Beltwide '06: Herbicide Resistance: Don't Panic, But Be Proactive

Herbicide resistance isn’t the end of the world, but it does present a very serious, expensive problem for any grower who runs up against it. If comments by North Carolina State University weed scientist Alan York’s are any indication, many cotton producers now face that risk.

Since last year’s Beltwide, glyphosate-resistant Palmer amaranth (PA) has been confirmed in Georgia and it’s suspected in North Carolina and South Carolina. The confirmed cases of resistant PA follow a four-year spread of glyphosate-resistant horseweed, which now includes instances in 13 states, including 6 cotton states.

One indication of how seriously the resistance topic is being taken now is the position York had on the program at this year's Beltwide Cotton Conference. Last year, herbicide resistance rated a symposium just before lunch on the conference’s second morning. This year, York’s presentation came in the first half of the conference’s opening session, just after the National Cotton Council and Cotton Incorporated gave their annual updates. In other words, that’s the point in the Beltwide when you’ll likely have the biggest crowd.

Horseweed has been sort of a warmup game for dealing with PA, another weed scientist told me later that day. It got everyone’s attention and forced people to think about strategies and options. As York noted, there are effective burndown options when for glyphosate-resistant horseweed, provided you know you have the problem. But if a grower doesn’t go with the right burndown program and then resistant horseweed after emergence, he has limited options.

Because resistant horseweed has spread so quickly, York has been “telling our no-till growers that if they have horseweed (they should assume it’s a glyphosate-resistant biotype and act accordingly.” Being proactive at this point might head off worse problems later, he explained.

PA is another beast, altogether. Left alone, this pigweed species can tower above the crop canopy and grow about an inch a day during peak periods. It also is a prolific seed producer, and the resistance trait can move easily into subsequent generations.

York cited a comment made by Ford Baldwin, a former University of Arkansas weed scientist who now works as a private consultant and contributor to Delta Farm Press (DFP). Earlier last year, Baldwin noted that soybean rust had stirred up concern among farmers. But Baldwin, writing in DFP, said that if glyphosate-resistant PA became established it would make soybean rust look like “a Sunday School picnic.”

Not long after that, Stanley Culpepper with the University of Georgia announced that glyphosate-resistant PA had been found in his state. York showed a slide that Culpepper was to include in a later presentation at the Beltwide. It showed three plots with resistant PA – an untreated check, a plot treated 3 times with a normal glyphosate rate and a plot treated 3 times with a 4X rate. There was very little visual difference among the images.

“If you have Palmer amaranth back home,” York said, “these slides should scare you.”

Discovery of glyphosate-resistant PA has “brought concern about herbicide resistance to a whole new level,” York added.

The trick now is to avoid the problem where it hasn’t developed, said York, because that’s easier than dealing with a resistant population after it becomes entrenched.

Keep in mind, too, that no new chemistries are in the immediate pipeline, said York. In the past, growers could be somewhat complacent about herbicide resistance because they assumed industry would introduce the next “silver bullet” as a remedy, York said.

He alluded to the fact that the dominance of Roundup Ready technology discouraged most other manufacturers from researching and developing new herbicides. What we have now in the way of chemistry will be it for a long time.

As York emphasized, growers must work to maintain the viability of all available herbicides, not just glyphosate. One concern are the ALS inhibitors, which can be an effective tool against glyphosate-resistant PA, but their use across multiple crops sets them up, as well, for problems. ALS resistance in PA already has been documented, York noted.

So, how do you deal with the threat of resistance? York outlined a number of recommendations:

First, understand resistance. Herbicides don’t cause resistance. Continued use of a material simply selects out biotypes that have developed resistance, either through mutations or other processes. An estimated one plant in a million might carry a resistant trait. It might take as long as 15 years for a resistant population to spring up or as little as 3 years, York estimated.

Know your weed problems. That’s the best way, said York, to spot resistant plants early and deal with problems before they get out of hand.

Minimize movement. If resistance develops on one farm, clean equipment and/or schedule work in a way that reduces the chance to move resistant seed to new areas.

Reduce selection pressure. There are plenty of options, said York, like shifting more weed control steps to preplant tillage and crop cultivation. Crop rotation, where possible, can add new modes of action, especially when rotational crops aren’t Roundup Ready. That forces new weed control measures into the system. Rotation with a perennial grass is another option. Maintaining a sound fertility program and close control of early insects also keeps cotton plants competitive and allows them to hold down some weed pressure, York added.

Some of his suggestions, York admitted, go against long-term trends. Farmers who’ve shifted to conservation tillage don’t want to return to cold steel as a weed control tool. But tillage may be the only option in some cases if heavy resistance develops, he said.

York showed a resistance management program that is being promoted for 2006 in Georgia, North Carolina and South Carolina. The program specifies three elements: a residual herbicide at planting, a residual herbicide during the post-emerge period and a residual herbicide at layby.
“We’re asking growers to pick 2 or those 3 options and include them in their plan along with their normal glyphosate program,” York said.

The approach, he said, is “slanted toward (resistance management) of PA because it’s such a potential problem and also because we have concerns about glyphosate resistance.” He also cautioned against over-use of ALS-inhibitors because of concerns about losing them as an option for controlling glyphosate resistant PA that might develop.

How do you detect herbicide resistance? York said a few common sense steps will help determine if a problem is developing. These include:

• Eliminate other possibilities. Most control failures are unrelated resistance. Before assuming anything, make sure that the proper herbicide was used at the right time, on the right species and at the correct rate. Also, be certain that problems with spraying equipment or washoff aren’t to blame.
• Study the survivors. Did some plants of the targeted species die while intermingled plants in that species come through the treatment? That’s a possible indicator that resistant biotypes are present. Also, how did the treatment work on other species in its spectrum? If it took out other targeted weeds but missed a portion of plants in one species – like horseweed or PA – that could be a sign of early resistance.
• Look for clumps or groupings. If a plant deposited seeds with resistant characteristics, you’ll probably see multiple survivors in spots or areas of a field.