At an Arkansas Plant Board meeting in late September, board members heard the current findings of multiple research projects looking at dicamba use in locations throughout the state. The gathered data from numerous studies were explained by Jason Norsworthy, University of Arkansas weed scientist, in a presentation that lasted well over one and a half hours.
The main takeaway: several studies show great potential for those interested in using the dicamba technology in the future.
And that isn’t all that’s happening on the dicamba front.
A big decision regarding dicamba was expected as the 2018 growing season morphed into harvest season. Now, well towards the tail-end of cutting, the EPA’s call on whether to approve the label for new dicamba formulations still hasn’t been announced.
“I don’t know when the EPA will make an announcement on renewal of the dicamba registrations, but I hope it is soon,” says Norsworthy. “I initially thought it would have been late summer, but August and September have come and gone — and folks nationwide had the expectation during that period that they would have some news to help guide their 2019 planting decisions.
“I do know that there is a wealth of data the EPA is assessing at this time, which include studies from Arkansas as well as other locations across the soybean production regions of the United States.
“The volatility results from previous years were again observed in 2018; however, we have had some major breakthroughs this year that are encouraging and may help us reduce the occurrence of dicamba off-target movement and that of other auxin herbicides. By better understanding all factors that contribute to dicamba volatility, we are able to minimize risk associated with this herbicide.”
Norsworthy’s presentation — titled “Learnings from 2018 on off-target movement of auxin herbicides” — “was not specific to dicamba, although dicamba was the majority of the conversation because more issues are associated with off-target movement of dicamba in Arkansas than any other herbicide in the state. Other auxins that caused some issues in 2018 included the new rice herbicide Loyant and Enlist Duo and Enlist One (2,4-D choline) applications to Enlist cotton.”
The first study mentioned was on “the use of Enlist One plus Liberty as a possible tank-mix in cotton. This study was conducted at the Lon Mann Cotton Research Station in Marianna in a 10-acre field in collaboration with Tom Barber (Arkansas Extension weed specialist)”. The two “looked at a labeled application of these two herbicides, using XtendFlex cotton (non-Enlist cotton) as an bioindicator of 2,4-D off-target movement.
“Of course, we knew Enlist One would kill XtendFlex cotton in the center of the field where one acre was treated with the mixture. We made the application on Aug. 6 to seven- to eight-leaf cotton.”
They then monitored the environmental conditions during and after application and later placed cotton plants from the greenhouse in the center of the field. That was done at half-an-hour, 24 hours, and 48 hours after application.
“We then removed those plants and brought them back to the greenhouse at 72 hours after application and looked for symptomology on these plants 14, 21, and 28 days after application.
“If Enlist One was volatilizing we would expect to see 2,4-D symptomology on cotton plants placed in the treated area similar to what we see when non-Xtend soybean is placed in dicamba-treated fields following treatment. We saw no injury symptoms on cotton on those plants later placed in the field.”
There were also air samplers placed in the field — “but we do not have the samples analyzed at this point.
“When we looked at the off-target movement of Enlist One, it was all in one direction. The 2,4-D moved only in the direction the wind was blowing at the time of application. That is a good indication that you are not dealing with volatility. If you have a volatile product, as wind shifts after application, you likewise have symptoms whichever way the wind is blowing.”
Another key component of the trial was covering non-Enlist plants in the downwind direction prior to application. “We kept those plants covered for 30 minutes after application. We had little to no symptomology on these covered plants.
“The interesting part of that was the plants right adjacent to covered ones had 70 to 75 percent injury. That cotton was almost dead from physical drift of 2,4-D.
“So, the conclusion from that trial is it appears any issues we have associated with Enlist One are likely not a function of volatility. They are most likely a function of physical drift, wind shifts during application, or tank contamination. But again for the problems in 2018, volatility was unlikely the cause.”
Norsworthy’s first field trial of the year was planned for northeast Arkansas to look at the ‘Blue River’ See-and-Spray spray system, which John Deere now owns. The See-and-Spray technology only sprays emerged pigweed and other weeds emerged in a crop. Most of the work to date has been in cotton.
“The trial was intended to go out the second week of June. Our cotton and soybean were planted and we were ready to go. But on June 11, I arrived in Keiser and found dicamba symptoms on soybean and 2,4-D symptoms on cotton.”
The trial was abandoned until later in the year. At that point, “we looked at the See-and-Spray with a hood and compared it to a broadcast application using a stand open boom. Basically, we had very little to no off-target movement using the See-and-Spray system. The area damaged from dicamba applied using the open boom was more than seven times larger than that from the See-and-Spray system.”
Norsworthy believes there are a couple reasons for the reduced off-target movement. “First, the technology utilizes an aggressive hood that minimizes physical drift. In addition, only the weeds are sprayed. By doing that, we were able to reduce the amount of dicamba applied to the field by better than 95 percent. By reducing applied dicamba you also lessen volatilization and physical drift.
“The See-and-Spray technology will not be commercially available in 2019, but I am encouraged by the progress made recently. The company has made great strides in improving the ability of the sprayer to recognize weeds since its first testing in Arkansas in May 2017. I do believe this technology, when combined with other strategies, does possibly provide lower risk for off-target movement in the near future.”
