The widespread adoption of Roundup Ready crops by Delta farmers may have helped prevent the further development of herbicide resistance in several Delta weed species. However, the rapid conversion of many growers to transgenic crops has created its own unique set of problems.
“From a weed management standpoint, the Roundup Ready system has been a great resistance management tool for us. The big issue now is whether or not we will develop weed species resistant to glyphosate,” says Dan Poston, weed scientist at Delta Research and Extension Center in Stoneville, Miss.
Another potential problem he sees with the Roundup Ready system is a potential species shift towards annual grasses and somewhat glyphosate-tolerant weeds.
Poston says, “Most resistance issues have been overshadowed by Roundup Ready, and those looming as problems a few years ago have been held at bay through the use of Roundup Ready technology.”
One such example is johnsongrass that could no longer be controlled by herbicides like Poast and Fusilade. Such problems have essentially been forgotten since the introduction of Roundup Ready.
In addition, the widespread development of weeds resistant to various ALS inhibitors has been slowed since the introduction of Roundup Ready. Problem weeds such as Scepter-resistant cocklebur and waterhemp, and Staple-resistant pigweed, are controlled with glyphosate in Roundup Ready crops.
“From a fitness standpoint ALS-resistant weeds are often very healthy, allowing them to grow and reproduce as well as normal plant populations,” Poston says. “If we didn't have alternatives to clean up resistant populations of these weeds we'd have a big problem on our hands, but glyphosate has made controlling weeds a whole different ball game.”
Poston believes rapid development of widespread resistance to glyphosate is not likely to occur for several reasons. Glyphosate doesn't offer any soil activity; therefore there is no constant selection pressure from a soil standpoint as was the case with many ALS inhibitors.
“This allows susceptible plants to emerge subsequent to glyphosate applications and produce viable seed thereby diluting the number of resistant plants in a population.” According to Poston, many weeds that potentially could develop resistance to glyphosate may also suffer a fairly severe fitness penalty making their survivability in a population fairly low in the absence of glyphosate.
Weeds that already possess a fairly high tolerance to glyphosate are more likely to emerge as problems. “In essence we may be substituting on problem for another,” he says.
“Instead of resistance problems, growers that rely exclusively on the Roundup Ready system are more likely to experience a shift towards weed species that are somewhat tolerant to glyphosate like morningglories,” Poston says. “As we fail to adequately control these weeds, the problem gets a little worse each growing season until we have a major problem on our hands.
“Growers may also see a shift to annual grasses, not because we don't do a good job controlling them with glyphosate, but because the Roundup Ready program offers no soil residual component.” Annual grasses are often a problem at harvest in early-planted Roundup Ready soybeans. Glyphosate products control emerged grasses, but annual grasses often germinate late in the growing season when soybeans begin to senesce thereby allowing sunlight to penetrate the crop canopy and new weeds to emerge if adequate moisture is available.
Poston recommends using a pre-emergence grass herbicide with glyphosate to reduce late-season infestations of grasses like barnyardgrass and broadleaf signalgrass.
For those growers facing an abundance of morningglories in their Roundup Ready soybeans, Poston suggests tankmixing glyphosate with Classic or FirstRate for improved control. “While various tankmixes offer improved control of problem weeds, there are often economic limitations to using some of these tools.”
Though not widespread, there have been reported cases of resistance to glyphosate in the United States. “The most current example is glyphosate-resistant horseweed (Conyza canadensis), which has been documented in Delaware and Tennessee,” he says.
To make matters worse, he says, “We're doing very little to delay the onset of potential glyphosate resistance or to manage potential species shifts in glyphosate-only weed control systems, and until economic situations change, or until cheaper alternatives emerge, that's likely to continue.