May 8, 2009

In weed control vs. planting, experts choose weed control

WEST LAFAYETTE, Ind. - Weed specialists at Purdue University and Ohio State University are worried farmers will put weed control on the back burner in order to get their crops planted, and it could come back to bite them later in the growing season.

If producers don't control giant ragweed, a very competitive weed, it can result in 50 percent to 100 percent yield loss, said Bill Johnson, Purdue Extension weed specialist. If the weed is not controlled before planting in a field with glyphosate resistance, producers are limited in what they can use to kill the weed postemergence in soybeans, he said.

Glyphosate is the active ingredient in herbicides such as Roundup and Touchdown, which are used for burndown weed control in no-till cropping systems and postemergence in Roundup Ready soybeans and corn. Since 60 percent to 80 percent of the soybeans grown in Indiana are no-till, weed specialists advise growers to burn down existing vegetation before they plant.

Marestail, a weed that's not as competitive as giant ragweed, also can present challenges because of the limited number of herbicides available to control it.

The challenge producers face is the short number of days suitable for fieldwork because of the wet soil conditions this spring in Indiana and Ohio. Producers now have to weigh whether or not to plant and get the crop in the ground or apply a burndown treatment and delay planting even more.

"The good news about delayed planting is more weeds have emerged by the time the ground is tilled or a burndown is applied, compared with early planting," said Mark Loux, OSU Extension weed management expert. "This makes things easier for the in-crop herbicide program, and later-planted crops develop more rapidly, allowing them to be more competitive with the weeds earlier.

"The bad news is as farmers become increasingly rushed to get the crop planted, it gets tougher to burn down and the opportunity to use 2,4-D ester is limited due to the seven-day preplant interval at the half-pound rate. However, the further we go into May without getting in the field, the taller the weeds get and the harder they are to control."

Glyphosate activity can be variable on large plants. In the absence of 2,4-D ester, populations resistant to glyphosate and/or acetolactate synthase (ALS) inhibitors can be almost impossible to control with soybean herbicides in May as plants get taller and older, Loux warned.

"We have documented cases of pretty good sized glyphosate-resistant marestail and ragweed now, 8 inches to 12 inches tall, and we essentially will not be able to control those effectively without the use of 2,4-D ester in the burndown," Johnson said.

Loux and Johnson recommended growers apply a burndown as soon as possible.

"Ideally, growers should use some 2,4-D with their burndown product, but if that's not possible there are other products growers can use without the seven-day preplant interval," Johnson said. "Those products include Classic-containing herbicides such as Valor XLT, Canapy DF, or First Rate-containing products such as Sonic and Authority First."

These products will provide decent burndown activity plus residual activity in the soil for later-germinating weeds, but Johnson warned these alternatives might not get the job done in fields with ALS-resistant populations.

If growers don't control these competitive weeds before planting they could suffer yield losses up to 30 percent this year, plus those weeds will produce seed that will infest other nearby areas, Johnson said.

Writer: Julie Douglas, 765-496-1050, douglajk@purdue.edu

Sources: Bill Johnson, 765-494-4656, wgjohnso@purdue.edu

Mark Loux, 614-292-9081, loux.1@osu.edu

Ag Communications: (765) 494-8415;
Steve Leer, sleer@purdue.edu
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