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Weed scientists develop plan to combat glyphosate resistance

Roundup Ready technology has created a revolution in cotton and soybean production. Eighty-five percent or more of the soybeans and 95 percent or more of the cotton in the Southeast is Roundup Ready, and Roundup Ready corn is catching on fast.

Over reliance on any one thing can often lead to problems. In the case of Roundup Ready, there is increasing concern over the potential for weeds to become resistant to glyphosate.

By now, most growers have heard of the problems with glyphosate-resistant horseweed (also called marestail) in western Tennessee, Arkansas, and the Delmarva region.

Horseweed resistant to glyphosate was confirmed in North Carolina in 2003, and the problem now exists in at least five counties. It is also suspected in northern Alabama.

During the summer of 2004, common ragweed resistance to glyphosate was found in Arkansas and Missouri.

Are these isolated problems? Or, are we just beginning to see the tip of the iceberg?

To investigate this concern, the Southeast Farm Press recently sat down with three top weed scientists in the Southeast: Alan York of North Carolina State University; Stanley Culpepper of the University of Georgia; and John Wilcut of North Carolina State University. Greg Elmore of Monsanto also offered insight about glyphosate resistance.

According to the weed scientists, herbicides do not cause resistance. Rather, herbicides select for resistance already present in the population.

Herbicide resistance, defined by the Weed Science Society of America (, is the inherited ability of a biotype of a weed to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type.

A biotype is a subgroup within a species that possesses certain traits or characteristics not common to the entire population. A biotype, for example, may be resistant to a particular herbicide that kills most individuals within that species.

According to York, there are two prerequisites for resistance to develop. First, there must be at least one plant (biotype) present which has the genetics that allow it to be resistant to a particular herbicide mode of action, and second, one must select for that biotype by using herbicides with that mode of action.

Herbicide-resistant biotypes are normally present in the population at very low frequencies — one in a million to one in 100 million. The higher the frequency, the more quickly resistance can develop. “A one in a million frequency, such as we see with ALS-inhibiting herbicides such as Scepter, Classic, and Staple, is a high frequency,” Wilcut says. “Within three years of registration, there was resistance to ALS materials reported.” The frequency of resistance to glyphosate is unknown but thought to be very low. In that regard, one might assume the potential for resistance to glyphosate is low. And, until recently, that was the prevailing opinion among weed scientists. The situation changed, however, with the wide-spread adoption of Roundup Ready technology. According to Culpepper, one must also consider the frequency of use. “The extensive use of glyphosate on multiple crops certainly increases the risk of resistance evolution.”

There are two strategies to deal with resistance. One is to simply gamble on it not happening, and then deal with the problem should it occur. This is an unacceptable strategy, for several reasons.

First, growers have come to expect regular introduction of new chemistry into the marketplace. When resistance to one product developed, growers historically switched to a new product. Unfortunately, development of new herbicide chemistry has almost ceased. As Wilcut points out, “There have been only two new herbicide modes of action in the U.S. marketplace in the last 20 years.” There is likely not a magic bullet coming to replace glyphosate.

Second, growers need to stop and think just how much they depend upon glyphosate. Growers have embraced Roundup Ready technology because of the broad spectrum control by glyphosate, the convenience, and the overall economics of the system. Moreover, the technology has been instrumental in wide-spread adoption of no-till or strip-till systems, which have reduced equipment and labor costs and allowed growers to expand acreage while enhancing environmental quality.

“The potential impact of glyphosate resistance is too great to ignore,” says Culpepper. “We simply can’t go back to where we were before Roundup Ready technology came on the market.”

Third, it is critical to keep glyphosate as an effective and viable tool because of resistance, or potential resistance, to some of the products that would be used as alternatives to glyphosate. For example, there are many known cases of resistance to ALS inhibitors (Classic, Staple, Scepter, others), organoarsenicals (MSMA, DSMA), triazines (atrazine, Princep), and the postemergence grass control herbicides (Poast, Fusilade, Select, Assure). “Resistance is not unique with glyphosate,” said York. “What makes glyphosate resistance so important is our level of dependence on glyphosate.”

A much wiser strategy would be to manage to avoid resistance evolution. Such a strategy centers on reducing selection pressure.

