The widespread adoption of glyphosate-tolerant crops by Mid-South farmers may have helped prevent the further development of herbicide resistance in several Delta weed species. However, herbicide resistant weeds have continually become more of a weed management issue for growers in the last 20 years.

The economics of transgenic production systems are driving rapid adoption, and with this comes concern, says Trey Koger, weed scientist with USDA's Agriculture Research Service in Stoneville, Miss.

“From a weed management standpoint, the Roundup Ready system has been a great resistance management tool for us,” says Dan Poston, weed scientist at Delta Research and Extension Center in Stoneville, Miss. “The big issue now is whether or not we will develop weed species resistant to glyphosate.”

Though not widespread, there have been several reported cases of true resistance to glyphosate. The most prominent example of this is resistant horseweed (conyza canadaensis), which has been documented in Tennessee, Missouri and Delaware in reduced tillage production systems.

“The problem of glyphosate resistant horseweed may be difficult to get a handle on because rotating crops and herbicides will likely have little value because its seed is spread through the wind,” Koger says.

Koger has fielded reports of both glyphosate resistant horseweed and smooth pigweed in Mississippi crop fields. However, he says, neither of these have withstood further resistance tests performed in his lab.

“In the case of the smooth pigweed, the suspect resistant plants treated with glyphosate had some injury, but those plants around them were dead,” he says. “So far, further tests of those plants suspected to be resistant have not shown glyphosate resistance. I believe it may have been more of a coverage issue, than a resistance issue.”

A suspected case of glyphosate resistant horseweed was found in the North Delta and brought to Koger for testing. “We grew out the seed in our greenhouse, but again we did not see any resistance. What we may be seeing are cases of potentially resistant biotypes, because all three “resistant” horseweed plants I tested required a full one pound rate of glyphosate for adequate control.”

In addition to a potential increased tolerance, some weed control problems may be caused by untimely applications and weed size at time of herbicide application.

“You've got to control these weeds while they are small. Growth stage does influence the level of tolerance. As plants get larger, the level of resistance gets higher,” he says. “Tillage and/or an application of 2,4-D are some really good options to control any of those more tolerant horseweed populations.”

While horseweed is a winter annual, Koger says its rosette is often six inches in size by mid-January, making it difficult to control at burndown. “Even with a one pound rate of glyphosate we are not completely controlling the six inch rosettes in our tests. Earlier burndown may help alleviate some of the problem.”

Before the advent of transgenic crops, Poston says, post emergence grass killers were effective for the most part, but some isolated weed populations developed resistance over time. One example is graminicide resistant johnsongrass, which can no longer be adequately controlled by herbicides such as Post, Select, or Fusilade. “Roundup is so effective in controlling johnsongrass, though, that the issue has really been forgotten about.”

The widespread resistance of many weed species to ALS inhibitors has also been halted by the advent of new pest management tools. Problem weeds such as Sceptor resistant cocklebur and waterhemp, and Staple resistant pigweed, are now fairly easily controlled by alternative herbicides.

“From a fitness standpoint these resistant weeds are very healthy, allowing them to grow and reproduce as well as any normal plant population,” Poston says. “If we didn't have alternatives to clean up the resistant populations of these weeds, growers would have a big problem on their hands, but glyphosate has made controlling weeds a whole different ball game.”

Because glyphosate doesn't offer any soil activity, there's no selection pressure from a soil standpoint. Also, many of the weed species that potentially could develop resistance suffer a fairly severe fitness penalty and their survivability is fairly low, according to Poston.

Exceptions to this rule are those weeds, which already possess a fairly high tolerance level to glyphosate. “As weeds, like morningglories, emerge as the new primary weed problems in a field the potential for substituting one problem for another gets a little worse each year,” he says.

“Instead of resistance problems, growers who have adopted the Roundup Ready system are more likely looking at a shift to those weeds, which already have some tolerance to glyphosate,” Poston says. To counteract the potential for annual grass infestations later in the season, Poston recommends adding a pre-emergence soil-applied herbicide to the Roundup Ready system, or tankmixing a herbicide product with soil-applied activity with their glyphosate treatment. Either option, he says, will effectively control annual grasses like barnyardgrass and broadleaf signalgrass.

“We're doing very little to manage resistance in glyphosate-only weed control systems, and until the economic situation changes, or until some cheaper alternatives emerge, that's likely to continue,” Poston adds.