After the recent cold mornings UT Extension Director John Goddard began notifying local cattle producers that the frost on Johnson grass sets local pastures up for prussic acid poisoning and cautioned them against pasturing cows to feed on frost-bitten Johnson grass. He said it doesn't take a lot of the grass to kill a cow.

"A good mouthful would do it," Goodard explained.  

Goddard said he has not had any farmers report dead cows but several farmers called in after the frost because they remembered there could there could be a problem with Johnson grass.

"I'm sure there are some cows out there who have died of it," he said adding, "When a cow dies a farmer feels defeated."

He said this can make them reluctant to report what they may view as a failure.  

Johnson grass has been prominent for the past two years because it is drought resistant allowing it to thrive when other pasture grasses couldn't handle the dry weather.

Part of the problem is the very thing farmers prayed for through the last two years - rain.  This year's heavy rains have caused the Johnson grass to flourish. The biggest part of the problem is frost. The heavy frost on Monday followed by two more frosts caused the prussic acid to form in the prevalent grass.

"This year we've had lots of rain and not much opportunity to put hay up," Johnson said.  

Prussic acid is a form of cyanide, a lethal ingredient that is used in rodent and vermin killers. Goddard said it is not only cows that are susceptible to the poison. Any animal that eats the grass would be effected.

"Sheep, horses, goats or anything else that eats it would be poisoned," Goddard said adding, "It would do the same to you and me." 

Anything that causes the plant cell to rupture will produce prussic acid. Plant cells can be ruptured by cutting, wilting, drought, crushing, trampling, chewing, chopping or, as is the case in Loudon County, freezing. Once plants containing prussic acid have been eaten, the toxin quickly enters the blood stream and spreads through the body of the animal. Prussic acid interferes the body's ability to process oxygen. 

In essence, the animal suffocates. 

While the poison can effect any animal that eats it, ruminant animals (cattle and sheep) are more susceptible to prussic acid poisoning than non-ruminant animals because their microorganisms have enzymes which will release prussic acid in the animal's digestive tract. 

Prussic acid is a potent, rapidly acting poison. Signs of prussic acid poisoning can occur within 15 to 20 minutes to a few hours after animals consume the toxic forage. 

The animals are often found dead. Signs of prussic acid poisoning, when noticed, occur in rapid succession. Excitement, rapid pulse and generalized muscle tremors occur initially, followed by rapid and labored breathing, staggering and collapse. There may be drooling, runny eyes and voiding of urine and feces. The mucous membranes are usually bright pink and the blood will be a characteristic bright cherry red. Though there are treatments if the animal gets help quickly Goddard said that was rare.

"Generally they'll die, because the farmer doesn't realize there is a problem before it's too late," he explained. 

Treatment is possible but rarely effective. For a veterinarian to treat an animal with prussic poisoning it must be taken in quickly.  Since most cows are out in the field it's hard for farmers to know in time that their animal has eaten the toxic grass which usually kills within 24 hours or less. 

The danger is still present and any cattle growers who have Johnson grass in their fields should beware. 

"Three days of frost has turned the grass brown so it's not nearly as attractive now for them to forage," Goddard said, adding other fields didn't get as heavy a frost "so it was still attractive" to cows.

Standing in a field of Johnson grass at Roger Huber's Loudon County farm, Goddard pointed out how much of the grass looks brown from the frost but the stalks were not affected and still looked green - and tempting for hungry cows. Huber has been keeping his cattle in the next pasture over where the grass has been cut down.  Though cows shouldn't eat the grass it doesn't have to be a total loss.

"A better idea would be to cut it for hay. The toxins will break down in 10 to14 days and then the hay would be safe to feed," Goddard said.

Huber reported one of his neighbors was busy doing just that ahead of more expected rain.