Beyond Trait Talk

By Jason Jenkins, Mill Creek Communications

Flip through the pages of soybean seed catalogs today, and the choices seem nearly endless. In 2020, growers are selecting from arguably the largest and most diverse set of herbicide-tolerant soybean technologies the industry has ever offered.

While characteristics — such as maturity group, soybean cyst nematode resistance, disease tolerance, insect resistance and, of course, yield potential — weigh heavily on seed choices, there’s no question that herbicide-tolerant traits play a large factor in what seed gets loaded into a planter.

According to the International Survey of Herbicide Resistant Weeds, 512 unique cases of herbicide-resistant weeds worldwide have been identified as of early 2020. Resistance was reported in 262 different species and in 93 crops across 70 countries. Of the 26 known herbicide sites of action, weeds have evolved resistance to 23 of them.

Maintaining weed control is vital to soybean profitability. While herbicide-tolerant traits have in many ways become the weed-control program, MU Extension Weed Scientist Kevin Bradley says it’s important for growers to implement a complete herbicide program to minimize weed pressure.

“Where we are today, it’s not like it used to be when we got started with Roundup Ready,” he says. “Growers need to think beyond just the trait for weed control.”

Pre-Plant Priorities

Regardless of the herbicide-tolerant traits that growers elect to plant, Bradley recommends they adopt a “two-pass approach” to their herbicide programs. While the specific products applied can vary, the goal is to gain weed control through the combination of both pre-emergent and post-emergent soil-residual herbicides with multiple effective modes of action.

“From some growers’ perspective, they think that if they have a dicamba system or a 2,4-D system or whatever the new trait technology happens to be, they may not need a two-pass approach, but I disagree with that,” he says. “To me, the most important herbicide pass you’regoing to make is that pre-emergentpass.”

A pre-emergent herbicide can be applied before, during or just after soybean planting. Depending on what herbicide resistance may present in a field, these products might include Authority, Valor, Fierce, Sonic, Boundary, Prefix, Zidua Pro or Trivence, to name a few. The timing of this application should be based on field conditions.

“Ask yourself, ‘What am I putting this on?’” Bradley says. “Is it a no-till field, one that’s been lightly tilled, or do you have cover crops to deal with?”

Previous research has found little difference in the timing of the application of pre-emergent herbicides between no-till and more traditional tillage systems. However, new research from the University of Missouri — published in the January-February 2020 issue of Weed Technology — found that the timing of cover crop termination affected the quantity of pre-emergent herbicide that reached the soil.

After establishing several cover crop species in the fall, the researchers applied a combination of 2,4-D and glyphosate either 21 days or 7 days before soybean planting to terminate the cover crops. A pre-emergent herbicide, Authority Maxx, was included in the burndown mixture. The concentration of Authority in the soil profile was measured immediately after application and throughout the growing season to determine how much was intercepted by the cover crop and how much actually reached the soil to provide residual weed control.

When averaged across cover crop species in the experiment, there was about a 50 percent increase in cover crop biomass between the early and late termination dates. This increase in biomass reduced the amount of Authority directly reaching the soil by more than half. The end result was a 20 percent reduction in waterhemp control between the two termination dates.

“Based on our research, we say keep the pre-emergent application out of your burndown if you’re terminating cover crops that are waist high or taller,” Bradley says. “Too much of that residual herbicide is getting tied up by the cover crop. It won’t be available for soil uptake, so you’re wasting money there. It will be better to come back later with the residual.

“Now, if you’re burning down that cover crop earlier, say when it’s no more than knee high, our data says it’s perfectly fine to include your pre-emergent,” he adds. “You’re going to get the benefit out of the herbicide.”

Bradley is often asked what pre- emergent residual is best for controlling waterhemp, Missouri’s top herbicide- resistant weed. The weed scientist says soybean growers do have choices.

“Look, I’m not trying to shy away from taking a stand,” Bradley says with a little laugh. “There are numerous products that provide equivalent waterhemp control, and we’re lucky that’s the situation.”

Instead of focusing solely on waterhemp, Bradley says producers should select a pre-emergent product that will control the pigweed species, while also targeting the weed of second-most concern in a field. In some fields, that might be a grass species. In others, it could be marestail or even ragweed.

“Yes, start with your worst weed, but then work backward from there as far as your pre-emergent selection,” he adds. “That’s how you can start to differentiate between products.”

The State of Traits

In 2020, a familiar offering of herbicide-tolerant seed technologies is available to soybean producers. Most varieties that will be planted in Missouri fall into one of three genetic platforms: Roundup Ready, LibertyLink and Enlist.

This will be the sixth growing season that the original glyphosate-resistant soybean, Roundup Ready 1, will be off patent, allowing growers to save seed, if they so choose. The trait still is available through some university breeding programs and smaller seed suppliers, even though Monsanto, now owned by Bayer, phased the trait out of its stock years ago. Roundup Ready 2, which entered the market in 2009, also remains a standalone option for growers.

