Herbicide-resistant weeds continue to drive weed management decisions. There are currently 131 unique cases of herbicide resistance in the United States with 13 different herbicide-resistant weeds in Maryland (Heap 2023). Furthermore, there has been a 40% increase in the last decade in the number of weed species resistant to at least 2 herbicide sites-of-action (Heap 2023). In Maryland these include Palmer amaranth and common ragweed.

Previous studies funded by the Maryland soybean board demonstrated several pre- and postemergence options for controlling these resistant weeds (Vollmer et al 2022; Beale and Leslie 2022). Conversely, future management practices should aim to abate further resistance development in order to preserve effective herbicides. Tank mixing herbicides with different sites-of-action is one way to assuage the onset of herbicide resistance. This is due to a weed population having a lower probability of developing resistance to two or more sites-of-action, but there is a caveat. Both herbicides used must 1) target different sites-of-action, 2) must be effective alone, and 3) must target the same life stage of the weed. Previous research funded by the Maryland Soybean Board has shown 2,4-D plus glufosinate to be an effective treatment for managing emerged herbicide-resistant common ragweed and Palmer amaranth, but these herbicides do not provide residual weed control (Figure 1). An effective postemergence with residual program is necessary for full-season control of weeds like Palmer amaranth. However, growers may forego a residual herbicide at the time of the postemergence application or may rely on a single site-of-action for residual control. Including herbicides with both foliar and residual activity, such as fomesafen (Reflex), in tank mixtures with other effective foliar and residual herbicides may help to preserve the utility of existing herbicides in Maryland soybean production.

In addition, there is little data from Maryland studies about the efficacy of OMRI approved herbicides and herbicide tank mixtures for control in organic soybean. These herbicides act on contact, similar to glufosinate, causing rapid cell desiccation. However, it is not recommended that they be applied over-the-top of emerged soybeans and effectively incorporating them into a postemergence weed control program can be challenging.

1) demonstrate the efficacy of herbicide tank mixtures for herbicide resistance mitigation,
2) evaluate the efficacy of early and late postemergence applications for full season weed control,
3) evaluate treatment effects on seed viability and the potential for resistance development.

The study will compare early-postemergence (2 to 3 weeks after planting, WAP) and postemergence (4-6 WAP) treatments (Table 1). All herbicide treatments will compare the efficacy of single products to tank mixtures in controlling emerged weeds and extending residual weed control. A flame weeding treatment will also be included to evaluate a non-chemical
option. Treatments will be evaluated for weed control 1, 2, and 3 weeks after treatment and at harvest. To assess the effects of these treatments on overall seed production, weed density will be recorded and seed from surviving plants of selected species will be collected prior to harvest and germination tests performed to determine overall seed viability.

Updated August 1, 2024:
Two field studies were established in June of 2024 at the University of Maryland Wye Research and Education Center in Queenstown, MD and a grower field in Sudlersville, MD. A residual herbicide application was made prior to planting Enlist E3 soybeans at both locations. Postemergence applications were made at the Wye location on June 20 and July 3. Although the Sudlersville location was evaluated weekly for weed emergence, weed pressure was insufficient to conduct an adequate evaluation. Therefore, the study was abandoned once soybeans canopied. Results from the Wye location were presented at the Maryland Weed Management Twilight Tour on June 26 and the Maryland Commodity Classic on July 25.

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This research will benefit Maryland soybean growers and the soybean industry by expanding knowledge of potential strategies to help control these species prior to seed set. This project will address weed control in terms of effectiveness, resistance management, as well as its adaptability to Maryland’s specific needs.