Johnsongrass has now “joined the party” for glyphosate-resistant weeds, with documented hot spots in two Louisiana parishes, says Daniel Stephenson, Louisiana State University assistant professor of weed science at the Dean Lee Station at Alexandria.

“In Rapides Parish, it took 7.2 times the normal rate of glyphosate to kill 50 percent of the resistant johnsongrass population,” he said at the annual meeting of the Mississippi Agricultural Consultants Association a Mississippi State University. “In Pointe Coupee Parish, it took 10 quarts to kill half the resistant population of rhizome johnsongrass, and 3 quarts just to kill the seedlings.

Research has shown, Stephenson says, that just two johnsongrass plants per row foot of soybeans can reduce yield by 16 percent when the weed is allowed to compete for four weeks.

“For 50-bushel beans at $12, that’s a 7.2 bushel loss and $86.40 per acre you’ve cost yourself by leaving johnsongrass in the field and allowing it to compete.

“Just one johnsongrass plant per row foot of cotton can reduce yield as much as 40 percent. At 1,000 pounds of lint, that’s $300 per acre you’ve lost.”

Johnsongrass, once labeled world’s worst weed, first started to be tamed in the era of recirculating sprayers and

a 2X rate of Treflan, Stephenson says. “That did a good job of controlling johnsongrass.

“Then came the graminicides, such as Poast, Fusilade, Assure, Select, and others that gave over-the-top control. Those materials still have utility, but we’re now seeing resistance.

“In 1996, Roundup Ready crops came along, and johnsongrass was no longer an issue because glyphosate worked so well. But unfortunately, we’ve overused it and now we’re having problems with resistance similar to that in Palmer amaranth and other weeds.

“Research has given us tools with which we can manage glyphosate-resistant Palmer amaranth. We’re now finding that glyphosate-resistant johnsongrass is difficult to control because of the limited tools we have.

“And as with Palmer amaranth, there’s no single solution for fighting resistant johnsongrass — you’ve got to rotate your crops and herbicide chemistries.”

In many areas of Louisiana, where no-till production is not utilized, he says, tillage can further spread rhizomes of glyphosate-resistant johnsongrass.

The resistant weed was first found in the state in 2008, but “has been spreading rather slowly, because growers and consultants have learned a lesson with glyphosate-resistant Palmer amaranth — if they see johnsongrass surviving a glyphosate application, they’re quick to get out there and remove it. They’re doing all they can to shut down resistant johnsongrass and keep it from spreading.”

Materials such as Fusilade, Assure, and Select “are beginning to lose effectiveness on this resistant population,” Stephenson says. “Thus far we’ve not documented multiple resistance, but if we keep banging away with graminicides the way we have the last couple of years, we unfortunately are likely to see multiple resistance develop. Past experience has shown that when we rely on one chemistry too long, we have problems.”

In cotton, if johnsongrass is not controlled, he says, “you can have all kinds of problems with picker heads when you start harvesting the field.