With new opportunities for weed control in soybeans for 2017 comes new responsibilities for management and stewardship. Glen Newcomer of Williams County is planning to incorporate some new technology to continue to stay a step ahead of weeds.
“Like many farmers in Ohio, we used Roundup Ready technology in our operation. We were one of the
first seed growers in our area to use Roundup Ready technology back in 1996. So for over 20 years we have been using that type of technology. Since then, we have expressed interest in Liberty Link technology, which is a different mode of action and different type of weed control. Now we have the opportunity to change that mode of action once again with Extend soybean technology,” Newcomer said. “With dicamba as an option. there will be a tremendous responsibility on the producer’s part to familiarize themselves with the different products, the different restrictions in regard to how they are going to spray it and how they are going to manage it. At the same time they need to familiarize themselves with the strengths and limitations are for the technology.”
Resistant weeds are a problem in the area and inclusion of the dicamba technology will have advantages.
“In northwest Ohio I see a tremendous amount of weed resistance. The difficult weeds we have to contend with are marestail, giant ragweeds, and waterhemp. The Palmer amaranth has been a problem in some areas around dairies that use the cottonseed meal,” Newcomer said. “In the fall of the year, because we do not use cover crops, we try to integrate a program to control winter annuals. That would typically involve a 2,4-D, dicamba and glyphosate mixture. We look at the weed spectrum we have out there and try to control all the winter annuals and any perennial weeds going into the winter. In the spring of the year those fields are generally very clean, however we still apply a pre-emerge product using another mode of action so we get a broader spectrum level of control. We mainly do the pre-emerge products so we address the concern with marestail and typically we use something like Envive or we also use six to eight ounces of metribuzin for the additional burndown. I do believe in residual products to compliment the post products. By having that in place, it really diminishes the potential for resistant weeds on our operation.”
The dicamba application will fit into Newcomer’s spray program the same as postemergence (POST) applications with Roundup.
“We always POST our soybeans just prior to or at R1. To me that is the last opportunity that we should be putting a herbicide on a crop. We’ll look at the same restriction using the dicamba soybeans as well. If there are weeds out there after R1 I believe you are too late,” he said. “Later on in the season we go back and apply fungicide, insecticide and foliar fertilizer to our soybeans as well to preserve the health and yield potential of our crop.”
In general, weed control in soybeans is more challenging (and critical) than with corn, particularly for the most problematic weeds, said Ohio State University Extension herbicide specialist Mark Loux.
“The more problematic weeds we deal with in Ohio — marestail, giant ragweed, and waterhemp, and then also common ragweed and Palmer amaranth in some areas, are more difficult to control in soybeans than in corn for several reasons. The biggest reason is that we have had only two to three foliar herbicide sites of action with activity on these weeds for soybeans, and they have developed resistance to one or more of these sites of action in most fields. Whereas there are still several sites of action used in corn on these weeds for which resistance has not developed,” Loux said. “We are right in the middle of several years when the options for weed management in soybeans are greatly expanding, with the introduction of soybeans resistant to dicamba, 2,4-D, and isoxaflutole. As we noted in the recent story on weed control in corn, these traits will allow use in soybeans of several sites of action and herbicides that have been used primarily in corn until now, or at least not used in soybean POST treatments. And while these traits can be a good thing for weed control, going forward they will require appropriate management so that they do not become our next resistance problems.”
Appropriate management relies on diversity to keep weeds guessing.
“Weed scientists across the region promote a multiple-application program of preemergence (PRE) and POST herbicides for weed management in soybeans in order to account for the more complex biology of these weeds and the resistance that already occurs, and to reduce risk of additional resistance. It’s not really possible anymore to obtain adequate weed control in soybeans with a single-application program, or multiple applications of the same herbicide, or without residual herbicides, and it hasn’t been for a while,” Loux said. “Residual herbicides are needed in both tilled and no-till fields, to reduce selection for resistance, help with control of more problematic weeds such as giant ragweed and waterhemp, provide flexibility in the POST application window, and reduce risk of yield loss when weather prevents timely PRE applications. There seems to be some occasional thinking that because tillage usually takes care of marestail for the season, residual herbicides are no longer needed, which is erroneous.”
Ohio soybeans are typically grown under no-till conditions. Loux recommends this basic program:
1. PRE burndown + residual — both burndown and residual components should include at least two different herbicide sites of action; and
2. POST with activity on annual grasses, volunteer corn, and the broadleaf weeds from the list above that are present in the field.
