Early planted soybean weed management as effected by herbicide application rate and timing

Authors: MILLER, LOGAN - University Of Illinois; Landau, Christopher; Williams, Martin; HAGER, AARON - University Of Illinois

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2024
Publication Date: 1/13/2025
Citation: Miller, L.R., Landau, C.A., Williams, M., Hager, A.G. 2025. Early-planted soybean weed management as affected by herbicide application rate and timing. Weed Technology. https://doi.org/10.1017/wet.2024.103.
DOI: https://doi.org/10.1017/wet.2024.103

Interpretive Summary: Soybean is being planted earlier than ever, yet little is known about how this change impacts weed management. We conducted field experiments in early planted soybean to begin to address this knowledge gap. We found that applying the full rate of commonly used soil active herbicides at the time of planting was critical to maximizing the utility of common postemergence herbicides. The impact of this work is that it provides some of the first insight into how to design effective weed management systems that account for recent changes to earlier soybean plantings.

Technical Abstract: The opportunity to maximize soybean yield has prompted Illinois farmers to plant soybean earlier than historically done. Extending the growing season with an earlier planting date might alter the relationship between soybean growth and weed emergence timings, potentially influencing herbicide application timings to protect crop yield potential and ensure minimal weed seed production. The objective of this research was to examine various herbicide treatments applied at different timings and rates to assess their effect on weed control and soybean yield. Field experiments were conducted in 2021 at three locations across central Illinois to determine effective chemical strategies for weed management in early planted soybean. Preemergence (PRE) treatments consisted of a S-metolachlor plus metribuzin premix applied at planting or just prior to soybean emergence at 1/2x (882 + 225 g ai ha-1) or 1x (1,764 + 420 g ai ha-1) label recommended rates. Postemergence (POST) treatments were applied when weeds reached 10 cm tall and consisted of 1x rates of both glufosinate (655 g ai ha-1) and glyphosate (1,260 g ae ha-1) plus ammonium sulfate, with or without pyroxasulfone at a 1/2x (63 g ai ha-1) or 1x (126 g ai ha-1) rate. Treatments comprised of both a full rate of PRE and POST provided the greatest and most consistent weed control. The addition of pyroxasulfone to POST treatments occasionally but not consistently reduced late-season weed emergence. The lack of a consistent effect by pyroxasulfone could be attributed to suppression of weeds by soybean canopy closure due to earlier soybean development. The full rate of PRE extended the timing of POST application 2–3 weeks for all treatments at all locations except Urbana. Full-rate PRE treatments also reduced the time between the POST application and soybean canopy closure. Overall, a full rate PRE reduced early season weed interference and helped preclude soybean yield loss.