Friday marked a major win for dicamba detractors and a home-turf loss for St. Louis-based Monsanto as the pesticide was temporarily banned in both Missouri and Arkansas. This is a big deal: veteran ag reporter Chuck Abbott called it “stunning” in his daily Ag Insider newsletter.

The damage the wayward weedkiller has allegedly caused is also unprecedented, according to a University of Tennessee weed specialist who told NPR’s Dan Charles he’d “never seen anything even close to this.”

Quick recap: Dicamba is a strong herbicide that’s been in use for decades. But recently, Monsanto started selling dicamba-resistant cotton and soybean seeds. So now, farmers who plant Monsanto’s dicamba-resistant crops can spray more dicamba, destroying hard-to-kill weeds that have evolved to survive even after being sprayed by common herbicides like Roundup (glyphosate). Both Monsanto and a German company called BASF sell versions of dicamba, though Monsanto is the only company that sells seeds with the resistance trait.

Monsanto’s dicamba isn’t yet approved in every state—Arkansas, for instance, doesn’t allow it to sell the herbicide (though Monsanto can sell the modified seeds). The company got a lot of flak when it marketed its dicamba-resistant seeds before the accompanying herbicide had been approved by the Environmental Protection Agency. Critics alleged that by introducing the weedkiller-resistant seeds, Monsanto was nudging farmers toward using illegal versions of the herbicide while it waited for its own dicamba product to be approved. Dan Charles at NPR chronicled that drama last year.

As a weedkiller, dicamba apparently works great. There’s just one problem: It also damages regular soybean plants, the ones that aren’t Monsanto’s genetically-engineered dicamba-resistant variety. Farmers who live near dicamba-spraying neighbors have complained that the pesticide drifts onto their land in the wind and heat, causing damage to their crops.

It’s the scale of that pesticide drift that caused Tennessee weed specialist Larry Deckel to tell NPR he’d never seen anything like it. Normally, drift is a problem he sees in only a handful of fields each year. But NPR cites an estimate that dicamba drift could affect 2 million acres this year. In Missouri and Arkansas alone, officials have already received more than 700 complaints of crop damage caused by rogue dicamba.

Hence the ban: It’s not fair for farmers who don’t buy Monsanto’s dicamba-resistant seeds to lose crops because their neighbors are spraying the herbicide. But the news is also a blow for farmers who did invest in dicamba-resistant crops: Now they can’t use the weedkiller they paid all that extra money to be able to use, and they’re left without a way to combat the scary-as-hell “superweeds” that continue to thrive in Roundup-soaked soil.

Reuters reports Monsanto has invested $1 billion in a dicamba factory and projects its dicamba-resistant seeds will cover 55 million acres by 2019. The Arkansas ban is scheduled to last for 120 days, and Missouri has ordered an “immediate halt” to the use of dicamba. Monsanto has called the Arkansas ban “premature” in a statement, and is encouraging farmers from other states to share their dicamba success stories. We can only imagine the social media campaigns it’ll launch in the coming weeks. #DreamingOfDicamba forever.