Soybean herbicides for residual control of marestail
The two major components of a marestail management program in no-till soybeans are ensuring that the existing emerged marestail are controlled prior to soybean emergence, and using residual herbicides to control later-emerging marestail for another six to eight weeks after planting.
There are several strategies that can be implemented to achieve this, and more information can be found in the OSU/Purdue fact sheet, “Control of Marestail in No-till Soybeans” (http://agcrops.osu.edu/specialists/weeds). The purpose of this article is to provide more in-depth information and rationale to help in the selection of residual herbicides. The variable emergence pattern of marestail, and it’s tendency to emerge late in the season, seem to mean that there are no completely “bulletproof” marestail management programs. Not all residual herbicides are effective for marestail control, so it’s possible to inadvertently use or recommend residual herbicides that ultimately make the program even less “bulletproof.” The bottom line in marestail management is that we are trying to avoid having to control it with postemergence herbicides, since they largely do not work (exception – Liberty in LL soybeans).
We have a number of residual herbicides that are relatively effective for control of marestail, but the presence of ALS resistance in many marestail populations reduces the number of viable options. When selecting residual herbicides, most effective control will generally result from: 1) assuming that the population is ALS-resistant, and 2) making sure that the residual herbicide treatment contains herbicide(s) that are effective on ALS-resistant populations, and are used at high enough rates. Table 10 in the “2013 Weed Control Guide for Ohio and Indiana”, which shows residual herbicide effectiveness, has ratings for both ALS-sensitive and ALS-resistant marestail.
To go into more detail - the ALS inhibitors cloransulam (FirstRate) and chlorimuron (Classic) are among the most effective herbicides for control of marestail, and their activity contributes greatly to the effectiveness of various premix products of which they are a component. For chlorimuron, these are Canopy/Cloak, Valor XLT, Envive/Enlite, and Authority XL, and for cloransulam they include Gangster, Authority First, and Sonic. However, the chlorimuron or cloransulam is not effective on ALS-resistant marestail populations, so in that case the second component of these premixes carries the load for control - the sulfentrazone (Authority/Spartan, flumioxazin (Valor), or metribuzin. The increasing frequency of ALS resistance in marestail, combined with the lower rates that are commonly used for these premix products, is partly responsible for the variability in residual control that we hear about. Another reason for this is that chlorimuron and cloransulam have a longer period of activity in soil than the other three herbicides. We have observed more variable control of marestail in our research when relying on just the single non-ALS component of these premix products.