Split preplant/pre-emerge application
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.