Editor’s note: This is the second story on dicamba-related research undertaken by University of Arkansas weed scientist Jason Norsworthy and colleagues during 2018. The first can found here.
Over the last several years, Xtend crops and new dicamba formulations have been fraught with controversy. Might the strife generated by off-target movement and regulatory edicts be dialed back?
New research is showing hope that may be the case.
“We have conducted low-tunnel trials for several years,” says Jason Norsworthy, University of Arkansas weed scientist. “In those, we spray soil and then place the soil in between rows of soybean or cotton in the tunnels. The products then volatilize and we remove the flats of soil at 48 hours after application and measure the response of soybean, or if it is 2,4-D, cotton to the various treatments.
“We did some work on that this year — some still ongoing. We saw some very exciting things.”
The researchers worked with Engenia plus Roundup PowerMax II in a treatment. “We also sprayed Engenia Pro (Engenia plus Zidua) without the Roundup PowerMax II. What we saw was a substantial reduction in volatility if Roundup PowerMax II (glyphosate) was taken out of the tank.
“Another thing we saw — and are now trying to go back in and repeat — is when we sprayed XtendiMax plus Roundup PowerMax II on soil and compared it back to soil we had sterilized prior to application, there was a reduction in volatility. That tells us it appears there may be something soil-mediated that is also having an impact on the volatility of dicamba.”
The third thing seen in the same trial “is there was an experimental formulation of dicamba that was much, much improved over the current forms of dicamba in the marketplace. There was a tremendous improvement in reducing the volatility in this low-tunnel trial.”
Norsworthy is unable to talk more about the experimental formulation “other than to say I am excited to see positive results and, yes, it was mixed with glyphosate.
“We also did work where we looked at adding Liberty to XtendiMax plus Roundup PowerMax II. We know the addition of Liberty increases the volatility of dicamba. But the real goal was to see if volatility was still occurring four days after application and did spraying Xtend cotton result in an increase in volatility over an application to soil.
“The amount of dicamba movement in the low-tunnels was farther when we sprayed flats of cotton with the three-way combination than when we sprayed only soil, indicating that the plant surface of cotton may be slightly increasing dicamba volatility. Bryan Young, a colleague at Purdue University, has data showing dicamba volatility is greater from leaves of soybean than from soil. There was also work done in the late 1970s that showed the same holds true for dicamba applied to corn leaves versus a soil-applied treatment.”
When the researchers sprayed cotton leaves versus only soil and then placed them under the tunnels four days after application, “injury to soybean was later observed — albeit less than when placed beneath tunnels immediately after spraying.”
Additionally, Tom Barber, Arkansas Extension weed specialist, “led research looking at volatility of Clarity, Engenia, XtendiMax, and Engenia plus Outlook — formulations similar to what we did in 2017. Not surprisingly, he got similar results.”
Barber also had Loyant in the trial. “There were off-target issues that occurred this growing season with Loyant moving off rice fields and damaging soybean, which led to the thought that Loyant may be volatile.”
What Barber saw was that “Loyant applied to soil and placed beneath low-tunnels did not cause leaf-cupping on soybean. That tells us volatility is not an issue with Loyant. The damage seen from Loyant seems to be a result of physical drift — it is not gassing off after application,” says Norsworthy.
Volatility and glyphosate
Another trial Norsworthy conducted looked at the possibility that glyphosate is increasing volatility of dicamba.
“Almost all in-crop dicamba applications are mixed with Roundup (glyphosate) for grass control. If you mix XtendiMax and Roundup, it is only the dicamba killing glyphosate-resistant pigweed. So, if glyphosate is increasing volatility could we replace Roundup with SelectMax, a grass herbicide, and not increase the volatility of dicamba?
“We do not have all the results yet, but what we have seen is when Roundup is added to dicamba, the pH of the spray solution decreases. As pH of the spray solution decreases, the ability of the dicamba acid to form increases, which is known to be highly volatile. We found SelectMax did not impact the pH of the dicamba spray solution while the addition of Roundup PowerMax II did lower the pH.