“This program usually takes care of a lot of the easier weeds well, but then requires specific ‘tweaks’ to ensure effectiveness on the tougher, resistant broadleaf weeds,” Loux said. “On a general note, going with one of the soybean traits that allows for use of glufosinate or dicamba can help greatly in management of certain types of resistance, but there are other considerations to take into account for the use of these as well.”
Some of the Loux “tweaks” by weed include:
Essentially all of it is glyphosate-resistant and most of it has at least some degree of ALS resistance (site 2). Strategies to ensure a weed-free start at planting and/or make sure the standard “glyphosate + 2,4-D” burndown works are:
- Fall herbicide application
- Adding Sharpen and/or metribuzin
- Swap in dicamba for 2,4-D, or mix both if approved
- Swap in Gramoxone or glufosinate for the glyphosate.
Strategies to ensure control after planting include:
1. Use comprehensive residual with two non-ALS actives that has chance of holding until soybeans close canopy. For example, use a Valor or Authority product plus metribuzin, or metribuzin + Sharpen at 1.5 ounces
2. Use Liberty Link soybeans and apply glufosinate POST
3. Use Xtend soybeans and apply a dicamba product POST.
Most populations have lost at least some response to glyphosate and actual higher-level resistance is becoming more frequent. Glyphosate-resistant populations are often ALS-resistant also. Strategies to ensure a weed-free start at planting are:
2. Include 2,4-D or dicamba in burndown.
Strategies to ensure control after planting are:
- Use residual herbicide premix or tank mix where chlorimuron, cloransulam, or Scepter is one of the components
- Add a fomesafen product to POST glyphosate applications
- Use Liberty Link soybeans and apply glufosinate POST
- Use Xtend soybeans and apply a dicamba product POST
- The most effective POST approach is to apply first POST on six- to 10-inch ragweed plants, and follow with second POST application three weeks later. We do not recommend trying to make one POST application work unless the ragweed population is low. A two-application POST approach will ensure a lower frequency of surviving seed-producing plants at the end of the season, and ultimately reduce ragweed populations faster.
This has become a resistance problem primarily in northwest Ohio, where there are populations with resistance to glyphosate and ALS inhibitors. Use of PRE Valor products in combination with POST applications of fomesafen and Cobra/Phoenix is driving these populations to become resistant to PPO inhibitors as well. The number one recommendation for growers with this issue is to diversify the rotation to deemphasize Roundup Ready and non-GMO soybeans. Plant Liberty Link or Xtend soybeans, and plan on using glufosinate or a dicamba product POST.
While this weed can be found throughout the state, the problems with resistant populations tend to be more concentrated in west central/northwest Ohio. In these areas, all populations are resistant to ALS inhibitors, many are resistant to glyphosate, and some are becoming resistant to PPO inhibitors such as fomesafen. Waterhemp cannot be managed except through use of a comprehensive program of PRE and POST herbicides that incorporates as much diversity in herbicide use as possible. Waterhemp will adapt and become resistant to any herbicide site of action that is used repeatedly in POST applications, even with a year of corn in between, so diversification of soybean traits and POST herbicides over time is essential.
Strategies for waterhemp include:
- PRE residual products that contain two non-ALS herbicides provide the longest control (see tables in “Weed Control Guide for Ohio, Indiana, and Illinois)
- Apply a combination of glyphosate and fomesafen or Cobra to reduce selection for glyphosate resistance
- Use Liberty Link soybeans and apply glufosinate POST
- Use Xtend soybeans and apply a dicamba product POST
- Apply POST herbicides when waterhemp plants are less than four inches tall
- Include residual herbicide such as metolachlor, Warrant, or Zidua/Anthem in first POST applications to minimize need for a second POST application.
Loux also suggested some nonchemical weed control methods.
“The consensus among weed scientists seems to be that waterhemp and Palmer are easier to manage with a combination of tillage and herbicides than with herbicide alone, whereas giant ragweed can be easier to manage in continuous no-till,” Loux said. “The faster soybean canopy closure that occurs with narrow-row soybeans —15 inches or less — helps with suppression of late-emerging weeds. Weed scientists also acknowledge that cover crops in combination with herbicides can aid in control of certain weed species, including Palmer amaranth, waterhemp, and marestail. One role of weed suppression by cover crops is to simply reduce the number of weeds that are treated with herbicides, thereby reducing the rate of selection for resistance.”
Loux said winter cereals have been among the more effective covers for this purpose, especially for the pigweeds where survival of the cover into spring is essential.
“The key to marestail control with covers seem to be more along the lines of ensuring maximum biomass production and competition with weeds by late fall, which can eliminate the need for a fall herbicide treatment,” Loux said. “It’s important to recognize that the control provided by cover crops is variable, and that they require management based on soil type and spring weather to ensure viable integration into cropping systems.”