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Research Highlights

Research Highlights
Ready 2 XtendFlex trials successful

By Kentucky Soybean Board

The University of Kentucky Weed Science Program has been evaluating the utility of the Roundup Ready 2 XtendFlex soybean on Kentucky acres infested with Palmer amaranth and waterhemp. The project, funded by the Kentucky Soybean Promotion Board, was conducted during the summer of 2018 in three locations and is continuing into a second year in 2019.

The Roundup Ready 2 XtendFlex soybean includes tolerances to glyphosate, dicamba, and glufosinate. The addition of glufos­inate to the tolerances in this soybean inherently gives farmers and applicators more flexibility in making postemergence appli­cations. This is especially important for those using the soybeans on acres with either Amaranthus species, as glyphosate resis­tance is widespread in both species in Kentucky. The Roundup Ready 2 Xtend soybean has already brought some relief to farm­ers who are dealing with these species over the last two years, but has brought another challenge in applying dicamba later in the season under extremely rigid, if not impossible, restrictions.

Even with these restrictions, off-site movement of dicamba has continued to occur and has been a major issue for Kentucky farmers. The large acreage of tobacco in the state also brings concerns of movement of dicamba from Roundup Ready 2 Xtend soybean fields during and after dicamba applications. With the addition of glufosinate, the XtendFlex soybean brings an addi­tional postemergence option to farmers and applicators to con­trol Amaranths while also allowing less restrictive applications and a safer alternative for neighboring fields with tobacco or other sensitive crops. As with all herbicide applications, caution should always be taken in mitigating any off-target movement.

While the addition of glufosinate brings a lot of utility to these soybean, UK weed scientists were interested in the best systems approach to using the technology. Field experiments in 2018 evaluated a systems approach, in which every treatment re­ceived a preemergence herbicide followed by one- or two- pass postemergence applications of dicamba or glufosinate (or com­bination) with and without layby residuals. The preemergence herbicide treatments included Zidua, Fierce XLT, and Intimidator; each representing a different number of effective SOA (sites of action) at one, two, and three, respectively. Evaluations of weed densities just prior to trial destruction in mid-July of 2018 re­vealed that all treatments were successful and reduced water­hemp and Palmer populations by at least 97% as compared to an untreated control.

Furthermore, no statistical differences were observed between treatments. Evaluations of the entire systems approach also re­vealed a lack of difference between a PRE followed by POST, PRE followed by 2x POST, or PRE followed by POST with a residual.

Despite a lack of differences in weed population reduction across treatments or systems, further analysis from the 2018 season did reveal one area where differences did occur. That area was the preemergence herbicide applied and number of effective sites of action. Plots receiving an application of Intimidator or a product with three effective sites of action had significantly less waterhemp and Palmer at the end of the trial than plots re­ceiving the single site of action preemergence herbicide across all three sites, regardless of what postemergence herbicide was applied to the plots. This finding further emphasized the need for robust preemergence herbicides for control of Amaranthus species, even with the expanded postemergence options these soybeans offer.

Similar experiments are ongoing in 2019 again evaluating the Roundup Ready 2 XtendFlex soybean system, although this year a series of treatment without a preemergence herbicide is being added for further comparison against the single, double, and tri­ple SOA preemergence herbicides.

The Roundup Ready 2 XtendFlex soybean will not be the only additional herbicide tolerant soybean available to farmers in coming years, as the Enlist E3 soybean and LLGT27 soybean are already available to farmers this year. All of these herbicide tol­erant soybean packages offer expanded postemergence options for control of herbicide resistant weeds and arguably bring great value to farmers dealing with these weeds.

Farmers must stay vigilant in their fight against Amaranthus species and not depend on postemergence herbicides only, as this only selects for further herbicide resistance. This research only further supports that message that we must continue to use

robust multiple site of action pre’s, even with these new herbi­cide tolerant soybeans.

University of Kentucky Weed Science also stresses that other management strategies such as hand-roguing of escapes at the end of the season are still essential for Palmer amaranth and waterhemp control. The initial research showed that although every treatment used on the XtendFlex soybean did reduce the population by at least 97%, the remaining 3% is still capable of producing seed and thus should be managed through tactics such  as hand-rouging.  

To find research related to this Research Highlight, please visit the National Soybean Checkoff Research Database.