Research Highlights

Tackling Weed Control in Early Planted Soybeans

By Dr. Travis Legleiter

Soybean farmers in Kentucky continue to experiment with earlier planting dates with some pushing soybean planting dates up to as early as March. This trend of pushing soybean planting date earlier into the growing season will likely continue to increase in Kentucky with more and more farmers trying it each year. 

As with all changes in our production systems, with change comes new challenges and that does not exclude weed control.  One of the principles of cultural weed control is adjusting cropping growing cycles to be outside of peak problematic weed emergence, although this trend is pushing the soybean season further into some of our peak weed emergence events. This creates the question of how to maximize the efficiency of our weed control products as we continue to see soybean planting dates get pushed earlier and earlier each year. The University of Kentucky Weed Science Program, based at the UKREC in Princeton, has set out to answer this question with a focus on two of Kentucky’s most problematic weeds: waterhemp and marestail.

Marestail Control in Early Planted Soybeans

Marestail was one of the first weed species to come up in our discussions when talking about particularly troublesome weeds in early planted soybeans. Marestail is a problematic weed in all soybean cropping systems simply because of its diverse emergence window. Marestail can emerge throughout the year in Kentucky, with emergence only being suppressed in the coldest winter months. Despite this expanded emergence pattern, it is largely considered to have peak emergence events in the fall, early spring, and early summer, or when we are planting the majority of soybeans in Kentucky. 

When considering marestail control in our traditional soybean planting dates, we often focus on effective burndown programs weeks or even months prior to soybean planting. The focus of these traditional programs is to control whatever marestail has emerged and controlling it prior to plants becoming too large to control.  When we move the soybean planting date up to April or even March, we eliminate or largely shrink that window of time to make these effective burndowns in the spring prior to soybean planting. This also creates a scenario in which we continue to see peak marestail emergence within the early parts of the soybean growing season. 

One of the tactics the University of Kentucky explored for controlling marestail in early planted soybeans was the implementation of fall burndown programs. Fall burndowns containing a tank mix of glyphosate plus 2,4-D plus dicamba were applied in the fall of 2020 prior to planting soybean on March 10 and April 12, 2021. The fall burndown tank mixes also included varying rates of metribuzin from no metribuzin to 8 oz. of a 75DF metribuzin formulation.

The use of a fall burndown program was extremely beneficial for the March soybean planting date as marestail densities in each plot with a fall burndown at planting was zero. In contrast, marestail was present at the April planting date, although densities were significantly lower in plots with a fall burndown as compared to those without a fall burndown. The inclusion of metribuzin in the fall burndown at any rate did not create a significant change in overall marestail control, and thus we would recommend saving residual herbicides like metribuzin for use at soybean planting.

A second variable evaluated was the use of residual herbicides in the at-planting burndown application for both the March and April planting dates. The use of residual herbicides is always recommended in soybeans, regardless of the weed, but in this trial the question was about the amount of utility that a residual provides. In the case of both the March and April planting dates the use of a metribuzin-based residual herbicide, such as Canopy, significantly reduced marestail populations at the time or the post-emergence application. These results further emphasized the value of residual herbicides in soybeans regardless of planting date, especially against weeds like marestail.

Overall, based on the initial research conducted in 2021 and our previous knowledge of marestail biology, the University of Kentucky recommends the following keys for successful marestail control in March and April planted soybean:

• Utilize a fall burndown that consists of effective foliar herbicides such as a tank mix of glyphosate plus 2,4-D, and/or dicamba
• Implement a burndown at soybean planting that contains an effective foliar herbicide such as 2,4-D, dicamba, or saflufenacil (Sharpen)
• Use a robust residual herbicide that contains an effective active ingredient such as metribuzin
• Maximize the residual herbicide by applying maximum labeled rate for your soil type.

Waterhemp Control in Early Planted Soybean

Another major weed species for soybean producers in Kentucky is waterhemp, as it has encroached onto more acres every year over the past half-decade. Waterhemp emergence begins in mid-April in Kentucky and typically peaks in May and declines in late June, although waterhemp can be observed emerging in the late summer and fall months in areas where a crop canopy is not established. The concern for weed scientists with pushing soybean planting dates into April and March is the potential loss in utility of some of our key residual herbicides.

The key to waterhemp control in soybeans has continued to be the use of robust residual herbicides that contain two or three effective sites of action. There are several pre-mixed herbicides that offer these combinations for soybean growers, and a complete list of those and their ingredient breakdown can be found in AGR-259.  In the traditional soybean planting window, a farmer will be applying these robust residual herbicides at the peak of waterhemp emergence in the month of May and/or early June. This traditional timing maximizes the utility of these residual products with a suppression of the bulk of that growing season’s waterhemp flush. The issue with pushing the soybean planting window to earlier dates is that these robust residuals can only be applied prior to soybean planting and the maximum window of control can occur prior to peak waterhemp emergence.  

Initial research conducted in 2021 reinforced these thoughts of losing the value of residual herbicides. Soybean plots planted in both April and March had equivalent waterhemp densities at the time of post-emergence herbicide applications regardless of if a residual herbicide was applied at planting or not. In other words, the residual herbicides did not provide any waterhemp suppression as would be expected. Additionally, the postemergence applications (based on weed size) occurred on the same date for all March planted soybean plots and the April plots with a residual herbicide were only delayed in postemergence application by one week. Whereas we typically see a two- to four-week delay in need for post-emergence applications in plots that receive a residual herbicide as compared to those that did not. These results reinforce that we may be losing the value of residual herbicide for waterhemp control in April and March planted soybeans as compared to traditional soybean planting dates in May.

The utility of fall burndowns was also evaluated for use in early planted soybean plots for waterhemp control. The premise behind using a fall burndown for these trials was to establish a clean planting bed in the fall, especially for the plots planted in early March. While the goal of creating a clean planting bed for both the March and April soybean planting dates was accomplished, the lack of winter annual weed cover on those plots also created a scenario in which waterhemp emergence began much earlier in the plots with a fall burndown. Overall season-long waterhemp control was less in plots that received a fall burndown, especially within the March planted soybean plots.

The initial year of research focusing on waterhemp control in March and April planted soybeans confirmed our thoughts that we may lose value in residual herbicide, but also raise some new questions and reinforced the need for further research. Further research with the support of Kentucky Soybean Board is planned for 2022 looking deeper into these questions as they pertain to waterhemp, including the evaluation of utilizing layered residual herbicides.

Reprinted with permission from the Kentucky Soybean Board.

Published: May 16, 2022