Glyphosate-resistant weeds By Kevin Bradley

The following article was written concerning glyphosate-resistant weed development and was prompted, in part, by the of official announcement from Dr. Reid Smeda about the discovery of a glyphosate-resistant common ragweed biotype in Missouri.

So far, this common ragweed population is confined to approximately 20 acres in a field with a history of glyphosate use in long-term soybean production. In initial greenhouse experiments, this biotype appears approximately 10 times more resistant to glyphosate than a susceptible common ragweed biotype. In field trials, the addition of lactofen (Cobra or Phoenix) to a standard in-crop glyphosate treatment significantly enhanced control of this species.

It was also discovered that glyphosate-treated ragweed plants were infested with a stem boring insect called the ragweed borer, and it is not known if the insect compromised some of the glyphosate activity in the field allowing plants to survive. However, all greenhouse dose-response experiments were conducted in the absence of the ragweed borer.

In addition, resistant common ragweed plants exhibit a shorter growth habit, and Dr. Smeda is currently conducting research to determine if herbicide retention is altered, possibly contributing to resistance.

It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere five years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone.

A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates.

The comments in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. However, the question that a person should ask about these studies is whether they can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test whether resistance will develop.

For example, the University of Wisconsin has a seven year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in these plots.

Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio or Tennessee or Delaware is not truly resistant? Obviously not. Small scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small fraction relative to total glyphosate. The true test to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year.

Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a "high selection pressure" is maintained. In this case, high selection pressure refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices.

This potential was confirmed at the North Central Weed Science Society meeting where glyphosate-resistant common ragweed was officially reported. This is the first report of glyphosate-resistance for common ragweed. It has been identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. Midwest weed scientists believe in the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance. Ideally, we recommend:

1. Tank mixing glyphosate with another mode of action like 2,4-D in burndown treatments when glyphosate will be applied in the subsequent crop,
   
2. Alternating glyphosate use with other herbicide modes of action between years, and


3. Incorporating appropriate integrated weed management practices

Kevin Bradley, Agronomy
(573) 882-4039