Scientists said on Friday they have confirmed expanding weed resistance to a key ingredient in Monsanto’s widely used Roundup herbicide, a troubling development for farmers and fresh fodder for Monsanto critics.

Kansas State University said scientists had found five kochia weed populations in western Kansas that have been confirmed to have become resistant to glyphosate.

Kochia, also called fireweed, is a drought-tolerant weed commonly found on land in the western United States and Canada where crops are grown and cattle are grazed.

“This complicates and may increase control costs for those growers who may have a resistance problem, but there are other herbicides,” said Kansas State weed scientist Phil Stahlman.

Stahlman and other university researchers are recommending farmers use other herbicides to try to control the weeds.

Monsanto said it was working with university scientists on a multi-state effort to keep evaluating the problem and advise farmers how to respond.

The company declined to answer questions about how significant the resistance problems are to date, and if resistance is expected to expand further.

Weed resistance to glyphosate, a key ingredient in Roundup herbicide, has been mounting across the United States in recent years as Monsanto’s genetically modified “Roundup Ready” corn, soybeans and other crops have gained popularity with farmers.