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. 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, KY has set out to answer this question with a focus on two Kentucky’s most problematic weeds: waterhemp and marestail.
Marestail Control in Early Planted Soybean
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.
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 consist 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 scientist with pushing soybean planting dates into April and March is the potential loss in utility of some of our key residual herbicides.
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 postemergence herbicide applications regardless of if a residual herbicide was
applied at planting or not. Another 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 1 week. Whereas we typically see a two-to-four-week delay in need for postemergence 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 soybean 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 soybean 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.