Common waterhemp with "potential resistance to glyphosate herbicide" has been found in two soybean fields in northwest Missouri by a weed scientist at the University of Missouri.

"Waterhemp grown from seed collected from suspect fields in 2004 shows high tolerance to glyphosate in two greenhouse dose-response trials this summer," said Kevin Bradley, MU Extension weed specialist. "The weeds were found in fields planted to Roundup Ready soybeans continuously since the new varieties were introduced in 1996.

"Common waterhemp is our No. 1 weed problem in corn and soybeans in most of Missouri," Bradley said. "With the introduction of Roundup Ready soybean varieties, glyphosate (the active ingredient in Roundup and similar herbicides) became the No. 1 herbicide used in soybean fields."

Roundup Ready varieties are genetically modified to tolerate the herbicide. Farmers spray glyphosate after the crop is growing, killing the weeds, while the modified soybean plants are unharmed. The U.S.
Department of Agriculture reported that 89 percent of Missouri's 2005 soybean acreage was planted to these genetically modified soybean varieties.

Herbicide resistance develops in weeds when a plant in a weed population has natural resistance to an herbicide, Bradley said. The plant is not killed and produces seeds. Over time, with continued use of the same herbicide, resistant plants dominate the population in a field as herbicide-susceptible plants are killed.

Histories from the suspected fields supported the theory that the weeds might be resistant, Bradley said. They have been in continuous soybeans, and glyphosate has been the sole herbicide used since 1996.

"Problems developed over the last three years at one site and the grower continued to use increasing rates of glyphosate," Bradley said.

To prevent development of herbicide resistance in weeds, extension specialists first recommend alternating herbicides with different modes of action. Second, Bradley said, a farmer can rotate growing corn and soybeans. "Generally, when you rotate crops, you also rotate herbicide modes of action."

MU specialists also encourage farmers to scout fields after spraying to find any "escapes," and then eliminate these with mechanical or spot treatments.

Common waterhemp, also known as pigweed, produces thousands of tiny seeds per plant. "Although waterhemp is not as competitive as some other pigweeds, high numbers reduce yields in corn and soybean fields,"
Bradley said.

In MU greenhouse tests, some waterhemp continued to grow after being sprayed with rates as high as 6 pounds of glyphosate acid per acre. "To put this in perspective, the recommended rate for the size waterhemp that we were spraying in these trials is 0.75 pounds of glyphosate acid per acre," Bradley said.

By definition, an herbicide-resistant weed must have the inherited ability to survive and reproduce following an application of an herbicide that normally kills it.

While his greenhouse studies confirm survival following labeled applications, Bradley is beginning inheritance studies to see if the resistant trait is present in seeds collected from surviving plants. He also will grow and treat plants under field conditions next season and conduct surveys of the suspected fields.

In 2004, MU weed scientists confirmed a case of common ragweed, from central Missouri, resistant to 10 times the normal rate of glyphosate. If Bradley confirms inherited resistance in common waterhemp, that weed will join ragweed, marestail and ryegrass as U.S. weeds found to have developed glyphosate resistance.

Bradley has been called to fields where glyphosate resistance was suspected. "Usually, we find some other cause for weed survival. Often weeds came up after the last glyphosate application."

Glyphosate has no residual activity. It must be applied to growing weeds to be effective.

Weeds grown in another suspected field in southwest Missouri did not show glyphosate resistance in the greenhouse.

In his study, Bradley compared six biotypes of common waterhemp. The control was seed from waterhemp in plots at the MU Bradford Research and Extension Center near Columbia that have never been sprayed with glyphosate.

The three suspected biotypes were compared with two lines of waterhemp that had shown "some tolerance" in tests several years ago. "All except the two biotypes from northwest Missouri were killed with the recommended rates of herbicide."

"This does not mean I recommend anyone moving away from Roundup Ready soybeans altogether," Bradley said. "It is very effective and economical for growers.

"However, the system must be used responsibly to sustain the technology."