Soybean herbicides for residual control of marestail (horseweed)
The two major components of a marestail management program in no-till soybeans are 1) ensuring that the existing emerged marestail are controlled prior to soybean emergence, and 2) using residual herbicides to control later-emerging marestail for another 6 to 8 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.
The application rate and timing of application becomes more of a factor in ALS-resistant populations, along with the overall complexity of the residual herbicide treatment. Applying earlier in spring, or using the lower “Roundup Ready rates” of products can reduce the residual activity in late May and June when marestail is still emerging. Metribuzin, sulfentrazone, and flumioxazin seem to be relatively similar in their residual activity on marestail. We conducted a study in 2012 where metribuzin greatly outperformed the other two herbicides, and that has led us to recommend that growers consider adding at least some metribuzin to treatments that contain sulfentrazone or flumioxazin (see accompanying article on metribuzin in this C.O.R.N. issue). The addition of metribuzin can improve burndown and the use of two herbicides with activity on marestail may also result in more consistent residual control. In an attempt to boil all of this down to a few key points, we offer the following suggestions:
– use a residual herbicide treatment that includes metribuzin, sulfentrazone, or flumioxazin. When using a premix that contains one of these, increase rates above the base “Roundup Ready” rates in order to provide more of these components. Consider adding some metribuzin to products that contain sulfentrazone or flumioxazin.
– products that contain flumioxazin: Valor, Valor XLT, Envive, Enlite, and Gangster. It is possible to use lower rates of the premix products, but add more Valor in order to improve marestail control.
– products that contain sulfentrazone: Authority XL, Sonic, Authority First, Authority Broadleaf, and Spartan. It is possible to use a low rate of one of these and add more Spartan to increase the sulfentrazone, but this may not be cost-effective compared with just increasing the premix rate.
– when using metribuzin as the primary residual for marestail, increasing rates can improve burndown and the longevity of control. We suggest aiming for rates of 10 to 12 oz of 75DF where soil type allows. Consult the labels for information on rate based on soil texture and organic matter.
– products that contain at least some metribuzin but need more added to be consistently effective: Canopy/Cloak DF, Matador, Intimidator, Boundary.
– it is possible to split the spring residual herbicide treatment into two applications – one in early spring where about half of the residual herbicide is applied with burndown, followed by the rest at the time of soybean planting with additional burndown as needed. This may help maintain control later in the season, compared with applying everything in early spring.
– products that are ALS-inhibitors and may be effective in ALS-sensitive populations: Python, Canopy/Cloak DF and EX, and FirstRate. We don’t recommend this approach. Don’t guess wrong about whether a population is ALS-resistant.
– products that we do not consider effective for residual control of marestail: Scepter, Prefix, Pursuit, Optill, Optill Pro, Sharpen, Outlook, Verdict, metolachlor, pendimethalin, and Warrant. Sharpen can contribute some residual but is generally not adequate unless mixed with another effective herbicide. It is possible to add a full rate of metribuzin to any of these to create a mix that works.
– Labels no longer allow mixtures of Sharpen with products that contain flumioxazin or fomesafen (Prefix, Intimidator), and we do not expect this to change between now and the planting season. Sharpen can be mixed with Authority products (see label for rates based on application timing), along with metribuzin and the other products mentioned in this article.