Resistant weeds are nothing new, not necessarily evolving because of increased and almost exclusive use of a particular herbicide, but have always been around, says Louisiana State University Extension weed specialist Eric Webster.

Webster, addressing a field day crowd recently at the Texas AgriLife Research and Extension Center in Beaumont, said the resistant plants “are already in nature and already resistant to certain herbicides. But we select for them.”

He said resistant weds have been showing up for the past few years but the resistant plants were already there, waiting for an opportunity.

“Reduced rates don’t cause the resistance,” he said. “High rates select for resistance.”

He said the first year a farmer might identify one plant that escapes herbicide treatments. The next year, two might survive and then a problem begins as those plants reproduce. “But the resistant plant was already there,” Webster said.

“We don’t have many resistance issues in Louisiana because we have enough weed species that we need to rotate chemistries anyway. But in some areas, farmers use a lot of Roundup and Roundup Ready varieties and they select for resistance.”

He believes very few pure pigweeds exist anymore. “Most are outcrosses.”

Webster said farmers also may see “size resistance. They change their timing (with new chemistries or technology) and apply herbicides when weeds are too large to control.”

He said herbicide resistant technology in Louisiana’s rice production area changed production practices. “Years ago, we never planted rice after rice. Now, some fields go three years in a row. And they use only one herbicide resistant technology, Clearfield.

“Some farmers have already lost their technology to resistance.”

Webster said farmers who think they have resistant red rice and are planting rice hybrids should rotate away from rice in those fields for at least two years, “probably longer.

“If they suspect resistance, they should assume they have it and do everything they can to get rid of it. If they have to Roundup an entire rice field it may be worth the loss to prevent resistant weeds from infesting the next year.”

email: rsmith@farmpress.com