Does the 95 percent reduction hold with other herbicides?
“Yes. You are only spraying the emerged weeds versus making an application across 100 percent of the field. You could use this technology in any wide-row crop for any postemergence herbicide such as glufosinate, paraquat, glyphosate, 2,4-D, etc. The percentage reduction in herbicide use for a field will be solely dependent upon the weed density in that field. The more weeds you have, the more herbicide will be applied to the field.
“With the See-and-Spray technology, we’d not only be looking at minimizing off-target movement but evaluating the environmental ramifications. We would be reducing the herbicide load on the environment.
“Also, from a cost standpoint, you could be reducing 95 percent of the product you normally put out. In weedier fields, you might end up using 90 percent of what you use without the system. In turn, your purchases of herbicide could be seriously curtailed.”
One current drawback is that the system is approximately 40 feet wide. “Most typical ground applicators are covering 100 feet or more with each pass of the sprayer. So, yes it is a smaller system, but it is still extremely exciting in terms of potential.”
Large field trial
Another trial conducted was in a 240-acre commercial soybean field — “one of the largest field trials I have ever conducted and likely one of the largest in the country on off-target movement of dicamba. I was fortunate to be able to work with some outstanding grower cooperators near Proctor. In the center of the 240-acre field, the grower planted 38.5 acres of Xtend soybean. He then surrounded the Xtend soybean with approximately 200 acres of Roundup Ready soybean.”
On July 9, “we worked with Monsanto (now Bayer CropScience) to set up air samplers in the field to measure how much dicamba would be coming off the treated portion of the field. The air sampler data are still being analyzed. Along with air sampling, the environmental conditions during and following application were monitored. These included wind speed and direction at several heights and air temperature.
“Unfortunately, on July 9, we did not have sufficient wind speeds to spray based on the federal label. My crew and several Monsanto employees stayed at the field for an additional six days waiting for conditions to be conducive for spraying. By July 15, both the Xtend and Roundup Ready soybean began to flower.”
That’s why Norsworthy decided “to make the application of Xtendimax plus Roundup PowerMax II plus Warrant plus Intact (drift retardant) on July 16 at 2:58 p.m. — otherwise the 240-acre study would have been lost.”
When the field was sprayed “there was an average wind speed of 2.8 miles per hour. Immediately prior to spraying the 38.5 acres in the center of the field, we covered Roundup Ready soybean on all sides of Xtend soybean with 12 feet wide by 25 feet long tarps adjacent to the treated area.”
Those tarps were put out with the idea that anything covered should have no damage from physical drift, and instead give an indication of contribution of volatility or secondary movement to injury to Roundup Ready soybean. “If most of the movement is due to physical drift and not volatility, you would expect the tarps to protect the covered plants; hence, the covered plants would appear healthy with little to no symptoms.
“I also covered plants during application and up to 30 minutes following treatment along three transects every 50 feet out to the edge of the field in the downwind direction — about 850 feet. That was to further assess volatility.”
So, what happened?
“Two weeks after treatment, dicamba symptomology was observed on all four sides of the field under the tarps and the extent of injury was comparable with that outside tarps. What that indicated is dicamba movement occurred in all four directions. Secondly, volatility or secondary movement was a major contributor to the injury observed in the field.
“A similar response was observed every 50 feet along each transect all the way to the edge of the field in the downwind direction. The extent of dicamba-induced leaf-cupping along these transects was comparable to the leaf-cupping of adjacent non-covered plants.
“Plant residue on the edge of the field was also burned during application to document that an inversion was not occurring. Additionally, a nearby weather station was used to verify the absence of an inversion based on air temperature at 18- and 120-inch heights at application. I can say with confidence that there was no inversion present during or soon after the XtendiMax application.”
That isn’t all the research found. In addition to seeing dicamba symptoms on all sides of the field, “we were also surprised to see increased injury on the side of the field where irrigation water moved from Xtend to the non-Xtend portion of the field. The first irrigation event occurred 9 to 10 days after application. So, the irrigation event itself appears to have moved dicamba and caused increased injury — something observed by myself and several other colleagues including industry representatives.
“The take-home for that trial: after application, within 24 hours, we had wind blowing in all four directions. We then had dicamba symptomology on Roundup Ready soybean located on all four sides of the treated field. Based on what we saw on soybean plants beneath and outside of coverings, volatility was a major contributor to the injury and for at least one side of the field, water from irrigation — or possibly rainfall from a later date — led to increased injury to Roundup Ready soybean.”
What does that potentially mean for management?
“Those wanting to use re-lift or tailwater recovery systems — say you are pumping out of a ditch or canal — might want to consider if that water came off a dicamba-treated soybean field if the water is to be placed on a non-Xtend field. If contaminated water is applied, it is quite likely you could see dicamba symptoms on soybean.”
Cammy Willett, an environmental weed scientist with the University of Arkansas, “conducted some research this year looking at different concentrations of dicamba in irrigation events, simulating contaminated recovered tail-water. At several concentrations, dicamba symptoms were observed on soybean when an irrigation event occurred at either V3 or R1 growth stage of soybean. As expected, the extent of injury was dependent upon the concentration in the irrigation water and a function of the length of the irrigation event.”