Weed management issues were evident in 2017 across Iowa. More so than the year before, weeds were poking above the soybean canopy at harvest. Untimely spring rains prevented some farmers from getting preplant residual herbicides applied. Others skipped burndown treatments and planted instead. By the time they could spray postemergence, the weeds were too big. Weed escapes left a lot of weed seed out there for 2018.

Adding to the number of escapes, weed populations with evolved resistance to herbicides continued to escalate statewide. And populations of a relatively new weed in Iowa, Palmer amaranth, were identified in new counties. Palmer populations likely will eventually be found in all Iowa counties.

“Weed management remains a major concern for Iowa agriculture and addressing these burgeoning problems requires greater diversity of tactics beyond herbicides,” says Mike Owen, Iowa State University Extension weed management specialist.

Controlling herbicide-resistant weeds
Most herbicide labels now include sections describing management of herbicide-resistant (HR) weeds. They describe various best management practices important for diversifying weed management programs. Typically, there is a statement that suspected HR weed populations should be reported to the company for investigation. “Hopefully,” says Owen, “the BMPs as suggested on the labels will gain traction, and more growers will adopt more diverse tactics to manage weeds.”

Farmers are hoping new herbicides will be developed to control these problem weeds. For 2018, there are no new herbicides with novel mechanisms of action being introduced and none are anticipated to come on the market in the near future, he says.

The introduction of new soybean varieties and corn hybrids containing HR traits represents a continuation of herbicide-based weed management. Eventually, weed resistance to the herbicide will evolve if it is used continuously on the same field. “That’s not saying we don’t need new herbicides with new modes of action,” Owen says. “We need them. But they must be managed and rotated with other herbicides that have different modes of action to avoid resistant weeds evolving.”

ISU dicamba recommendations for 2018
2017 was the first year the new dicamba herbicide formulations for soybeans were used in Iowa, and there were plenty of problems with drift onto non-tolerant soybean varieties in nearby fields.

For use on dicamba-tolerant soybeans, the new dicamba formulations provided good weed control in 2017. “But they are a challenge for managing off-target dicamba movement,” Owen says. “Given the problems with dicamba movement attributable to particle drift during application and movement after application from volatilization, it is clear the adoption of the dicamba-based technology has greater risk than past herbicide technologies.”

ISU Extension weed scientists are recommending only preemergence applications of dicamba on Roundup Ready 2 Xtend soybean fields in 2018 after widespread reports of off-target damage occurred in 2017. “We are fully aware that this very much limits the use of the technology,” Owen says. “But especially for postemergence application, we feel the benefits do not outweigh the risks.”

Applying dicamba postemerge too risky
Off-target movement of dicamba is complex and involves a number of factors, some that can be addressed with better application techniques. Nozzle type, boom height, application speed, wind speed and direction can be addressed by applicators.

However, other factors cannot be addressed by applicators, such as the inherent chemical characteristics of dicamba, high sensitivity of susceptible soybean varieties and other non-target plants, and effects of rain, temperature, relative humidity and inversions, and not just the day of application but for several days following application. This increases the risk of adopting the dicamba-based technology, notes Owen.

Off-target dicamba damage in 2017 has been attributed to a number of reasons, including particle drift, contaminated sprayer tanks, volatilization and spraying during temperature inversions. “The new formulations of dicamba don’t appear to have solved the age-old problem of dicamba volatility,” says Bob Hartzler, Owen’s ISU colleague. There have been documented cases of dicamba injury more than a mile from where the herbicide was sprayed.

EPA issues additional label requirements
The three new low-volatility dicamba formulations are Monsanto’s Xtendimax with VaporGrip technology, BASF’s Engenia, and FeXapan by DuPont. The Environmental Protection Agency has added additional requirements on the labels of these products for use beginning in 2018, to try to address the widespread drift issues.

The label changes include classifying these herbicides as “restricted-use products.”

“Beginning in 2018 only certified applicators with special training to better apply dicamba products are allowed to apply them,” Owen says. “Dicamba-specific training is required. Applicators are required to keep extensive records for two years, and these records must be made available to the Iowa Department of Agriculture, USDA and EPA upon request. Applicators must also keep the receipts for dicamba purchases.”

Application parameters have also been modified. Applications can be made in daytime only, from sunrise to sunset, and wind speed during application is restricted to 3 to 10 mph. Label language regarding sprayer cleanout and proximity of susceptible crops to dicamba-treated fields has been expanded. “We feel these label changes are appropriate, but we are concerned they do not address the issue of off-target movement due to volatilization,” Owen says. He advises reading the dicamba product labels for specific information about the changes.

Managing herbicide-resistant weeds
The need for different technologies to address the increasing problem of HR weeds must be considered in relation to the risks associated with the technology. If you are committed to planting dicamba-tolerant beans, keep in mind that preemergence applications of dicamba with dicamba-resistant soybean varieties represent the least-risky strategy, Owen says.

Early post-applications in May, when temperatures are typically relatively cooler, have greater risk than soil applications. The greatest risk from dicamba-based weed management is with postemergence application in June and later. “The greater risk of off-target movement is why we don’t recommend using dicamba-based weed management for post application,” Hartzler adds.

Regardless of pending changes in herbicides and crop traits, the ISU specialists emphasize that weed management diversification beyond herbicides must be considered in order to support the tools currently available to farmers. Owen sums it up: “Iowa agriculture will not be able to resolve weed management issues by simply spraying herbicides.”

For more information, download the 2018 Herbicide Guide for Iowa Corn and Soybean Production.