With the widespread occurrence of herbicide-resistant weeds across Wisconsin soybean and corn production acres there has been increased adoption of soil residual herbicides for improved season-long weed control. The primary way growers are bringing residual herbicides into their soybean and corn systems is through PRE-emergence (PRE) applications. The WiscWeeds team firmly believes that PRE herbicides are the foundation for effective chemical weed control (see “Residual Control of Waterhemp with PRE Herbicides in Soybean” and “Herbicide Comparison for Residual Weed Control in Corn”). Unfortunately, PRE herbicides alone may not be enough to achieve season-long control of troublesome weeds such as waterhemp. Waterhemp emerges throughout the bulk of the growing season and is forcing growers to consider adopting a ‘layered residual’ approach with their herbicide programs.
The layered residual approach is a strategy where herbicides with soil residual activity are applied PRE (i.e., near the time of crop planting) and then at the time for a POST-emergence (POST) application an additional residual herbicide is added to the tank to overlap soil residual control of weeds that are yet to emerge. The layered residual concept is summarized nicely in the Iowa State University blog post by Dr. Hartzler & Anderson: “Achieving Full-season Waterhemp Control in Soybean”). Oftentimes, the residual herbicides tank-mixed with a foliar POST application in soybean consist of Group 15 herbicides (e.g., acetochlor [Warrant], S-metolachlor [Dual II Magnum], dimethenamid-P [Outlook], and pyroxasulfone [Zidua]). The WiscWeeds program has gathered years of data suggesting that the layered residual approach can result in season-long control of glyphosate-resistant waterhemp (see pgs. 62-65 in “2020 WI Weed Science Research Report”).
One complication that goes along with the adoption of the layered residual approach would be herbicide rotational restrictions for winter wheat. According to Dr. Shawn Conley, most of the winter wheat acres planted in Wisconsin follow soybeans (see CoolBean.info blog post: “Choosing Soybean Varieties for Rotation with Winter Wheat”). We are aware that growers who plant winter wheat following harvest of their soybean crop may have difficulty including a layered residual herbicide in their soybean POST program as many products require 4 months or more after application before planting winter wheat. Dr. Conley recommends the window of September 20th to October 10th as the ideal window for winter wheat planting (see CoolBean.info blog post: “Top 8 Recommendations for Winter Wheat Establishment in 2021”). Assuming an October 1st winter wheat planting date, many of the commonly used residual herbicides as part of a POST program would have to be sprayed by June 1st to remain on-label. One opportunity that we have identified for growers looking to adopt the layered residual approach and planting winter wheat following soybean harvest would be using a reduced Zidua SC rate (3.25 fl oz/acre or less) which requires only a 1-month rotational restriction for winter wheat. Adjusting the time of the POST application in soybean (i.e., spraying earlier) and slightly delaying winter wheat planting to match rotational restrictions represent another opportunity.
Given that multiple agronomists and growers have reached out to us with this question/concern, the WiscWeeds team put together a short report summarizing commonly used soybean residual herbicides and their winter wheat rotational restrictions according to their respective labels. The intention of this resource is to help guide growers in a soybean-wheat rotation who might be battling herbicide-resistant weeds, including waterhemp, with herbicide selection.
Click HERE to download the PDF report: “Winter Wheat Rotational Restrictions Following Common Soybean Residual Herbicides”
Please remember to always read, follow, and understand the pesticide label.
Authors:
- Nick Arneson, Weed Science Program Outreach Manager, Department of Agronomy, University of Wisconsin-Madison
- Ryan DeWerff, Weed Science Research Specialist, Department of Agronomy, University of Wisconsin-Madison
- Rodrigo Werle, Assistant Professor and Extension Cropping Systems Weed Scientist, Department of Agronomy, University of Wisconsin-Madison