“Honestly, I don’t hear a lot of people just sticking with plain old Roundup Ready because glyphosate isn’t effective against waterhemp at all,” says Bradley of the Group 9 herbicide. “Most are going with Xtend beans.”

Roundup Ready 2 Xtend soybeans, which are tolerant to both glyphosate and dicamba, received USDA approval in 2015. Last year, the trait accounted for 60 percent of the soybean crop planted in the United States, covering roughly 54 million acres. The technology has faced heightened scrutiny: Dicamba injury from off-target movement the past three seasons has resulted in a new set of restricted-use labels for the herbicides.

“The federal registration for the approved herbicides all expire this December, so they’ll be up for re- registration,” Bradley adds.

While not yet commercially available, the next generation of the Roundup platform — XtendFlex — could find its way into some fields in 2020, pending regulatory approvals. This trait provides triple-stacked tolerance to glyphosate, dicamba and glufosinate, the active ingredient in Liberty herbicide.

Glufosinate, a Group 10 herbicide, is the over-the-top active ingredient to which LibertyLink soybeans are tolerant.

First planted in 2009, the LibertyLink platform still remains a standalone option for this growing season. However, there are new developments in the LibertyLink family that may soon be available.

BASF, which owns the LibertyLink platform, has launched Credenz soybeans with both the LibertyLink and the new GT27 trait. These soybeans are tolerant to glufosinate, as well as glyphosate and the Group 27 herbicide, isoxaflutole.

“It’s the active ingredient in Balance, which we’ve used in corn since the late 1990s,” Bradley says. “If you sprayed that on other soybeans, you’d kill them, but it’s designed for use with their trait pre-emergence. It’s a different mode of action that we don’t use in soybeans, so that’s one way it might be helpful.”

BASF is still awaiting approval of Alite 27, its trademarked isoxaflutole herbicide, for use on LibertyLink GT27 soybeans.

Bradley adds that Credenz is the first example of a soybean trait that confers pre-emergent herbicide tolerance, rather than tolerance to a product sprayed post emergence. He says more such traits will likely be coming in the future.

The third and most recent technology platform available to Missouri soybean growers is the Enlist E3 system, an offering from Corteva Agriscience first available in 2019. These beans have a trait that confers tolerance to glyphosate, glufosinate and the Group 4 herbicide 2,4-D choline. Corteva currently has two herbicide products registered for use on Enlist soybeans: Enlist One, a 2,4-D choline product, and Enlist Duo, a glyphosate and 2,4-D choline pre-mix. Bradley anticipates that 90 percent or more of Missouri soybean producers will utilize one of these three technology platforms — Xtend, LibertyLink or Enlist — in 2020. While each is unique, he says they all have one thing in common.

“None of them are like Roundup used to be. They are all very sensitive to weed height,” says the researcher. “When Roundup was working, it could kill something a foot or two tall the same as something four inches tall. With any of these systems today, you’re not going to kill 12-inch-tall waterhemp as effectively as four-inch-tall waterhemp. You really have to pay attention to weed size and make your application at the right time.”

Each also has a specific set of application parameters and guidelines that growers absolutely must follow. This includes everything from application rates and environmental conditions to the correct nozzles and tank cleanout procedures.

“We really have to pay attention to those details a whole lot more,” Bradley says. “It’s not just a matter of following the label. It’s the law.”

Of the choices currently available to suppress weed pressure and management against herbicide resistance, Bradley says he’s a proponent of those that offer multiple modes of action against weeds.

“Look at the Enlist system. You can spray Liberty and 2,4-D together, so that’s two effective modes against waterhemp,” he says. “It’s all a numbers game with resistance. Anytime we’ve sprayed just one effective mode in the past, it’s usually resulted in resistance at some point. By spraying two, I’m taking that potential for resistance and minimizing it greatly.”

Mindful of Management

Regardless of the technology platform
a producer elects to plant, adopting the two-pass approach is key to maximizing the efficiency of any herbicide program. When successful, the soybean canopy closes and weed control is achieved. One to two weeks after the post-emergence application, fields should be scouted, and any surviving weeds should be rogued before they reach reproductive stage.

“Rogueing out anything that didn’t get controlled is your best bet at a clean field in the future,” Bradley says. “It may not be all that big of a deal from a yield standpoint this year, but from a seed standpoint and what’s going into your soil next year, it’s a huge deal.

“Growers who are willing to go to that next step, who aren’t satisfied with a weedy field, who have an attention for detail, who mix effective modes of action, they’ll have the cleanest fields.”

EDITOR’S NOTE: This is the second article in a yearlong series examining the past, present and future of weed control in the production of soybeans in the Show-Me State. This issue, we look at current options growers have for chemical control of weeds in their soybean fields. To learn more, visit iwilltakeaction.com.

Photos courtesy of United Soybean Board. Find the entire April issue of Missouri Soybean Farmer here.