GARDEN CITY, Kan. – Clinical signs of prussic acid poisoning in cattle might include labored breathing and staggering. Prussic acid poisoning is a condition that occurs when cattle ingest forages with high levels of prussic acid, which inhibits the blood’s oxygen-carrying capacity and causes cattle to die very quickly. Producers should be on particular lookout for the condition this time of year.
“When we move into fall and flirt with that first frost, we have risk potential for prussic acid poisoning in livestock,” said Justin Waggoner, beef systems specialist at K-State Research and Extension’s southwest area office in Garden City.
High levels of prussic acid are common in several forages native to Kansas that include sorghum, Sudan grass and crosses of those types. The high levels are caused by anything that damages the cells in plant leaves, including that first frost.
“When plant cells are damaged due to frost, the plant cell wall ruptures and releases prussic acid, or hydrocyanic acid, into the surrounding leaf tissue,” Waggoner said.
Timing Fall Grazing
When the first frost hits a particular field, it burns the leaves and prussic acid content rises, Waggoner said. Over a period of time, five to seven days later, the prussic acid will volatilize. Once the plant becomes dormant, the risk of prussic acid is gone.
But, because prussic acid is volatile, it is hard to determine if it dissipates after one frost. If the plant doesn’t become dormant, there is still a risk for high prussic acid levels if another frost comes along.
Waggoner said producers often like to turn their cows out to graze sorghum stocks before a hard killing freeze, which could be risky for cattle.
“If you graze too early, the situation you run into is you get that early frost that doesn’t burn the entire field and make the plants uniformly go into dormancy,” Waggoner said. “Prussic acid levels may be high in one part of the field and relatively lower or non-existent in another part. We get another frost, and prussic acid just continues to spike and decline until all plants go into dormancy.”
Handling Forage Samples
Testing forages could help prevent prussic acid poisoning in cattle, but samples must be handled properly. Waggoner, along with K-State agronomist J.D. Holman, recently completed a study that examined the effectiveness of different sample handling methods for forage samples intended for prussic acid analysis.
In the study, they handled forage sorghum samples in different ways. The goal was to think like a producer and how the samples might be collected and managed under a variety of different scenarios. One sample was sent directly to the lab on the day of collection, while the other samples were placed in the refrigerator, freezer, a sealed plastic bag and an unsealed plastic bag. The bag samples were left in a pickup for seven days.
Waggoner hoped to find the best management practice if producers couldn’t get the sample directly to the lab, or if the sample had to be shipped by mail to the lab.
Compared to the fresh sample taken immediately to the lab, refrigerated and freezing showed no difference in prussic acid content.
“To stabilize the sample if you have to ship it, freezing would be preferred based on this experiment,” Waggoner said.
The plastic bags placed in the pickup were exposed to environmental factors, including temperature, for seven days. Waggoner said the open bag sample placed in the pickup had 400 parts per million (ppm) less prussic acid compared to the fresh sample, which showed that it was not an efficient sampling method.
“Interestingly enough, in the plastic bag we sealed, prussic acid content was basically maintained,” he said. “It was slightly lower, so we did lose a little bit of the prussic acid. But, at the same time, it was still relatively high compared to the sample delivered directly to the lab.”
This find, Waggoner said, lends itself to the potential need for prussic acid sampling in packed forages, such as silage.
“Silage is packed relatively tightly, and it doesn’t go through the same wilting process that hay does, when hay is allowed to dry in a field,” Waggoner said. “So the prussic acid content could still be relatively high. You might want to run a sample if you put up a forage for silage that was potentially high in prussic acid.”
Knowing Appropriate Levels
The haying process does allow forages to wilt and prussic acid to dissipate, so Waggoner said hay samples are typically not submitted for prussic acid testing. When testing fresh forages or silage, less that 500 ppm of prussic acid is acceptable for cattle to eat. He said anything between 500 ppm and 1,000 ppm is potentially toxic but should be fine to use for feed as long as it is not the only feed source. Anything that tests above 1,000 ppm for prussic acid should not be used to feed cattle.
Waggoner’s research about prussic acid sampling methods and the specifics about his findings are available in a K-State Research and Extension bookstore publication (http://www.ksre.ksu.edu/bookstore/pubs/SRP1086.pdf). The bookstore also has more information about prussic acid in forages (http://www.ksre.ksu.edu/bookstore/pubs/mf3040.pdf).