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.

The trial in 2023, funded by Delaware Soybean Board, was the first systematic method of looking at soybean injury in the past ten years. We were surprised by the results. We did not anticipate such good crop with metribuzin, nor did we anticipate the amount of crop injury observed with chlorimuron containing herbicides. In addition, UD Weed Science had two other trials from herbicide manufacturers, that included metribuzin and chlorimuron. In these industry trials, we observed the same trends, good crop safety with high rates of metribuzin and a noticeable amount of injury with chlorimuron, although chlorimuron rates were not consistent across all the trials.

In general, our soybean 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.

Based on the observations from 2023, it is important to include planting date in the 2024 trial. Planting later in the season with warmer soil temperature may allow soybeans to metabolize the herbicides to avoid injury (or less the amount of injury).

This proposal was developed as a result of the priorities outlined by the Delaware Soybean Board in their request for proposals as well as conversations with soybean farmers and ag businesses. This proposal is also heavily influenced by the results of the 2023 project.

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
• effect of planting date on crop safety

Procedures to Achieve Objectives:
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 will focus on herbicides that have the highest potential to cause soybean injury. Herbicides are selected based on previous UD Weed Science Herbicide Trials and observations from 2023 research. This research will focus on s-metolachlor (Dual Magnum), flumioxazin (Valor), sulfentrazone (Authority products), chlorimuron (is active ingredient in Canopy, Fierce XLT, and Authority XL), and metribuzin.

At least two soybean varieties will be used, with differing levels of sensitivity to metribuzin and flumioxazin (Valor). Soybeans will be planted no-till in early April to increase the likelihood for injury due to slower germination and emergence. A second planting date will include the same twelve herbicide treatments. 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.

The 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:
Trial will be conducted as a small-plot replicated trial. 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 August 5, 2024:
All research treatments have been applied, plots have been rated on a regular basis. Research has been discussed at the Weed Science Field Day in June 2024.

This research will benefit Delaware’s soybean growers and the soybean industry by characterizing potential for crop injury when using preemergence herbicides and the influence that environmental conditions at planting can have on soybean safety.