Was dicamba spray window too small in 2018?

FARGO, N.D. -- It will likely be early July before farmers and crop experts see whether the 2018 dicamba spray system work without off-target impacts seen in 2017.

Gared Shaffer, field extension weeds field specialist at the South Dakota State University Extension, urges farmers to follow SDSU recommendations not to spray dicamba post-emergence in the last week of June. Photo taken Feb. 27, 2018, at Aberdeen, S.D. (Forum News Service/Agweek/Mikkel Pates)

FARGO, N.D. - It will likely be early July before farmers and crop experts see whether the 2018 dicamba spray system work without off-target impacts seen in 2017.

"It's an open question, what the results might be," said Andrew Thostenson, North Dakota State University Extension pesticide program specialist. North Dakota farmers can spray through June 30. South Dakota farmers don't have a deadline. Farmers in Minnesota crossed the finish line for their state deadline on June 20 - a new deadline in that state.

Those specific dates are different than federal guidelines, which prevent spraying after the R1, or first reproductive phase.

NDSU weed experts had advised North Dakota Agriculture Commissioner Doug Goehring to impose the June 20 cutoff to protect neighboring farm crops from getting hit by dicamba drift. Non-dicamba-resistant soybeans are susceptible to so-called "direct" spray drift, but also from the "volatilization" as dicamba molecules can vaporize and move downrange in light or variable winds, for a period up for up to two to three days after the application.

Thostenson said farmers would have been "very, very, very hard-pressed" to have met a June 20 cutoff in North Dakota this year, had it been imposed.


Tri-state slow

Gared Shaffer, weeds field specialist at South Dakota State University Extension, said a lot of intended dicamba applications after the crop emerge in South Dakota had not happened as of June 20. This was especially true in the southeast, where crops still aren't planted because of excess rain.

SDSU Extension had recommended a June 21 cutoff, but the South Dakota agriculture secretary did not a set a deadline.

Shaffer said some used it pre-emergent, which appears to have worked well. Most of the beans in the north are only in the second or third "trifoliolate" stage, or "true leaf." This late bean development will likely mean farmers will apply the chemical in the last week of June or in the first week of July, which can cause off-target movement due to volatilization.

Jeff Gunsolus, a University of Minnesota Extension weed specialist in St. Paul, said that state's June 20 deadline was imposed because 75 percent of damage reports in 2017 occurred in applications after that date, and 50 percent occurred after July 1 applications.

Gunsolus said farmers in the Interstate 90 corridor from Worthington, Minn., to Rochester, Minn., have seen late planting conditions and untimely rains and weather that would prevent the use of dicamba before the June 20 deadline. Conditions farther north in Minnesota have been more favorable but there are "pockets" where it has been prevented, mostly due to late planting dates.

"It's still very overcast and drizzly at Worthington," Gunsolus said June 20, adding that many fields there were too wet to apply it even if the weather was fit.

Liz Stahl, a regional extension educator in crops at Worthington, Minn., says the federal guidelines allow dicamba spraying on soybeans only until the so-called R2, or second reproductive phase. Technically, this is when an open flower is at one of the two uppermost nodes on the main stem that has a fully-developed leaf. With varying planting dates and development stages, however, Minnesota set the June 20 deadline because that's when most of the complaint started that year. It also makes it simpler. "People don't have to figure out the exact stage of their crops," or the neighbor's crop which may be different and susceptible.


Pressed for time

Thostenson noted that North Dakota farmers saw a very late spring, which meant corn, soybeans and even wheat and sugar beets all were planted in a compressed planting schedule. In turn, this means the spray treatments also had to go on in a compressed schedule. He said just because farmers can spray until June 30 in North Dakota, doesn't mean "it's just fine" that they do. "We're still concerned about higher temperatures," he said.

Forecasted temperatures of highs in the 70s and lower 80s through end of June may bode well for safe applications.

North Dakota farms saw their third-warmest May on record, which means crops "jumped out of the ground relatively quickly," Thostenson said. Farmers contended with new wind speed and time frame requirements. Wind had to be between 3 mph and 10 mph. North Dakota added spray times limited to one hour after sunrise and one hour before sunset, with prohibitions against temperature inversions, where the ground temperatures are colder than the temperatures at 10 feet.

Gunsolus said the new dicamba formulations are best used on small weeds - 4 inches or less, as an "early post-emergent" technology. It is prone to off-target movement into July as a rescue treatment for large weeds.

5-7 days later

Last year, some soybeans not resistant to dicamba in North Dakota started experiencing "leaf-cupping" damage from July 4 to July 6, Thostenson said. The damage reports escalated July 10 to July 15. Companies that make the formulations were very active in a stepped-up education program to avoid problems.

"I anticipate that if we have problems this year it's going to be five to seven days later than last year," Thostenson said.


One of the factors in the situation is that the dicamba-resistant soybean seed technology may have captured an estimated 45 to 55 percent of the soybean acres in North Dakota - up from only 20 to 30 percent a year ago at this time, Thostenson said, citing reports from seed dealers.

Gunsolus said he doesn't know percentages of dicamba-ready soybean varieties planted in either 2017 or 2018, or by region.

Both Gunsolus and Thostenson said they've heard some farmers planting the new varieties, available from three companies, as a defensive measure - not intending to use the herbicide. Gunsolus said some bought the varieties because dicamba is effective with certain herbicide-resistant or otherwise difficult weeds. Waterhemp, giant ragweed and common ragweed are the usual targets.

Thostenson understands that intent, but thinks farmers are unlikely to "leave that tool in the toolbox" and not spray dicamba if they see a flush of waterhemp or common ragweed coming, and they haven't seen control from their pre-emergence or post-emergence herbicides.

NDSU crop experts generally are apprehensive about seeing off-target herbicide issues, including non-dicamba problems. There also has been some drift of mesotrione herbicide from corn into sugar beets.

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