Increase of herbicide-resistant weeds has led to a renewed emphasis on preemergence herbicides applied within a few days of soybean plantings. The availability of many products and combination products has increased concerns about
crop safety. In addition, soybean planting dates have occurred earlier for many operations that further increase the likelihood of soybean injury from preemergence herbicides.

UD Weed Science Program has not looked at crop injury in a systematic method in the past ten years; nor has any of the regional universities. Information from other regions is not appropriate for Delaware due to our soil type and production practices. There is a need for this type of research.

Many of our trials are planted in mid- to late-May to maximize weed pressure and better evaluate weed control. Ratings and notes on soybean injury from preemergence herbicides are taken in all of our trials; but with the later planting dates that we use the potential for herbicide injury is reduced. When we do observe injury, often times there is a limited number of herbicides in the trial and it is difficult to draw conclusions across a range of active ingredients.

Information on soybean sensitivity to herbicides is lacking. For instance, Pioneer® Seed is one of the few companies that makes this information available in their agronomic tech sheets. It is much more common for corn hybrids to supply this
information than for soybeans. As a result, there is little information that soybean farmers and consultants can rely upon to make decisions on herbicide selection.

It is also challenging when trying to design this type of experiment. A comparison of more than one soybean variety is critical to determine the consistency of results across a range of genetics. However, when changing a variety to develop a
range of varietal sensitivity, it is not possible to keep other variables consistent. As a result, this type of research often sacrifices other agronomic attributes to achieve a range of herbicide sensitivities. As a result, information needs to be compared on a relative basis, which herbicides are most likely to cause crop injury. To determine sensitivity of individual soybean varieties to such a range of herbicides is beyond the scope of this project.

This proposal was developed based on the priorities outlined by the Delaware Soybean Board in their request for proposals as well as conversations with soybean farmers and ag businesses. This research will benefit Delaware’s soybean growers and the soybean industry by knowing potential for crop injury when using preemergence herbicides. In addition, this project was designed to address weed control in terms of effectiveness, adaptability to Delaware’s specific needs, as well as herbicide-resistance management.

The goal of this research will be to provide growers with better recommendations for soybean safety; this research will pertain to both early planting dates as well as traditional planting times. The objectives of this research will be to evaluate:
• the relative crop safety of many of the common soybean herbicides
• potential interactions if herbicides are used in combination with other herbicides
• early-season weed control of single active ingredients versus herbicide combinations

Evaluating all herbicides and combinations is not practical given the number herbicides and the range/ratio of rates that manufacturers use with premixed products. Thus, this research is designed to identify those products that have the highest potential to cause soybean injury. Herbicides are selected based on previous UD Weed Science Herbicide Trials as well as conversation with numerous farmers voicing concern about crop safety with specific herbicide. This research will focus on smetolachlor (Dual Magnum), pyroxasulfone (Zidua or Anthem products), flumioxazin (Valor), sulfentrazone (Authority products), pendimethalin (Prowl), linuron (Lorox), and metribuzin.

Four soybean varieties will be used, with differing levels of sensitivity to metribuzin and flumioxazin (Valor) (Table 2). Soybeans will be planted no-till in early April to increase the likelihood for injury due to slower germination and emergence. Soybean varieties will be selected based on herbicide tolerance and may not be the best option for early-planting; but data is limited on herbicide tolerance for most soybean varieties and it will be difficult to find a range of tolerances to these herbicides while keeping soybean agronomic characteristics consistent. Field will be weed-free (clean) at planting and an effective postemergence herbicide will be applied at 5 weeks after planting. A second postemergence treatment will be made if needed.

Methods: All herbicides will be applied at the 1.5X recommended rate for the soil type. Applications will be made at 20 g/A. Plots will be evaluated for crop response and weed control on a weekly basis from emergence until 5 weeks
after planting. Further ratings will be made as needed. Plots will be harvested to determine impact of injury on yield.

Data Analysis: All trials will be conducted as small-plot replicated trials. Data will be analyzed in a statistically sound procedure, based on the design of each study. Yield and injury for a specific variety will be compared to the untreated plots of the same variety. All trials will be replicated four times.

Dissemination of Results: Reports of this study will be disseminated through extension presentations and be used to develop weed control recommendations. Research plots will be highlighted on field tours at the Research and Education Center. Information will be included in the Mid-Atlantic Weed Management Guide. An annual report will be available through the UD Weed Science Website (http://extension.udel.edu/ag/weed-science/weed-science-reports/).

Updated July 19, 2023:
Study was planted on May 5, 2023. The field was no-till and planted with four soybean varieties, differing in their tolerance to metribuzin and PPO herbicides. The plots have been rated four times. Plots will be maintained for harvest.

Updated December 19, 2023:
The study was harvested and data has been entered. We have initiated preliminary data analysis. Results will be discussed at the 2024 Delaware Ag Week.