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.
Application of burndown plus residual herbicides in early spring (late March/early April). This accomplishes the goal of applying early enough to ensure that burndown of emerged plants is not an issue. We have had consistent results with 2,4-D applied this early for marestail control. Applying all of the residual herbicide at this time can result in late-emerging marestail escapes in late May or early June if the residual herbicide activity runs out by then.
Keys to making this approach work:
Application of burndown plus residual herbicides in late April or close to planting (whenever that is). This accomplishes the goal of applying the residual later in the season, which increases the potential for adequate control of late-emerging marestail plants. The disadvantage to waiting this late, especially as applications are delayed into May, is increased variability in the burndown of existing plants.
This variability can be minimized by increasing burndown herbicide rates or by using a more effective mix of burndown herbicides. Some examples:
Split preplant/preemergence application of herbicides. This can take various forms, but in our thinking would most likely be something like: late March/early April application of glyphosate + 2,4-D + a low rate of residual herbicide (several ounces of metribuzin or a low rate of Canopy DF); followed a second application at the time of soybean planting consisting of the majority of the residual herbicide (Valor XLT, Gangster, Sonic, etc) plus whatever additional burndown is needed.
Need for additional burndown could be minimal with a late-April planting, but fields should be scouted to determine this, and burndown adjusted accordingly. It would also be possible to use products such as Valor XLT, Envive, Authority XL, etc., applying half the total rate in early March/late April, and the rest at the time of planting.
The obvious drawbacks to this approach compared with the single-application approaches are increased cost and time, and soil conditions in early spring may be unsuitable for traffic. The primary benefit is more consistent control of marestail across a range of marestail population densities and emergence patterns, weather conditions, and planting dates. By making two applications, there is flexibility built into the second application that should reduce the chances of a significant in-season marestail problem.
There is of course a continuum between the first two approaches, since preplant applications can conceivably be made anytime between late March and when soybeans are finally planted. Application timing is dependent upon weather and soil conditions as well. The main things to keep in mind are that burndown of marestail becomes more variable as we move from early to late April and beyond, while applying later improves chances that residual herbicides will control marestail that emerge in late May and June.
Consult Table 13 in the current “Weed Control Guide for Ohio and Indiana” for information on residual herbicide effectiveness. Best results occur by using herbicides that are rated an 8 or higher in the “ALS-resistant” marestail column.
More information on marestail management can be found in OSU Extension Bulleting 789, “Weed Control Guide for Ohio and Indiana” and the OSU/Purdue fact sheet, “Control of Marestail in No-till Soybeans”. These are available at OSU Extension County offices and online.