OSAGE COUNTY, Oklahoma - There's a poisonous weed that is killing Oklahoma cattle, and it's been especially bad this year.

With rainy weather this summer the toxic plant is growing fast, and the more mature it gets, the more attractive it is to cattle to eat. The bigger it gets the more toxic it becomes.

Rancher Gary Chambers' pasture should have cattle on it, but he's moved them away, and the toxic weed is the reason. The weed killed 16 head of cattle.

“It kind of threw us in a tizzy, trying to figure out what was going on,” Chambers said.

The weed is called Perilla Mint. It's bitter most of the time, so cattle usually won't eat it, but when it blooms it become more palatable and it took a toll on Chambers' herd.

"Well, when you're sitting there and watching about a $2,500, $3,000 animal die, and knowing you can't do anything with it, and the financial part of it, it hits you pretty good,” he said.

The OSU extension service is warning ranchers to watch for the Perilla Mint, and urges them to use a broadleaf pesticide, relocate cattle, or even pull up the toxic plant by the roots if they have to.

Brushogging can also help.

Will Cubbage, OSU Extension Educator said, "But it needs to be done in such a way that it's really finely chopped."

The weed isn't native to Oklahoma, but made its way to our state from southeast Asia some time ago. This year it's worse.

"It's another case of something that was brought here as an ornamental plant a long time ago, and just escaped,” Cubbage said.

Cattle develop pneumonia type symptoms. The toxin hits hard and hits fast.

"The toxin goes into the bloodstream, and goes straight to the lungs and causes like emphysema. They can breathe in, but they can't breathe out and the animal essentially drowns,” said Chambers.

Experts say the plant usually grows in shaded areas, near water. It's more prolific in eastern Oklahoma, but some has been found as far west as Ponca City. It's also toxic for other livestock.

Only a few pounds of the Perilla Mint can be deadly.