Dicamba drift a major herbicide concern

Ray Mueller

Fond du Lac — During 2016, there were 124 documented illegal uses of dicamba herbicide in Missouri, 47 in Tennessee and 28 in Arkansas. Those cases affected 42,000 acres of nine different crops that are vulnerable to the active ingredient in dicamba, which is applied to control weeds in soybeans.

Not only were crops such as corn, alfalfa, peas, cucurbits, grapes, and other vegetables and fruits damaged by the dicamba which drifted during application, but one case on the border of Missouri and Arkansas escalated into one farmer murdering another.

To reduce the possibility of similar incidents in Wisconsin, the state's Extension Service Integrated Pest Management program outreach specialist Dan Heider is sounding the alert to farmers, crop consultants and service providers. One setting in which he did that was at the three area soybean conferences held in January.

Dan Heider

Reading the labels

Heider suspects that the incidents of drift in three south central states in 2016 probably affected 120,000 to 150,000 of crops adjacent to the fields on which dicamba tolerant soybeans and cotton were grown. He pointed out that in Wisconsin the annual count of complaints about the effects of drift from all herbicide products is about 120 to 150 and that 40 to 50 field investigations are carried out.

With the recent registration of new dicamba-based herbicides with the trade names of XtendiMax (with Vapor Grip technology) and Engenia for application on the tolerant Round-Up Ready 2X soybeans to control broadleaf weeds such as water hemp that are resistant to other herbicides, Heider remarked that complying with restrictions has become very complex and carries a load of legalese. He noted some water hemp populations in Illinois have already developed resistance to five different modes of herbicide action.

At a minimum, Heider implores all users of dicamba products to carefully read the labels and follow the instructions precisely. He quipped that his presentations at meetings and through other venues do not substitute for observing what's stated on the product labels.

Application restrictions

The registered post emergent dicamba herbicides are governed by a federal Section 3 plus a supplemental label, Heider pointed out. A new requirement is that any user or applicator must check an information website for any updates within seven days before any application.

With both the XtendiMax and Engenia products, no sprayer tank mixes are allowed, no adjuvants or ammonium salts can be included and no aerial application is permitted, Heider noted. Another factor that could pertain to many applicators is that only a Ttl1104 nozzle can be used.

Although the labels specify that no application is allowed if wind speeds are above 15 miles per hour (mph) at any time or above 10 mph if the wind direction is toward nearby sensitive crops, Heider would limit it to 12 mph at all times. He bases that in part on research conducted at the Extension Service's Arlington farm in 2014.

Temperature inversions

In addition to wind speed, Heider is wary of the temperature inversions which tend to occur both in the early mornings and late evenings. That's because of ease at which the particles can drift under those conditions.

Although there are differences between the two commercial herbicides for identifying a safe buffer zone from sensitive crops, Heider indicated that the distances are 110 to 220 feet. Heeding those limits can be tricky because of the proximity of several different crops, the configurations of and the obstacles on the landscape and the wind direction and speed at the time of application.

In addition to following the rules at the time of application, Heider warned that cleaning of the tank used to make the application is both essential and very difficult. He suggested using ammonia to help with the cleaning.

Upcoming developments

Heider reported that the new 2,4-D product (Enlist Duo), which is far less volatile than the dicamba products, has been approved for weed control in corn for 2017 and will probably be approved for soybeans in 2018.

For the entire lineup of herbicides, Heider pointed out that 138 mixes of products are allowed and 28 different nozzle types are acceptable. With that group of herbicides, the inclusion of adjuvants and ammonium salts is permitted but in all cases there should be no application during winds above 15 mph and a 30-foot buffer zone from other vegetation must be observed.

For additional guidance through the increasingly complex maze of herbicide use, contact Heider at djheider@wisc.edu.