Achieving effective weed control in soybean cropping systems has become a challenge for farmers dealing with weeds resistant to commonly applied POST-emergence herbicides. As a result, glufosinate (e.g., Liberty) has shown potential to play an important role in POST broadleaf weed control in glufosinate-resistant soybean systems (e.g., XtendFlex, Enlist E3). However, the efficacy of glufosinate applied alone can be impacted by several factors such as weed size, application methods, and the environmental conditions during application (i.e., time of the day, temperature, and relative humidity) (Figure 1). For additional information about optimizing glufosinate applications, please check the following video: Getting the Most Out of Glufosinate.
In some of our previous experiments and according to recent results reported in the weed science literature (please see Takano et al 2020), PPO-inhibitor herbicides (Group 14) were presented as good candidates for tank-mix partners with glufosinate, as together they helped enhance waterhemp and Palmer amaranth control. Moreover, having more than one herbicide site-of-action in the tank brings value to delaying herbicide resistance evolution.
This blog post uncovers the results of two field experiments conducted in 2020 and 2021 in southern Wisconsin (2 sites) and western Illinois (1 site) evaluating the influence of glufosinate tank-mix combinations with PPO-inhibitors (flumiclorac, fluthiacet-methyl, fomesafen, and lactofen; Group 14), and also with other herbicides (2,4-D choline, Group 4; bentazon, Group 6) on waterhemp control (study #1) and on soybean development and yield (study #2). Herbicide treatment information is presented in Table 1.
PRE herbicides were applied in both experiments (except in nontreated control plots) within 3 days of soybean planting. For the waterhemp response study #1, Valor (flumioxazin) was applied at 2 oz/a, whereas in the soybean response study #2, Fierce (flumioxazin +pyroxasulfone) was applied at 3 oz/a. All POST applications were applied between V4-V6 soybean growth stage, depending on experimental site-year. Different PRE herbicides were chosen for each study due to the different objectives; soybean response study #2 needed a stronger PRE to help maintain the plots weed-free whereas for study #1 we wanted waterhemp to establish to evaluate its control with our POST-emergence treatments.
Study #1: Waterhemp Control
Fourteen days after POST application (DAT) a visual assessment of waterhemp control was taken. The results from 6 experimental site-years (3 sites x 2 growing seasons) indicated that waterhemp control 14 DAT was generally enhanced when glufosinate was tank mixed with flumiclorac, fomesafen, lactofen, bentazon, and 2,4-D choline. The enhancement of waterhemp control seems appealing, but the important aspect of this and the main question we have received from farmers and agronomists when presenting these results is: “what about soybean crop response and yield?”. That was the focus of the second experiment, which we will discuss next.
Study #2: Soybean Development and Yield
Farmers are often concerned if their herbicide programs will impact development and yield of their crops. At the end of the day, we are all after optimizing weed control, crop yield, and profits. Study #2 focused on crop response to glufosinate tank-mix combinations in the absence of weeds (the study was kept weed-free). At 14 DAT we collected herbicide injury ratings, both visual and camera based (using the Canopeo software) assessments. Across site-years, the highest injury was observed with lactofen and glufosinate + lactofen treatments, the latter being one of the top-ranked treatments for waterhemp control in study #1. This was expected, as lactofen (Cobra) is known to “burn” the soybeans more than other PPO-inhibitor counterparts (Figure 2). Across the 6 site-years, only at one site-year soybean yield was negatively impacted by one of the treatments (glufosinate + lactofen; this could be due to a delayed POST application at this site-year which happened at the V6 soybean growth stage); no yield impact was detected otherwise.
Take-Home Message
In a nutshell, glufosinate-based herbicide programs seem promising for POST-emergence broadleaf weed control in glufosinate-resistant soybean cropping systems. It is important to note that some of the programs tested in this study can be a relatively expensive investment and we encourage weed management decision makers to evaluate the costs associated with these programs and how they fit on a field-by-field basis. Mixing glufosinate with a PPO-inhibitor herbicide would be advised in a scenario where waterhemp may be getting “out of control” early season in a soybean crop (hopefully that is not happening often out there).
Tank-mixing glufosinate with other herbicides, such as PPO-inhibitors, bentazon, or 2,4-D choline, can enhance waterhemp control without impact on soybean yield.
It is important to note that certain herbicide combinations may result in higher crop phytotoxicity risk (e.g., lactofen) thus it is crucial to follow all label recommendations to determine the most effective and safe herbicide combinations for your specific field conditions.
The soybean response study (study #2) was maintained weed-free, and potential herbicide injury might outweigh the loss of soybean yield if soybeans were competing with weeds.
The research reported herein is being led by Nikola Arsenijevic (WiscWeeds PhD student). Click here to see Arsenijevic’s slides presented during the 2022 North Central Weed Science Society Meetings (December 2022) in St. Louis, MO.
Additional Resources
This article was written by Nikola Arsenijevic (UW-Madison PhD Student), Nick Arneson (UW-Madison Outreach Program Manager), and Rodrigo Werle (UW-Madison Assistant Professor).
Ryan DeWerff (UW-Madison Research Specialist), Dan Smith (UW-Madison NPM Outreach Specialist), and Dr. Mark Bernards and his team at Western Illinois University participated in this project.