New 2,4-D and Dicamba Data

ROCKVILLE, Md. (DTN) -- University of Missouri Extension weed scientist Mandy Bish unveiled the results of the group's latest research on new auxin herbicide technologies at the annual MU Crop Management Conference, held virtually Dec. 1-2.

MU weed scientists, who led the charge on early investigations of dicamba volatility and temperature inversions, have now turned some of their attention to 2,4-D-choline as its use on 2,4-tolerant Enlist crops ramps up.

Here are four big takeaways for farmers to keep in mind for the 2021 spray season:

1. DICAMBA AND 2,4-D HAVE BEEN FOUND IN RAINWATER

One study presented by the MU team involved collecting rainwater weekly from 12 sites around the state in 2019 and testing it for dicamba and 2,4-D. The preliminary results show that dicamba was detectable in rainwater throughout the season at some sites. In the sites located in the southern corner of Missouri, known as the Bootheel, the dicamba amounts in the rain were high enough to injure sensitive crops, especially with multiple exposures.

The results directly correlated with adoption rates of Xtend crops, Bish explained. Where adoption was higher, dicamba was more frequently detected in the rainwater, and at higher rates. Data from nearby weather stations also indicated that lack of wind and inversion conditions likely influenced dicamba's presence in rainwater.

"It doesn't paint a good picture of dicamba technology's success in the Bootheel, even with label changes," Bish said.

The researchers also detected 2,4-D in some of the rainwater samples, despite little to no Enlist acreage in the state that year, and Bish warned that could increase as Enlist acreage expands.

"We do detect more dicamba right now, but I anticipate that, going forward, 2,4-D amounts in rainfall will increase in the Bootheel," she noted. "Of course, it takes a lot more 2,4-D to injure soybeans, but that doesn't bode well for tomatoes or grapes or other sensitive crops in the area."

2. ENLIST PRODUCTS ARE LESS VOLATILE BUT STILL PRONE TO PHYSICAL DRIFT

Along with weed scientists from other universities, the MU researchers did multiple studies assessing the volatility of the over-the-top dicamba formulations (XtendiMax and Engenia) as well as 2,4-choline (Enlist One) and 2,4-D-choline and glyphosate premix (Enlist Duo). The studies will be repeated in 2021, as well.

MU results showed that, in field low tunnel studies, the dicamba formulations were more volatile once glyphosate was added to the tank, a common finding among weed scientists in recent years. As a result, some visual injury symptoms to surrounding soybeans were still visible at 28 days after application. (Keep in mind that these studies used the 2018 versions of the dicamba herbicides; the 2020 registrations require new volatility-reducing agents to be used with XtendiMax and Engenia).

While some volatility of Enlist herbicides was observed 14 days after application, by 28 days after application, no visual injury symptoms were visible on surrounding cotton plants from any of the Enlist spray mixtures.

The MU team also tested Enlist Duo and XtendiMax in larger field studies, sprayed on the same day and in the same field. Fourteen and 28 days after the application, the researchers could find no visual injury symptoms on the soybeans surrounding the Enlist application, but dicamba injury symptoms to surrounding soybeans radiated from the Xtend field in all four directions, regardless of wind direction.

"It does seem that Enlist One and Enlist Duo are less volatile than dicamba products," Bish said. "But 2,4-D can still be moved in the air, and it is still susceptible to temperature inversions -- I want to remind people that those risks do not go away with reduced volatility."

3. LIBERTY AND DICAMBA ARE NOT A GOOD MIX

With three-way-tolerant XtendFlex soybeans ready for a commercial launch in 2021, the MU team added another ingredient to their low tunnel volatility study -- Liberty (glufosinate).

The results were not good. Mixing glufosinate with Engenia or XtendiMax caused significant volatility, with the tank mixes producing visual injury symptoms on 30% to 40% of the surrounding soybeans nearly a full month after application.

Bayer and BASF are still working to put out new tank-mix-approved products for XtendiMax and Engenia for 2021, but in the past, glufosinate has not been an approved partner for either.

4. WINDLESS DAYS ARE A CONCERN FOR BOTH 2,4-D AND DICAMBA

Wind and its ability to contribute to physical drift of herbicides is well known, but Bish is more worried about clear, windless days for auxin herbicide applications. "Some wind is needed to disperse herbicide particles, which remain in the air following applications," Bish told DTN. "If there is no wind, herbicide particles can get trapped in stable air."

One common cause of stable air masses is temperature inversions -- the atmospheric phenomenon where a mass of cool air gets trapped near the ground and suspends air particles, whether they be dust, water vapor -- or pesticides. "We know from our previous work that more dicamba gets suspended in the air when it is applied during an inversion, and we have no reason to believe 2,4-D choline would be any different," Bish warned.

Any herbicide droplets or volatilized particles that get caught in inversions can easily move off target when wind finally arrives to end the inversion. But even without wind, they can slowly seep into low-lying parts of a field, as the cold air mass seeks the lowest point around, a type of air movement known as "cool air drainage," Bish said.

In an effort to limit this risk, current dicamba labels ban spraying when winds are below 3 mph and for one hour after sunrise and two hours before sunset. Enlist product labels caution against spraying in an inversion but have no such restrictions.

MU scientists' past analysis of weather stations from seven states shows that inversions occur frequently in the afternoon and evening hours of May, June and July. Newer data from Tennessee and Missouri show that those inversions frequently form earlier than two hours before sunset -- the current cutoff for applications on the 2020 dicamba labels.

"Sunset is not a good predictor in every location," Bish said. "I'm still in favor of applicators using other known indicators of an inversion." Those include clear skies, lack of wind, and odors and noises traveling long distances. See more on inversions here: https://www.dtnpf.com/….

"We know from our past research that geographies with less wind have more off-target dicamba movement issues," Bish said. "And when you also have high adoption in these geographies with less wind, you have even more issues."

The off-target movement of 2,4-D wasn't as prominent in Missouri as dicamba in recent years, but with the state's inversion frequencies and increasing Enlist acreage, Bish believes that could change. "As higher Enlist adoption occurs and more product is sprayed, stable air could become an issue for it, too," she warned.

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

Follow her on Twitter @Emily_Unglesbee

(c) Copyright 2020 DTN, LLC. All rights reserved.