ROCKVILLE, Md. (DTN) -- Ready or not, the 2017 spraying season is about to begin. Winter annual weeds like marestail, henbit and chickweed and even some early emerging summer annuals like giant ragweed and kochia are off to the races.
"Given the mild winter, the growers who will have the most trouble with spring marestail control are those who skipped or missed their fall-applied herbicide program," said Purdue University weed scientist Bill Johnson.
Many cover crops are also rapidly nearing their termination dates, Johnson warned. Last year, many growers in Indiana struggled to terminate robust cover crop stands after weather and other priorities kept them out of the field early in the season.
Growers should strive to have their cover crops terminated a full two weeks before planting, Johnson said. For growers in southern Indiana and into the mid-South, that means burndown applications need to occur shortly.
"That [two weeks] gives growers a chance to follow up with another herbicide treatment or tillage if they haven't completely killed it," Johnson said. Last year, some Midwest growers were horrified to discover that their cover crop mixes had been contaminated with glyphosate-resistant canola -- but it was too late to spray.
Killing the cover crop weeks before planting also gives any insects in the field time to move off the dead vegetation and out of the field in search of food, rather than moving immediately to the newly sprouted seedlings, Johnson said.
WEEDS AND RESISTANCE
Marestail will be the biggest winter annual weed to watch this spring for Midwestern growers. Some of the most problematic marestail emerges in the fall and overwinters as small rosettes. If growers don't control it with fall-applied residual herbicides, the weed has an excellent head start on the spring growing season, especially after a mild winter.
Indiana growers can safely assume the majority of their marestail populations are glyphosate-resistant, and about half of the populations in Indiana are resistant to ALS herbicides, too, Johnson said.
"There are also places where 2,4-D, at the one-pint [per acre] rate, is not reliable anymore against marestail" in southern Indiana and much of the mid-South, Johnson added.
Aggressive burndown treatments may be essential this year. Johnson is recommending a "two-shot burndown," for growers who won't be planting until late April or May. The first pass will kill the marestail and other winter annuals that made it through the winter as soon as they emerge and before they get too large for control. The second pass -- preferably with different modes of action -- will ensure a clean field at planting.
Other winter annuals to watch for are henbit, chickweed, pennycress and Shepherd's purse. Some early-emerging summer annuals can also clog fields in the spring, such as giant ragweed, lambsquarter, foxtail and kochia, for western growers. Giant ragweed was already sprouting in Indiana alongside marestail, but the recent cold snap has likely killed those weeds, Johnson noted.
Keep in mind that kochia populations can carry a host of chemical resistance, University of Nebraska Cropping Systems Specialist Rodrigo Werle warned recently in a university newsletter.
"When selecting herbicides for kochia control, keep in mind that kochia populations resistant to Group 2 (ALS-herbicides), Group 4 (2,4-D and dicamba), Group 5 (atrazine), and Group 9 (glyphosate) have been reported in Nebraska," he wrote.
Growers have some new burndown options, as recently registered herbicides marketed alongside new herbicide-tolerant trait technology thatare sold this spring. Dow AgroSciences' Enlist Duo is a premix of glyphosate and 2,4-D choline. Monsanto's XtendiMax, DuPont Pioneer's FeXapan and BASF's Engenia are new formulations of dicamba.
All are labeled for pre-plant applications, but they come with plantback restrictions unless you are planting the corresponding herbicide-tolerant seed. Enlist Duo requires a 30-day plantback restriction for non-Enlist soybeans and a seven-day restriction for non-Enlist corn. Both XtendiMax and FeXapan require growers to wait 14 days for non-Xtend soybeans at the 11 ounces/acre rate and 28 days at the 22 ounces/acre rate. Growers using Engenia as a burndown must wait 14 days before planting non-Xtend soybeans at the 6.4 ounces/acre rate, 21 days at the 9.6 ounces/acre rate and 28 days for 12.8 ounces/acre.
Keep in mind that for the dicamba herbicides -- XtendiMax, FeXapan and Engenia -- the plantback interval doesn't even start until the sprayed field has accumulated at least an inch of rainfall or irrigation. That leaves growers with a "sliding window" that could delay planting even further, Johnson noted. The labels state that growers should not use these dicamba herbicides as a pre-plant application in geographic regions that average fewer than 25 inches of rainfall per year.
Growers planting Enlist and Xtend crops benefit from a tighter planting window. However, University of Illinois weed scientist Aaron Hager still advises farmers wait a few days following burndown applications before injuring weeds with the planting operation.
These new dicamba and 2,4-D herbicides cannot be tank mixed freely -- growers must consult the corresponding tank mix website to find a list of approved adjuvants and herbicides. So far, only Engenia has glyphosate listed as a tank mix option for 2017. "Even with that, there's going to be limited number of glyphosate products," Johnson added. "The challenge will be that if a glyphosate formulation is not labeled for use [with Engenia], that is a finable offense, even though the products may have the same active ingredient."
Enlist Duo's tank mix of glyphosate and 2,4-D combines two modes of action, but keep in mind that against widespread glyphosate-resistant marestail populations, it is essentially a single-mode of action, Johnson added.
For more information on winter annual weeds, see this publication from the University of Nebraska: http://bit.ly/….
For Werle's update on kochia control, see his UNL article here: http://bit.ly/….
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org.
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