As use of the herbicide exploded, resistant variants evolved, an assemblage of weed Avengers that can withstand higher concentrations of glyphosate or shrug it off altogether. Palmer amaranth, an Iron Man of superweeds, for instance, grows to eight feet and arms each plant with a million seeds.

“That is the much bigger issue,” said Scott H. Irwin, a professor in the department of agricultural and consumer economics at the University of Illinois at Urbana-Champaign. Whatever the cumulative effect of the lawsuits, he said, “in farmers’ minds, the weed-resistant question will determine whether they’re using it or not.”

That is one reason Bayer announced in June that it planned to invest $5.6 billion over the next 10 years in research on weed control. Whatever alternatives are developed, though, Mr. Condon at Bayer said glyphosate would remain a base product. And farmers seem receptive.

“I’ve used it for over 40 years, since I was first around the farm,” said Davie Stephens, who has 6,000 acres in Clinton, Ky., and is president of the American Soybean Association. “I’ve never had any problems whatsoever, and the farming community uses it more than anyone.”