“We also added Warrant to the XtendiMax and Roundup PowerMax II mixture and saw no further impact on pH, even when the mixture was allowed to sit for four days.”
Those findings, says Norsworthy, “are exciting as there might be some potential to look at XtendiMax and SelectMax as a mixture along with residual herbicides.”
Arkansas researchers also conducted a large field trial where a 2-acre block was sprayed in two 20-acre fields.
“In the first field, XtendiMax, Roundup PowerMax II, Quadris, and Intact were applied. Quadris, a fungicide, was used as a tracer because it is nonvolatile and thus can be used to differentiate suspended spray particles collected by air samplers versus those that volatilize from the XtendiMax portion of the application. There was also a XtendiMax, Quadris, plus Intact treatment — no Roundup.”
Then, three days after application, a 4.02-inch rain fell.
“Dicamba movement in water on the downslope side of both fields caused substantial damage to soybean, even though there was no wind movement in this direction during or soon after application. The dicamba appeared to move in the furrow, similar to the large trial conducted with Monsanto (now Bayer CropScience) earlier in the year at Proctor.
“When the dicamba that moved in the water was excluded and only the remainder of the field evaluated, the Roundup PowerMax II/XtendiMax mixture injured more than twice as much area compared with XtendiMax by itself. So, it circles back to what we saw in the low-tunnel trial: the addition of Roundup PowerMax II to XtendiMax, or Engenia, appears to be increasing off-target movement of dicamba. There is some pretty strong data indicating that.”
In other trials, Norsworthy partnered with Jeremy Ross, Arkansas Extension soybean specialist, to look at 21 soybean varieties — 4.8/4.9 maturity groups — common to the Mid-South. These are non-dicamba soybean with some being LibertyLink, Roundup Ready, and conventionals.
“The varieties were sprayed with a drift rate of dicamba to see if all germplasms would respond similarly to a low dose,” says Norsworthy. “We were surprised and pleased to see there are a couple of varieties with greater tolerance.
“Now, they do not allow for a 1X rate to be applied over-the-top, but they had greater tolerance to a low rate of dicamba. That means there might be some opportunities from a breeding standpoint to minimize injury to non-Xtend soybean caused by dicamba. We will harvest these beans in a few weeks and see if there are any differences in yield loss between those that had elevated tolerance vs. those that did not.”
Arkansas researchers are also involved in a joint project with Tennessee, Missouri, and others across the country looking at the frequency of inversions, especially in areas where injury from dicamba has been common.
“Many weed scientists believe that the occurrence of frequent inversions beginning in late evening in areas where dicamba is heavily used during the summer months may be exacerbating landscape-type damage we see, especially in southeast Missouri, west Tennessee, and northeast Arkansas. I placed weather stations in east Arkansas and will compare those back to ones placed in northwest Arkansas as well as data collected from other states. Mandy Bish from the University of Missouri is currently summarizing most of these data.”
Norsworthy sums up what has been seen in 2018 thus far — “and note we are still collecting data” — like this:
- Loyant damage to soybean does not appear to be a result of volatility.
- The damage to cotton from Enlist One in 2018 was most likely caused physical drift or tank contamination.
- The addition of Roundup PowerMax II to dicamba products increases volatility as well as off-target movement of dicamba.
- Irrigation from tail-water recovery systems — or just irrigation itself moving through a field — can lead to off-target movement of dicamba.
- The research continues to show that dicamba volatility “is a major factor we must deal with, at least during the summer months, to make the technology successful.”
“I was encouraged this year from my research findings in that dicamba may be used with less risk for off-target movement through advancements in application technology such as the See-and-Spray system, by replacing Roundup with SelectMax or possibly other grass herbicides, and through apparent dicamba formulation improvements that further lower volatility of the herbicide.”