Herbicide rotation just as important as crop rotation for sunflowers
By Jennifer M. Latzke
Kansas has historically been known as the Sunflower State. But with the recent confirmation of populations of glyphosate-resistant Palmer amaranth in the state, the most recognizable state symbol could someday be a weed. That poses a problem for sunflower growers.
"One of the major needs expressed by growers over the last four or five years is the need for better broadleaf weed control in sunflowers," said Phil Stahlman. "Palmer amaranth is a growing concern in sunflowers and kochia as well." Stahlman is a weed scientist at the Kansas State University Agricultural Research Center in Hays, Kan.
In fact, he said, in addition to the widespread presence of glyphosate-resistant kochia throughout western Kansas, there are limited pockets of glyphosate-resistant Palmer amaranth in parts of central Kansas. And there are even some populations of Palmer amaranth that are resistant to the HPPD class of herbicides, such as Callisto, Impact or Armezon, and Laudis.
Typically, a sunflower grower would ensure that his field was clan before planting, using a glyphosate burndown, but with glyphosate resistance in weeds, he needs to have other options available. And that doesn't mean just a crop rotation, he added.
"If we've learned anything from resistance management, it's that we need to do a whole lot better of a job with our herbicide rotations as well as our crop rotations," Stahlman said. "Yes, we certainly want to remind growers to rotate their modes of action, and simply rotating crops may not be enough--especially if they use the same products on the next crop, or even products out of the same family in successive crops. That's going to increase selection pressure."
Stahlman gave the example of a farmer raising soybeans and using the product Authority. Authority has the same active ingredient as Spartan, "so, if they're using Authority on soybeans, and planting back with sunflowers and using Spartan, they may not realize they are applying the same active ingredient in successive crops," he said. Farmers really must read those labels and check the active ingredients more than relying on product names."
Building on success
Relying on one successful chemical year after year is a sure path to building up a resistant weed population, so Stahlman and his colleagues continue to evaluate chemical ingredients to find a solution to weed control. What growers would like is an herbicide that could control broadleaf weeds in sunflowers, without damaging the sunflowers themselves.
A little more than a decade ago, the active ingredient sulfentrazone, or, its more common product name, Spartan from FMC, was found to be such an ingredient. Sunflower producers call it their workhorse because it is effective.
"Fortunately, sunflowers have quite a bit of tolerance to the product when it is used correctly," Stahlman said. "Sulfentrazone, we knew, worked well in soybeans when it was first developed, and we knew sunflowers had tolerance to it, but its effectiveness and sunflower tolerance were affected by soil texture." He explained that coarse textured soils with high pH tended toward more crop injury after application than a more neutral soil with finer texture.
"We've advised for years to reduce the risk of possible injury to apply sulfentrazone as a pre-plant application, rather than a pre-emergence application," Stahlman said. "It's good to have it on a couple of weeks prior to planting flowers to increase chance of rainfall activation and reduce risk of crop injury."
Sulfentrazone is a great active ingredient, to be sure, but researchers continue to look for the next ingredient or combination of ingredients to give farmers more tools in their weed control toolbox. Thus came a product called Broadaxe, also from FMC, which is a combination of sulfentrazone and S-metolachlor, the active ingredient in Dual Magnum. It was released for the 2012 sunflower season and so far it offers longer residual, better control of early season grasses, and a broad control spectrum of broadleaf and grassy weeds like Palmer amaranth and foxtails.
Broadaxe is an example of researchers' looking at new combinations of ingredients to address weed control needs. Sulfentrazone and S-metolachlor have both been known effective active ingredients for several years. S-metolachlor has been around in one form or another for about 30 years, Stahlman said. "Both are fairly established and older herbicides but have been put together in this new formulation," he said.
Because it contains sufentrazone, much like Spartan, growers must be aware of their soil conditions before using Broadaxe on sunflower fields. "And, really, if the soil is too coarse, you really should reconsider growing sunflowers on that ground in the first place," Stahlman said. For those farmers who have no other options but to plant on that ground, they could try a formulation of Pendimethaline herbicide, commercially known as BASF's Prowl H2O, Stahlman said. "There's also some generics out there that are straight S-metolachlor that could work," he added.
The next chemical that may benefit sunflower growers is the active ingredient pyroxasulfone, which is the active ingredient in the commercial herbicide Zidua from BASF. Stahlman said this herbicide is currently only labeled and approved for use in corn, but there is work toward having regulatory approval for its use on sunflowers sometime in 2013.
"We started looking at it several years ago," Stahlman said. "We've been doing additional work with it and have found it quite effective in controlling broadleaf weeds and grasses. It works best in combination with sulfentrazone. Most applications have been done as a pre-emergence treatment, but based on what we know it should be effective as a pre-plant treatment. We just still need more data to back that recommendation up."
There's also some products from FMC and Valent in the pipeline that could help sunflower growers handle broadleaf weeds, too, Stahlman added.
Growers becoming aware
Stahlman said growers are becoming more and more aware of their weed populations, more so than they were even three or four years ago. And they're paying attention to their control methods like never before.
"For example, we have known that we have a big problem with glyphosate-resistant kochia, so we had a graduate student survey crop consultants in Kansas about their issues with kochia," he said. "These consultants scout more than a million acres in Kansas and eastern Colorado and the Oklahoma Panhandle. We asked them about three time periods: pre-2007; 2007 to 2010; and 2011 to 2012.
"We didn't have confirmed glyphosate-resistant kochia before 2007," Stahlman explained. "We confirmed it in 2007 and it escalated in three years. In 2011, the population exploded."
The crop consultants said that before 2007 slightly less than half of the crop land in western Kansas had kochia infestation, around 47 or 48 percent. "By 2011 and 2012, that number grew to more than 70 percent of the fields scouted," he said. "They were seeing an increase in kochia's presence overall. Of that 70 percent, in 2011-2012, not quite one-third of those infested fields were considered to be resistant." Stahlman added that the use of glyphosate increased by 50 percent over that five-year period, and consultants reported that they were using higher glyphosate rates and more frequently.
"These are things we know that are important for selecting for resistance in weed populations," Stahlman said. "I'm happy to say, though, that starting in 2011 consultants were trying to do things differently. There are relatively fewer consultants totally relying on glyphosate to control weeds and they're starting to mix in other modes of action, using different timing of applications. Some have even resorted to tillage."
"We may never rid ourselves of the resistance issue in kochia, because it will be found in roadside ditches and fencerows," he continued. "But, kochia, unlike Palmer amaranth, has a weak point. The seed doesn't retain viability in the soil more than a couple of years. So, if we can prevent plants from going to seed and replenishing the seed bank and do that for a three-year period or longer, we can dramatically reduce the population."
It's a step in the right direction, and one that may help keep the state symbol a bright and sunny one for years to come.
Jennifer M. Latzke can be reached at 620-227-1807 or firstname.lastname@example.org.