“Selection pressure is the key,” says York. “You have no way of knowing whether that ‘one in a 100 million’ plant is present or not. You just assume it might be present and your resistance management strategy becomes one of reducing selection pressure on that plant.”

Reducing selection pressure means reducing one’s dependence on a particular herbicide mode of action, in this case glyphosate. This can be accomplished by using a non-glyphosate program in crops rotated with Roundup Ready crops or integrating other chemistries into a program for Roundup Ready crops.

When one is rotating crops, Culpepper advises a non-Roundup Ready crop in the rotation. A high percentage of the cotton and soybeans in the Southeast are Roundup Ready. However, peanuts are not Roundup Ready, and most of Georgia’s 750,000 acres of peanuts are rotated with cotton. “A peanut-cotton rotation is an excellent way to break the glyphosate cycle”, says Culpepper. “There are eight herbicide modes of action to use in peanuts, all different from glyphosate.”

The weed scientists are much more concerned with continuous planting of Roundup Ready crops, such as back-to-back cotton, or Roundup Ready cotton and Roundup Ready soybeans in rotation, or even a soybean and corn rotation now that Roundup Ready corn is catching on fast.

Cultivation is a very effective resistance management tool, but most growers are no longer cultivating. And, it is not practical to expect growers to abandon the Roundup Ready technology. Hence, with continuous planting of Roundup Ready crops, the only practical resistance management strategy is to integrate other chemistry into the system.

The reasoning behind using multiple chemistries in the same crop is simple. The idea is that if a biotype resistant to one herbicide is present, the second herbicide will kill it before it can reproduce and build up.

York, Culpepper and Wilcut believe the effective life of glyphosate can be extended another 15 years by judicious use of other active ingredients in the battle against weeds.

It comes down to a selling job — whether the extra expense is worth the added value to the farmer, the weed scientists say.

“It is hard to ask a grower to spend additional money for other chemistry to use along with glyphosate to deal with a problem he currently does not have and may never have,” says York. And yet, each of the weed scientists agreed that a strong, proactive program to avoid glyphosate resistance is greatly needed to keep glyphosate working effectively into the future.

As York pointed out, however, integrating additional chemistry into a Roundup Ready program can bring immediate value, in addition to helping avoid resistance. “Cotton is a good example,” said York. “Many of our growers are already using other chemistry in the system to help with various problems. They may be putting down a pre-emergence herbicide, such as Prowl, to control Florida pusley or to buy some time before having to start back with glyphosate. Many growers are mixing Staple with glyphosate to boost morningglory control, or mixing Dual with glyphosate to provide residual control of grasses and pigweed and extend the window for layby application. Many are also still using conventional chemistry for layby, or at least mixing something else with glyphosate at layby to help with morningglories and maybe give some residual. In addition to helping with current problems and bringing value, these practices are also good for resistance management.”

University weed scientists are not alone in their concern over glyphosate resistance. Greg Elmore of Monsanto pointed out that his company takes this issue seriously. Monsanto is encouraging use of 2,4-D with glyphosate in burndown programs for horseweed and other tough to control weeds such as cutleaf evening primrose. The company also supports the use of residual herbicides, either applied pre-emergence or postemergence with glyphosate, where appropriate.

“Residuals have value,” says Elmore, “whether it is for tough weeds or time management, and many growers are including residual herbicides into their Roundup Ready weed management programs”.

Both Monsanto and the university weed scientists advised against cutting glyphosate rates. Monsanto’s Elmore pointed to a long-term study that looked at glyphosate rates in the Midwest. The conclusion: reducing rates created weed shifts. A connection between low use rates and resistance is less clear.

The weed scientists encourage growers and others to be on the alert for resistant weeds. Typically, about 20 percent of the population has to be resistant before a grower realizes he has a problem.

Unfortunately, “the horse may already be out of the barn” by then. Early detection and response can help in preventing spread to other fields and farms.

As the weed scientists point out, there are many causes of herbicide failures. Most herbicide failures are not due to resistance. When a control failure is noted, growers need to first eliminate the more likely causes, such as poor weather for herbicide activity, poor application technique, poor timing of application, etc.

If, after considering all the possible causes of herbicide failure, resistance is still suspected, growers are encouraged to seek assistance in verification from their county agent, dealer, or consultant.


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