Concerns about the world’s most popular herbicide continue to mount, as U.S. agricultural experts note spreading weed resistance to glyphosate. As the key ingredient in Monsanto Co’s Roundup herbicide products as well as about 700 other products, glyphosate is widely used on farms as well as residential lawns.

But the chemical has come under increasing scrutiny in recent years in part because scientists and environmentalists have warned that weed resistance to glyphosate has become a significant problem that impacts crop production.

In the latest account of glyphosate-resistant weeds, U.S. weed scientist Dallas Peterson said this week that resistance is increasing rapidly in the key farming state of Kansas. The trend is a worrisome sign as weed resistance spreads from the southern U.S. into the Midwest and Plains farming states, he said.

Peterson, who is both a weed scientist at Kansas State University (KSU) and president of the Weed Science Society of America, said Kansas soybean farmers in particular are experiencing weed problems, particularly with a type known as Palmer amaranth. Wet weather along with the weed resistance contributed to the problem, he said.

“It’s really kind of exploded,” he said.

Farmers in other Midwestern states, including Missouri, Nebraska, and Illinois have reported mounting problems with weed resistance as well.