We've previously advanced that the single most effective approach to marestail management is probably mid to late April application of a combination or burndown and residual herbicides.
Based on our research, it is possible for this approach to control marestail well enough, possibly leaving a few plants to emerge in mid-season.
However, growers have been frustrated by the inconsistency of control, which can occur where soybeans are planted later than intended or because the residual herbicide runs out before the soybean canopy develops well enough to shade out late-emerging plants.
For example, in the study we conducted here, we applied glyphosate + 2,4-D + Valor XLT on April 22, and planted soybeans a week later. The plants that were present at the time of application were effectively controlled, but the control in early June was only 73% due to later-emerging plants, which illustrates that even effective residual herbicide(s) can fail to last long enough.
In 2010, we conducted the first of several studies designed to get a better idea of what the right system, or combination of fall and spring herbicides, might be to ensure consistent marestail control.
We used LibertyLink soybeans, and worked in an area where the marestail population was resistant to glyphosate but not ALS inhibitors. The study included all possible combinations of three fall and four spring preplant treatments, and these were followed with a postemergence application of Ignite.
Fall treatments included none, no residual (glyphosate + 2,4-D) or residual (Canopy + 2,4-D). Spring treatments included glyphosate + 2,4-D, glyphosate + Sharpen, Ignite + metribuzin, and glyphosate + 2,4-D + Valor XLT. We also had a treatment that consisted of fall herbicide only – Canopy + 2,4-D.
We evaluated marestail control 3 weeks after planting to measure control 3 weeks after planting to evaluate burndown herbicide effectiveness, 6 weeks after planting (at time of POST) to evaluate residual control. The study was then treated with Ignite POST, and we evaluated again later in the season. Here’s what we found (results also shown in the table below):
Where we applied Canopy + 2,4-D in the fall without a follow up spring preplant treatment, we ended up with only about 50% control of marestail in early June when the POST Ignite was applied.
These results support our recommendation not to use residual herbicide only in fall where marestail is a problem. Even where the population is not ALS-resistant, it’s risky to assume that fall application of Canopy or another chlorimuron product can provide enough control the following year in the absence of additional residual herbicide applied in spring.
When evaluated 3 weeks after application, all of the spring burndown treatments effectively controlled the marestail that had emerged by April 22, even where herbicides were not applied in the fall.
This is not surprising given that the spring burndown treatments we used have a history of effectively controlling small plants in April.
In the absence of fall-applied herbicides, use of a spring preplant treatment that lacked substantial residual activity (including Sharpen and the low rate of metribuzin used here) resulted in less than 60% control of marestail in early June. Where Valor XLT (4 oz/A) was included in the spring treatment, control in early June increased to 73%.
This factors in that the burndown herbicides were effective, so the difficulty here was control of marestail that emerged in May, after soybean emergence. These results illustrate some of the problems occurring with marestail management programs in Ohio – even a relatively strong approach may not completely control marestail until soybeans canopy.
It was possible to improve the effectiveness of the spring residual treatments by applying herbicides the previous fall. Because this population was not ALS-resistant, where we applied Canopy in the fall and followed with any spring preplant treatment that had at least some residual (Sharpen (1 oz/A), metribuzin (4 oz/A), or Valor XLT), we ended up with 97 to 100% marestail control in early June.
What was surprising from our results, however, were the apparent benefits of applying at least something in the fall, even if it lacked residual activity. Where we applied glyphosate + 2,4-D in the fall vs where nothing was applied in fall, the control in early June increased from 73 to 92% for the spring Valor XLT treatment.
For the metribuzin and Sharpen treatments, control increased from an average of 50% to an average of 72%. What’s fairly strange about this is that there were not any differences in control at 2 weeks after planting. So it appears that the type of fall treatment did not influence the effectiveness of burndown treatments, but did alter the effectiveness of residual herbicides.
While Ignite is generally effective for control of emerged marestail, the effectiveness of the POST Ignite application in this study was somewhat dependent upon the level of management up to that point.
Three weeks after the POST application, control of marestail ranged from 95 to 100% for any treatment that included both fall and spring preplant herbicides.
Control was reduced to 78 to 82% where fall herbicides were omitted, and the spring treatment lacked substantial residual (glyphosate+2,4-D or glyphosate+Sharpen).
The soybean canopy did suppress remaining plants well enough that control at the end of the season exceeded 90% for all treatments. However, the results of this study show the importance of using an effective overall approach to marestail management even where the POST herbicide is capable of controlling marestail.
We will follow these articles with a third in the next issue of C.O.R.N. that summarizes viable approaches to marestail management, and lists common mistakes in marestail management.