Weeding Out Resistance
March 5, 2009 
The challenge in growing row crops is back. Corn and especially soybeans had almost become a management no-brainer for retailers and their grower-customers, especially with the adoption of glyphosate-resistant crops.
But all that reliance on glyphosate in the majority of U.S. corn and soybean acres has led to resistant and tolerant weeds. The other shoe has dropped, and as these problem weeds — along with the weeds that are resistant or tolerant to other herbicide chemistries — become more widespread, so does the need to keep an eye on them.
How are retailers dealing with this issue? CropLife® magazine talked to some CropLife 100 retailers to get their take on it.
Waterhemp Worries
MFA Inc. services most of Missouri, the state that documented the first case of triple-resistant weeds, so they know a bit about the problem. “We have major resistance issues across the majority of our territory,” says Paul Tracy, MFA’s director of agronomy services. MFA also serves portions of southern Iowa, eastern Kansas, and northern Arkansas, and its growers have encountered weed resistance to glyphosate, ALS (acetolactate synthase), PPO (protoporphyrinogen oxidase), and triazine herbicides. The No. 1 glyphosate-resistant weed in Missouri is waterhemp.
“In the northern part of the state, we’re probably looking at a rough estimate that 75% to 80% of the counties have some glyphosate resistance,” Tracy says. But “probably less than 20%, maybe less than 10%, of the acreage has glyphosate-resistant waterhemp.”
Horseweed, also called marestail, is problematic in southeastern Missouri, while weed resistance is just moving into the southwestern region.
Like many retailers, MFA recommends resistance prevention practices of crop rotation and mode-of-action rotation. “That’s old stuff; we’ve all been preaching it for years and years, but it doesn’t always sink in with growers,” Tracy says.
He surmises that one reason Missouri has more resistance than some other Midwest states is its abundance of continuous soybeans. “In some areas, we have a lot of continuous beans for eight, 10, 12, 15 years in a row,” he says. MFA now tells its growers to rotate to corn for a year or two and use atrazine and/or other corn herbicides to knock out the waterhemp.
MFA chooses to keep the strategy simple for growers, focusing on the use of older and new residual chemistry along with using other herbicide modes of action. “We have always recommended some residuals down with our Roundup Ready corn, and during the last two or three years, we’ve strongly recommended residuals with our Roundup Ready soybeans as well,” Tracy says.
“It is also a way to preserve crop yields if we cannot get across the field in a timely fashion with the first postemergence application of glyphosate,” says Kevin Bradley, University of Missouri Extension weed specialist.
Tracy points out that there are still a lot of fields that glyphosate works very well on. “We’re still going to be promoting glyphosate-tolerant genetics as the base of our weed control program,” he says. “We just have to deal with all these resistant weeds that are coming along.”
Pesky Pigweed
The resistance picture is somewhat different in the South. While marestail has been around for a few years, Palmer amaranth-resistant weeds have been building up even more in soybeans and cotton. Also known as pigweed, this resistant population is expected to “explode” this season, according to Ken Smith, University of Arkansas Extension weed specialist. Currently, 19 counties in the state have confirmed glyphosate-resistant Palmer amaranth, and estimates of some level of infestation in Arkansas are in excess of 650,000 acres.
“Farmers have started realizing things are going to have to start being done a little differently,” says Ben Branch, location manager at Ritter Crop Services in Marked Tree, AR. “We have resistant weeds that are popping up in pockets in our area, and we’re having to change our whole plan basically to manage that problem. These weeds didn’t survive thousands of years without being able to change, so they’ve adapted to the glyphosate chemistry.”
Ritter Crop Services’ 12 outlets serve growers in the northeastern quadrant of Arkansas with 10,000 to 15,000 acres of cotton and “tens of thousands of acres of beans,” Branch says. “Just about every farmer here probably has a resistant or tolerant species of pigweed in a field. And if the beans start canopying over a little bit and the herbicide doesn’t just penetrate down to the small pigweed tucked in underneath, just a little bit of Roundup might not kill it. Instead, all it does is just build the resistance up for the next year, for the next population from that plant.”
The retailer’s resolution is to put down residuals, even overlapping residuals and changing the herbicide mode of action as much as possible. “It’s kind of hard to do in cotton and beans, but crop rotation’s a big help,” Branch says. “You can go to milo or corn and use a different form of chemistry on your problem fields.”
They recommend one of Syngenta‘s Dual products — an older chemistry — and Syngenta’s Reflex and Valent U.S.A. Corp.‘s Valor, which are newer. This application would be followed by Roundup as needed.
“The first shot in the crop, we’re definitely talking about putting out some type of Dual product (an S-metolachlor from Syngenta), or some type of metolachlor, just to have a double whammy because once you get the weeds up, it’s hard to kill them,” Branch says.
Smith adds that his office recommends using a residual like Valor or Syngenta’s Prefix as a preemergence or preplant application, scout closely for escapes, and treat while small. Or rotate to a LibertyLink soybean program. “We have some very good recommendations for control of pigweed in LibertyLink soybeans,” he says. Including atrazine in the corn program is still providing excellent pigweed control in the state.
Ritter Crop Service’s staff has been sitting down with growers to talk about the process. “Some of them have already grasped the whole process, and they see that’s what they’re going to have to do,” explains Branch. The extra step of spraying weeds when they’re small is much like conventional soybean management, a practice many haven’t used since switching to Roundup Ready crops.
“It’s going to require a little more management,” Branch adds.
Lessons Learned
MFA’s growers have learned to deal with triazine resistance, which preceeded glyphosate resistance. “We still use atrazine on a large percentage of our corn acres,” says Tracy. “Just because we have resistance to a certain chemistry doesn’t mean we’re going to lose that chemistry, it’s just not as effective as it used to be.
“I always use atrazine as an example when people get really, really fearful of resistance with glyphosate,” he adds. “I use that as an example of something that we have resistance to and we’ve learned to deal with. After we hit field after field year after year with ALS chemistries, we got resistance that’s still out there. But some of those ALS products are coming back around as tools to fight glyphosate resistance.”
Some of MFA’s growers will be planting the recently approved LibertyLink soybeans, available on a limited basis for 2009. “That does give them another bullet in their gun to fight resistance,” Tracy says. “But the bottom line on this is unless we change modes of actions on herbicides or management practices one way or another, we’ll get resistance no matter what we throw at these weeds; we know that.”
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