No weed scientist can walk out into the field today, look at dicamba injury and tell you what the yield loss will be in that field, says Kevin Bradley, University of Missouri Extension weed scientist. Yet, farmers want to know, so researchers like Bradley are searching for answers.
Yield loss associated with off-target dicamba movement is a hot topic. Some farmers are reporting a reduction in bushels per acre, while others are seeing actual gains. Bradley's team looked at old and new technology to determine how much damage dicamba drift causes in soybean fields.
At MU's Bradford Research Center just east of Columbia, Bradley stands in front of a test plot where soybeans were sprayed with XtendiMax at 1/10, 1/100 and 1/1000 of the label rate of the new chemistry from Monsanto. "What we see happening is more injury expressing itself in the form of cupping at lower rates," he says. There was also significant damage at the higher rates.
The study is being replicated at six universities this year. Once the plots are harvested, Bradley says the universities will analyze the data and see just what, if any, type of yield loss is associated with these new products. "Right now," he adds, "we just don't know. Nobody does."
Graphic courtesy of Kevin Bradley, University of Missouri
In 2016, there were no U.S. EPA-approved dicamba post-application products labeled for use. Still, illegal applications occurred, causing damage to thousands of soybean acres in the Midwest. Farmers wanted answers to growing concerns over potential yield loss.
So Bradley, along with MU graduate student Shea Farrell, analyzed four Missouri fields with dicamba drift injury to determine if there was a loss of bushels per acre.
The two presented results from at just one field where the soybean injury occurred at the beginning of July 2016. A field assessment occurred at the R1 stage.
Using a handled GPS monitor, Farrell walked the soybean field, stopping every 25 meters to evaluate the plants and give each section an injury rating from 0 to 100%, zero meaning there was no injury and 100% meaning terminal injury. He was then able to create a color map to show visual injury in the field — green represents little injury and red represents extensive damage.
At harvest, the farmer provided yield data. Farrell compared the yield data to the visual injury report. Using the historical three-year yield average for this farm of 48 bushels per acre, he determined yields based on percent of injury. Here are the results.
Table courtesy of University of Missouri
It appears that from 0% visual injury to 40% visual injury, the soybeans make better-than-average yields. However, Farrell warns that is not always the case. "When we looked at the data across all four fields, we started seeing a 1% yield loss at 21% to 40% visual injury," he says. "And it increased from there."
According to the study results, the field yielded 3 bushels per acre less than the three-year average, when all other fields that year were above-average.
Other factors to consider
Proximity of a field to the spray event or field topography may affect yield loss.
In the Missouri field example, the greatest injury and loss were along the southern edge. Bradley points out that drift came from a field just to the south, roughly 100 yards away. A tree line and ditch separated the two fields. It did not deter dicamba drift. The greatest yield loss came along the edge, from the field directly adjacent the sprayed field.
However, a few pockets on the northern edge of the field recorded just 1 bushel per acre. "Everybody asks how far way dicamba is to the affected fields," Bradley says. "They think it has to be right next to it. But dicamba is moving miles."
Farmers may also see greater yield loss in low-lying areas or ditches. "These are areas where water pools and the soybeans are more stressed," Farrell says. "Plant stress during a drift event may increase yield loss."
More research needed
There is still more work to be done in determining just how much yield loss farmers can expect if dicamba drift damages their soybean acres.
Farrell continues to look for farmers with drift-damaged fields who are willing to share their yield data to become a part of the MU project. "If you want to see the correlation of yield loss and injury," he adds, "we need more data."
Farmers can contact Farrell at firstname.lastname@example.org.