Weeds Now Showing Paraquat Resistance

By Patrick Cavanaugh, Farm News Director

There’s been weed resistance to glyphosate herbicide, but now some weeds show resistance to paraquat herbicide.

“When we see paraquat resistance at often a very high level of resistance, it doesn’t cause any visual injury, which is different than a lot of cases,” said Brad Hanson, a UC Cooperative Extension Weed Specialist at UC Davis.

“We’re dealing with a lot of glyphosate resistant weeds right now, and when you spay glyphosate on the resistant weeds, they are slightly injured and then they recover,” said Hanson. “With paraquat resistance, it’s very stark. You often don’t see any symptoms at all. I describe it almost as immunity. It’s such a high level of resistance.”

“We have glyphosate resistance to annual ryegrass and hairy fleabane, and we have some populations of those same weeds that are resistant to paraquat,” Hanson said. “We see a very temporary reduction in the photosynthetic ability, but within a few hours it bounces back to normal.”

The plant is taking that active herbicide molecule, and it is being sequestered. It’s taking that molecule and putting it somewhere in the cell where it cannot get to the photosynthetic apparatus that’s its target site. It’s a very unique and interesting kind of resistance challenge.

“I think we’re talking about multiple resistance to paraquat and glyphosate. The grower likely switched to use paraquat in order to kill the glyphosate resistant weeds, and then the weeds selected paraquat resistance on top of that.”