Soybean producers looking to manage waterhemp and horseweed may be able to add a new weapon to their arsenals, but a University of Illinois weed scientist warns that it may not be available for the full 2016 season.
On Feb. 3, Monsanto announced its commercial launch plans for soybean varieties resistant to the herbicides dicamba and glyphosate (designated Roundup Ready 2 Xtend varieties). Many weed management practitioners hope this new technology package will provide improved control of problematic weed populations, including those with evolved resistance to glyphosate and herbicides from other site-of-action families.
The weed science program at the University of Illinois evaluated the new technology for several years, conducting field research that has improved understanding of the technology and how it might be best utilized in Illinois soybean production systems.
“We believe the technology can be a useful new tool for weed management, but are less confident that soybean farmers can realize its full utility during the 2016 growing season,” says Aaron Hager.
Currently, there are no federal or state labels for any dicamba-containing product that allow applications at soybean planting (preemergence) or after the soybean crop has emerged (postemergence). Without approved labels, applying a dicamba-containing product to these soybean varieties would constitute a violation of both state and federal laws.
Hager notes, “We’re not sure whether approved federal and state labels will be granted in time to allow application of dicamba-containing products on these varieties during the early portion of, or perhaps even much of, the 2016 growing season.”
Some have posed the question of whether or not dicamba can be applied prior to planting dicamba-resistant soybean varieties.
“The answer is ‘yes,’” says Hager, “but remember, this type of application must follow the herbicide label guidelines regardless of the soybean variety planted.”
For example, dicamba-based Clarity can be applied to control existing vegetation prior to planting, but one inch of precipitation must accumulate after application; once precipitation occurs, a waiting interval of 14 to 28 days – depending on the concentration applied – is then required before planting can occur. This use pattern is governed by the herbicide label, not by the soybean variety planted.
Herbicide-resistant weed populations continue to be a common occurrence across most areas of Illinois. Waterhemp and horseweed (marestail) are the two most common glyphosate-resistant weed species in Illinois, and observations during 2015 suggest these species are likely to remain prevalent in 2016.
“There are no documented cases of dicamba-resistant waterhemp or horseweed populations in Illinois,” Hager notes.
Soybean producers planning to rely on dicamba and dicamba-resistant soybean to control glyphosate-resistant waterhemp in their 2016 weed management programs are encouraged to consider utilizing alternative strategies. Alternative strategies to manage weed populations with resistance to multiple soybean herbicides include rotating fields to a different crop, or planting soybean varieties resistant to glufosinate (i.e., Liberty Link) and utilizing glufosinate as a postemergence herbicide.
“Please keep in mind, however, that regardless of the crop planted, the variety selected, or the herbicide applied, the most sustainable solution to the challenges of herbicide-resistant weeds is an integrated weed management system that utilizes both chemical and non-chemical tactics to eliminate weed seed production throughout the growing season,” says Hager.
Source: University of Illinois
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