Plan B for Dicamba

Spraying Dicamba? Better Have a Back-Up Plan

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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There are a lot of reasons applicators might not get a post-emergence dicamba application done this year, which will complicate control of problem weeds like the waterhemp shown above. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- Bill Johnson can count the days it might be safe to spray dicamba postemergence this year on one hand.

Along with his colleague, Joe Ikley, the Purdue University weed scientist applied the revised 2018 dicamba label restrictions to weather conditions in west-central Indiana in June 2017.

They found that growers had only two eight-hour days to spray dicamba legally. Smaller one- to five-hour windows were sprinkled throughout the month, for a total of just 44 hours when the label technically allowed spraying in June.

There's no reason to think 2018 will be any different, Johnson warned. "We looked at wind speed data for the last five years, and 2017 was not an anomaly," he said.

Between the federal dicamba label, state restrictions and the usual summer weather, Xtend soybean growers need to be prepared for the possibility that they -- or their commercial applicator -- may not get into the field with dicamba this year.

Weed scientists recommend that growers beef up their pre-herbicide program if time allows, study up on local weed resistance and review their limited additional chemical options before summer begins.

OBSTACLES TO SPRAYING

Let's review the many parts on the 2018 federal labels for Engenia, FeXapan and XtendiMax that will limit applicators' opportunities to spray.

-- Wind speeds must be between 3 to 10 mph

-- Wind cannot be blowing toward a sensitive crop

-- No spraying before sunrise or after sunset

-- No rain permitted in the 24-hour forecast

-- No temperature inversion can be underway

-- Spraying must end at soybean's R2 growth stage

-- Weeds should be sprayed at 4 inches or smaller

Don't forget that some states have additional restrictions and cutoff dates, Johnson added. For example, North Dakota limits the dicamba spraying window from one hour after sunrise to one hour before sunset, forbids spraying when air temperatures surpass 85 degrees Fahrenheit and has a cutoff date of June 30. Missouri limits spraying between 7:30 a.m. and 5:30 p.m. and has cutoff dates that vary by county.

See the various state restrictions here:

-- Arkansas: http://www.agriculture.arkansas.gov/…

-- Indiana: https://www.oisc.purdue.edu/…

-- Iowa: https://www.iowaagriculture.gov/…

-- North Dakota: https://www.nd.gov/…

-- Missouri: https://agriculture.mo.gov/…

-- Minnesota: https://www.mda.state.mn.us/…

-- South Dakota: https://sdda.sd.gov/…

-- Tennessee: https://www.tn.gov/…

CHEMICAL OPTIONS

The time has passed for Southern soybean growers to adjust their pre-emergence herbicide program, but some Midwesterners still have time.

"For guys with un-emerged beans, really maximize how much residual herbicide you use, so you do as much damage to the weeds without dicamba as you can," Johnson said.

Once soybeans are up and growing, your postemergence options for control dwindle. Without dicamba, growers will have to treat RR2 Xtend soybeans like RR2 soybeans.

See this article on residual herbicides to consider after soybean emergence from United Soybean: https://unitedsoybean.org/….

"Go back to the old tank-mix partners or [cultivation]," Johnson said. "Do your homework on other things you can add to the tank."

That means knowing what herbicide resistance your problem weeds may carry. For example, in Indiana, most giant ragweed is resistant to glyphosate and ALS herbicides but will still succumb to PPOs. Waterhemp populations, on the other hand, are resistant to one or more of the following: glyphosate, ALS herbicides and PPO herbicides.

Outside of dicamba -- or glufosinate on Liberty Link soybeans -- there really are no choices for postemergence herbicide-resistant waterhemp control in soybeans, Johnson said. "It's either hand-weeding or cultivation."

Southern growers who miss out on dicamba applications will have the most to fear from waterhemp and Palmer amaranth, said University of Kentucky weed scientist Travis Legleiter.

"The majority of our Palmer amaranth is glyphosate resistant, and we have confirmed PPO resistance, too, which is where things do get a little scary," he said. "In waterhemp, we have glyphosate resistance and -- although we haven't officially confirmed PPO resistance yet -- it's probably close."

CHOP OR PULL

If chemical options are out of the question, and your rows are too narrow for cultivating, have a plan to remove weeds manually from your fields, Johnson said.

"If you're doing it in June, you can simply dislodge the weed and lay it on ground, and hopefully it doesn't rain the next day and re-root," he said.

"If you're doing it late in the year with Palmer and waterhemp, you'll have to carry them out of field" to avoid dropping viable seeds, he added.

See more from Indiana on dicamba restrictions and requirements this year here: https://ppp.purdue.edu/…

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/AG)

Emily Unglesbee