Ohio State University research on marestail management has shown that a systems approach, involving a combination of herbicide applications, provides the most consistently effective control and reduction in marestail population over time. Given that essentially all of the marestail populations in the state are glyphosate-resistant by now, and something like 25% of these are also resistant to ALS inhibitors (Classic, FirstRate), POST applications are often the least important application in a marestail control program. The exception to this is LibertyLink soybeans of course, where the POST Liberty application has considerable value when preceded by an appropriate preplant burndown/residual treatment.
In Roundup Ready soybeans, we have obtained the most consistently effective control with a combination of fall and spring preplant treatments, where the fall treatment contains a low rate of Canopy EX/DF that has some residual control, and the spring treatment contains effective burndown herbicides along with the majority of the residual herbicide (e.g. Valor XLT, Sonic, Authority First, etc).
Use of even non-residual herbicides in the fall seems to improve the effectiveness of spring treatments, although not in every situation, whereas failure to apply in fall introduces more variability in control. This is the situation we find ourselves in coming into this spring though, due to the lack of time and good weather last fall to get herbicide applied.
It is possible to obtain adequate control of marestail with a single spring application, but there is some inherent variability with this approach. It can also be difficult to determine an optimum time for application. There is a tradeoff that occurs with early versus late-spring applications in that applying early in spring (late March/early April) makes control of emerged plants more consistent, but can result in the residual herbicide activity not persisting into early summer when marestail are still emerging. Applying later in spring (late April/early May) can reduce the risk of the residual running out too early, but increases the risk of ineffective burndown of emerged plants, especially where there was no fall application and overwintered plants are present. With regard to minimizing soybean yield loss and marestail seed production, ensuring a weedfree start at planting is more important than ensuring near 100% control of later-emerging plants. As we found last year in a very wet spring, waiting too long to apply burndown herbicides can result in major control problems.
The bottom line here is that there is no one easy approach to marestail management that consistently optimizes both burndown and residual control of marestail. Several possible approaches are offered for your consideration here, with the caveat that any of them may work in a field with a low infestation level and the growing season progresses “normally,” but the more complex approach will help ensure control when populations are higher and the growing season less favorable.