MARE'S TAIL, a familiar nemesis for Missouri farmers, has reappeared with a new resistance to glyphosate-based herbicides, a University of Missouri weed scientist said.

Marestail, also known as horseweed in the Delta region, “isn't a new problem, but until recently, glyphosate controlled it,” said Andy Kendig, weed science specialist at MU Delta Research Center in Portageville, Mo. “Now, we see fields where everything is burnt down except horseweed. It's really erupted over the past two years.”

“Some of the marestail problem is due to glyphosate-only burndown treatments,” Kendig said. “The good news is that we have a couple of very good treatment options. The bad news is that it doesn't always get done.”

MU researchers recommend a March application of an herbicide such as 2,4-D or Clarity, he said. “These two herbicides are essential to control marestail — or primrose and several other troublesome weeds.

Even if a few late-germinating horseweeds escape, this application is still needed. Without the preplant burndown application, he said, “you have to go with mediocre cleanup options. There are only a few choices,” including “old-fashioned tillage.”

Kendig and other researchers on the Delta Center Weed Science Project will present their latest findings at the Delta Center Field Day scheduled for Sept. 2.

Anthony Ohmes, MU Extension agronomist in Mississippi County, Mo., will address the problems of weed control in wheat. “Ryegrass continues to be among our most difficult problems,” he said. “In some cases, ryegrass has become resistant to grass-only herbicides” traditionally used to control them.

Ohmes will discuss new herbicide compounds that can control ryegrass effectively as well as rotating herbicides and other resistance management techniques.