Iowa farmers are awash in Roundup weed killer.

That was a federal judge’s takeaway after seeing slides that “sort of seared into my brain” that the main chemical in Monsanto Co.’s Roundup is ubiquitous in the Hawkeye state’s cropland.

“I got the impression that everybody takes a shower in glyphosate every day in Iowa,” U.S. District Judge Vince Chhabria said in court, with a hint of hyperbole.

The slides were presented by Beate Ritz, a public health professor at the University of California at Los Angeles who is a witness on behalf of more than 700 farmers, landscapers and gardeners claiming that exposure to glyphosate — through skin contact or inhalation — caused their non-Hodgkin lymphoma.

Chhabria will be the first judge to weigh in on the toxicity of the world’s most popular herbicide, the source of a heated debate among scientists and regulators worldwide for more than 30 years.