Published on Wednesday, August 2, 2017
When it comes to growing double crop soybeans, one hurdle many growers face is weed control. One of the most challenging weeds to manage in a double crop soybean system is marestail. In fact, some biotypes of marestail have developed resistance to Group 2 (ALS) herbicides, which are the most commonly applied in wheat. If a population is resistant, the weeds will escape the herbicide application and thus, will be well-established at wheat harvest. The larger the weed, the more difficult it is for contact herbicides to have effective control. It’s also important to note that when wheat is harvested the platform cuts the top of the marestail plant. This causes it to regrow with multiple growing points, making management even more challenging, as shown in Figure 1.
Figure 1. Marestail regrowth 28 days after wheat harvest.
So what are your options? This year, Beck’s Practical Farm Research (PFR)® team in southern IL evaluated multiple different burndown options for marestail in double crop soybeans (Table 1) to determine which is most effective.
Table 1. Herbicides tested to evaluate marestail control in double crop soybeans.
Just like any other burndown program, starting with a clean field is critical for success. When deciding on what product to use, you may want consider one that won’t limit your crop rotation the following year. Selecting a product that won’t result in a plant back restriction may also be crucial. Other herbicide options that result in effective control are 2,4-D and dicamba, both of which are advantageous because they are systemic. Keep in mind that they both require a plant back restriction in most systems. However, Roundup Ready 2 Xtend® soybeans will not require a plant back restriction for dicamba. When moisture is limited in June, a delay may limit planting into prime seed bed conditions. One systemic product that does not have a plant back restriction would be Roundup PowerMAX®.
Figure 2. Marestail treated with Roundup PowerMAX® 7 DAT.
The new growth had developed chlorosis, or yellowing.
Products such as Sharpen® can be applied with no plant back restriction, however, the rate may need to be reduced depending on soil type. The products and rates that were tested in this study did not result in a plant back restriction for the soil types at Beck’s PFR site in SIL. As a result, the team was able to plant in June when ample moisture was available, versus in July when moisture was scarce.
The symptomology of contact herbicides such as Gramoxone®, Sharpen and Liberty® is very rapid when compared to other products such as Roundup PowerMAX (Figure 3B).
Figure 3. Control (A) Roundup PowerMAX® 7 DAT (B).
Thus, seven days after treatment (DAT) Gramoxone (Figure 4A), Sharpen (4B + 4C), and Liberty (4D) all achieved greater than 90 percent control (Figure 5). All treatments, with the exception of Roundup PowerMAX, resulted in greater than 90 percent control seven DAT. Adding metribuzin to Liberty increased control by two percent (Figure 4D). However, when metribuzin was added to Gramoxone, control increased by seven percent seven DAT. Treatments that contained Sharpen averaged 93 percent control by seven DAT. Roundup PowerMAX alone resulted in less than 35 percent control at seven DAT.
Figure 4. Gramoxone® (A) Liberty® + Sharpen® (B) Sharpen + Roundup PowerMAX®
(C) Liberty + Metribuzin (D) by 7 DAT.
At 14 DAT, the SIL PFR team observed that all treatments were effective with the exception of Roundup PowerMAX (Figure 5). Roundup PowerMAX resulted in less than 42 percent control at 14 DAT. However, adding metribuzin to Gramoxone resulted in 99 percent marestail control, which was four percent higher than Gramoxone alone. By 14 DAT, Liberty alone resulted in 98 percent control of marestail. With such effective control, the tank mix products did not show an additional advantage. Average control of marestail in treatments that contained Sharpen or Gramoxone averaged 97 percent. By 28 DAT, control of marestail was 100 percent for all treatments except Gramoxone alone and Roundup PowerMAX alone. Gramoxone alone resulted in an acceptable 98 percent control of marestail while Roundup PowerMAX fell below 20 percent control. The marestail treated with Roundup PowerMAX had regrowth at this point.
Figure 5. Percent Visual control of marestail in double crop soybean burndown
7, 14 and 28 days after treatment (DAT) in SIL.
Though these treatments are all very effective, there are a few things we need to keep in mind. The use of Sharpen on marestail resulted in effective control across all ratings and never fell below 92 percent control. Liberty combined with Sharpen could be an effective alternative to a Roundup and Sharpen combination when battling glyphosate-resistant weeds. When looking at the control of marestail, Liberty and Sharpen never fell below 95 percent at any rating date (Figure 6A).
Figure 6. Sharpen® + Liberty® (A) Control 28 DAT (B).
While Liberty offers very effective control of marestail, if you are using LibertyLink® soybeans, this would require 32 oz. of your total in-season allowance. Sharpen with Roundup resulted in greater than 92 percent control at all rating dates and can still be an effective option when battling glyphosate-resistant marestail. Just like in regular crop soybeans, a burndown in double crop soybeans provides the opportunity to use other modes of action or other herbicide actives. Gramoxone (Group 22) offers a great burndown option, but at each rating date we saw an increase in efficacy when metribuzin (Group 5) was added. Though Gramoxone and Liberty efficacy was greater than 90 percent, adding a tank mix partner may still be necessary. As marestail pressure increases, the addition of another mode of action to Gramoxone or Liberty becomes even more crucial in effectively controlling marestail.
Reducing marestail pressure in the wheat crop is the first step towards successful weed control in a double crop system. Marestail is very sensitive to tillage or soil disturbance, which can reduce emergence. While a fall burndown will help control fall emerging marestail, to control spring emerging marestail the use of POST applications may be necessary. If you suspect your marestail is resistant to Group 2 herbicides, try other options. Reducing the number of marestail plants reduces the weed pressure in your double crop burndown. No matter what platform you select for your double crop soybeans, you can still manage your marestail successfully. However, an effective burndown is the key to season-long management of marestail!
Dimetric® is a registered trademark of Winfield Solutions, LLC. Roundup PowerMAX® and Roundup Ready 2 Xtend® are registered trademarks of Monsanto Technology, LLC. Liberty® and LibertyLink® are registered trademarks of Bayer. Sharpen® is a registered trademark of BASF. Gramoxone® SL is a registered trademark of a Syngenta Group Company.
Author: Joe Bolte
Categories: PFR, PFR Reports