does not guarantee a pasture is safe.  Before turning in to one of these pastures, make sure the cows are full of hay and only turn a couple of cows in for the first 6 hours to make sure the pasture is safe.    

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