Glyphosate resistance has taken another step in its march across the United States. University of California researchers recently identified a marestail (horseweed) population able to withstand four times the labeled rate of glyphosate in a research center near Fresno.

Although marestail resistance is not new in the United States, this case spotlights how dominant glyphosate resistance can be. Proper management has become more important than ever. Kurt Hembree, a U.C. Davis cooperative weed management farm advisor in Fresno County, who helped to identify the resistant population, said, “The location where this population was identified had been treated intensively with glyphosate for 15 years, with three or four applications a year. In just the last five years, we’ve seen the population of marestail in the (San Joaquin) Valley increase 10 fold.”

Problem marestail can be found in vineyards, canal and irrigation banks, roadsides and gardens, and also poses a threat to conservation tillage programs in California. An overwhelming majority of the problem lies in perennial crops, such as orchards and vineyards, and has consumed roughly 600 miles of irrigation banks running nearby. Since the seeds are wind-blown, it’s easy for resistant weed seeds to travel long distances and quickly enter new areas of the state. Due to groundwater regulations, glyphosate has become the most efficient herbicide to use for controlling weeds in this area. Should it become unavailable, options are very limited.

 “Glyphosate is simply the world’s greatest herbicide but, as a result, we are over-using this great resource and resistance is appearing worldwide, especially in the U.S. where glyphosate resistant soybeans, cotton and corn are dominant,” said Dr. Stephen Powles, a leading glyphosate resistance expert from the University of Western Australia. “Glyphosate resistant marestail is now a problem in China, Spain, South Africa and reported in a number of other countries as well as the U.S., where it is already present on well over 2.5 million acres.”

Researchers throughout the U.S. continue to investigate new cases of marestail resistance and are testing other potential weed species for resistance as well. Marestail resistance is confirmed in 11 states and this past year alone has spread to an additional nine counties in Indiana. Illinois has highly suspected populations in three counties. While Ohio has had resistant marestail for three years now, they are seeing some of the worst cases ever this year.

This week, palmer pigweed has surfaced as “probable” resistance to glyphosate in Georgia. While not yet identified as resistant, preliminary findings in field and greenhouse trials show a lack of control at labeled rates.

Glyphosate-resistant common ragweed was recently confirmed in Missouri and Arkansas, while giant ragweed is being tested for resistance in Indiana and Arkansas.

The spread of glyphosate resistance continues to grow and affect more weed species and more acres across the U.S and around the world. The trend follows very similarly to other patterns of herbicide resistance. For example, eight years after the first ACCase-resistant weed was identified, six weed species had developed resistance. Four years later that number had more than doubled to 13 weed species, while another five years later added another 15 species to the count. Glyphosate has only been intensively used in the U.S. since the introduction of glyphosate-tolerant crops in the mid-1990s. And, seven years after the first glyphosate-resistant weed was identified, seven species were found resistant. 

Susceptible (S) marestail plants were clearly controlled in these trials at U.C. Davis. By comparison, the resistant (R) population withstood 4 lbs ai/acre (4x) and was still lush.

“What is important to recognize is that persistent use of the same herbicide provides the ideal conditions for resistance to develop,” Powles commented. “Greater diversity is required than using the same herbicide every year on the same land - diversity in herbicide mode of action and use of non-herbicide weed control tools are the best way to minimize the likelihood of resistance.”

Syngenta Crop Protection recommends not using more than two applications of glyphosate on an individual field in a two-year period in glyphosate-tolerant (GT) corn and soybean systems. In GT cotton, up to three glyphosate applications may be used per crop year of employing in-crop cultivation and/or residual herbicides. Proper management can help control potential resistance and alleviate future problems.

The recent activity in California and around the rest of the United States only accentuates the need for action. “Two years ago, we didn’t see any resistant marestail out here,” Hembree said. “We need to do something and we need to do it now.”

Syngenta is a world-leading agribusiness committed to sustainable agriculture through innovative research and technology. The company is a leader in crop protection, and ranks third in the high-value commercial seeds market. Sales in 2004 were approximately $7.3 billion. Syngenta employs some 19,000 people in over 90 countries. Syngenta is listed on the Swiss stock exchange (SYNN) and in New York (SYT).