"Glyphosate will be driven to redundancy over the next few years, and that is lamentable because the herbicide glyphosate is simply the world's greatest herbicide," said Steve Powles, Ph.D., University of Western Australia professor. Young weed scientists of today "will never again see a herbicide as good as glyphosate. It is a one in a 100-year discovery that is up there with penicillin's discovery."



Powles is one of the foremost herbicide resistance experts in the world, and he was in the U.S. recently to participate in the Pan-American Weed Resistance Conference conducted by Bayer CropScience. Approximately 200 university specialists from almost 20 countries participated.



"We should do everything to try and keep glyphosate sustainable," Powles said. "In some parts of the U.S., glyphosate is lost because the gene pool of resistance is too high. There are many parts of the world where that is not true."



Glyphosate is without sufficient effectiveness in many areas of the Mid-South and South and is quickly losing efficacy against major weeds in the main Corn Belt. Brazil and Argentina also have small areas of insufficient weed control and a rapidly spreading problem.



Most portions of the world still can use glyphosate because it is highly effective, including most of Canada and all of Europe where glyphosate weed resistance continues to be minor. Europe's situation is because Roundup Ready crops have not been approved for production.

Continous Use Problems

Continuous use of a herbicide leads to resistance, Powles explained; therefore, the U.S. has surpassed Australia as being number one in herbicide weed resistance. His estimate is that Australia and the U.S. both currently have 15 million hectares of herbicide-resistant acres, but the U.S. is rapidly increasing its acres because of glyphosate over use.



The cause of herbicide weed resistance is "lack of diversity" in production, with the same crops being grown and treated with the same herbicides. Australia earned the number one herbicide resistance distinction first for growing wheat year after year and having the same mode of action herbicides applied. The U.S. has now earned its distinction for weed resistance by growing glyphosate-resistant corn, soybeans and cotton continuously.



"Weed herbicide resistance is all about evolution," Powles said.



Broadleaf weeds and grasses evolve in developing resistance, he contends, by developing mechanisms for resistance. He went on to explain the difference between target sites resistance and target site enzymes and non-target site resistance and non-target site enzymes.



"There are some weeds that easily develop resistance because they have biology that enables them to do so. They are resistance prime species," Powles noted.



Usual contenders for this designation are ones that produce a lot of seeds or offspring and are "cosmopolitan" in that they are spread around the world. They are even susceptible through cross pollination to developing more than one mechanism of resistance.

Resistance Patterns Studied

Powles pointed to triazine herbicide resistance as occurring globally because of one mechanism or mutation of one gene, which resulted in a target site mutation.



This early resistance pattern strongly influenced herbicide resistance thinking that all resistance would be target-site based.



But the scientific view quickly changed when ALS herbicides began showing resistance, much of it in Australia.



"We saw 22 different mutations within the ALS enzyme — 22, not one as with trazines," he said.



The lesson learned was that scientists must select for all possible mutations including target site and non-target site mutations on a case by case basis.



"There are three different mechanisms recognized for resistance to glyphosate," Powles said. "Not too many years ago people said plants cannot develop resistance to glyphosate, but already there are three different mechanisms."



In speaking before a crowd of scientists and a handful of Bayer CropScience representatives, Powles noted that some people might think the loss of glyphosate efficacy is a good thing because other herbicides can step in to replace it. He contends that farmers using glyphosate until it fails is a completely unwise decision.



"We, as professionals, should do everything in our power to keep glyphosate working and used, except in those areas where it has already failed," he said.



Maintaining effectiveness should be the goal when recommending or using any herbicide. "Herbicides need to be used with diversity if we want these precious resources to last," Powles said. "Diversity is the key word."