About two-thirds of producers surveyed by Purdue University said they are not overly concerned that some weeds are shrugging off glyphosate, the active ingredient in the popular herbicide Roundup. The survey indicated farmers expect chemical companies to develop new herbicides for weeds that do not respond to glyphosate, said Bill Johnson, a Purdue Extension weed scientist and survey coordinator.

Both views are cause for alarm, Johnson said.

“About 90 percent of the soybean acres are grown in Roundup Ready varieties and about 20 to 30 percent of the corn acres will be in Roundup Ready varieties, yet less than a third of the Indiana growers we surveyed had a high level of concern about glyphosate resistance in weeds,” he said. “This is despite the fact that growers will be using the same herbicide year after year, particularly farmers who are rotating Roundup Ready beans with Roundup Ready corn.

“We find this a bit concerning, given the fact that most chemical manufacturers have de-emphasized new herbicide development, primarily because the Round-up Ready technology has taken a lot of the value out of the market.”

More than 600 Indiana corn and soybean growers who farm 499 acres or more completed the Purdue survey, which asked producers about their perceptions of glyphosate resistance and weed management strategies to prevent further resistance. A glyphosate replacement herbicide isn’t likely to come onto the market for at least five years, if ever, Johnson said.

“There aren’t any silver bullets in the pipeline,” he said. “Once glyphosate-resistant weeds become more widespread, we’ll be back to using the old herbicides that we used before the Roundup Ready technology.”

So far, only one glyphosate-resistant weed has been found in Indiana. Populations of glyphosate-resistant marestail have shown up in 28 counties, mostly in southern Indiana. Johnson is studying marestail samples from 15 northern counties to determine if glyphosate resistance is spreading.

Another weed, common waterhemp, is showing signs of glyphosate tolerance in Illinois, Missouri and Iowa, Johnson said. Meanwhile, Indiana field inspections indicate giant ragweed, common ragweed, common lambsquarter, velvetleaf and red root pigweed routinely escape control with glyphosate in Roundup Ready soybeans. The Purdue survey found concern about glyphosate resistance varied among farmers depending on how many acres they farm, and where.

Farmers who grow corn and soybeans on 2,000 acres or more, and those who farm in northwest, west-central and southeast Indiana - where herbicide resistance is an issue - expressed the most concern. Farmers with fewer than 1,000 acres, and those in northeast, southwest and south-central Indiana, were least concerned. In the south-central region, cattle and forestry are more prominent than corn and soybean production.

When asked what proven weed control practices they might adopt to prevent glyphosate resistance, farmers said they were more likely to rely on field scouting, tank mixing companion herbicides with glyphosate and treating fields with soil-applied residual herbicides. The survey also found:

•Only 48 percent of growers statewide scout fields for weed and pest problems. By region, southwest Indiana farmers lead the state in scouting (59 percent) due, in part, to the high-value fruit and melon production that exists there.

•Nearly 60 percent believe the repeated use of the same herbicide mode of action contributes to weeds developing herbicide resistance, while more than a third believe poor application technique or timing is a factor.

“That 60 percent ought to be 100 percent because the repeated use of the same mode of action is the predominant cause of herbicide resistance,” Johnson said. “On the other hand, poor application technique and timing has a minor affect on herbicide resistance.” Whatever their opinions, Hoosier crop producers should consider farming practices that keep herbicide resistance to a minimum, Johnson said.

“There are a number of resistance-prevention strategies, such as utilizing alternative modes of action on the worst weeds that you have in the field, not letting the worst weeds go to seed, rotating herbicide modes of action when you rotate crops, and utilizing scouting to pick the weed management tactics that are best suited for those species,” he said. “It’s all about managing your toughest weeds with more than one tactic.

“If we start instituting resistance-prevention strategies now, I think the development of glyphosate resistance in weeds will be slowed dramatically - and may not be important for 10 years down the road. If we go to weed management systems that rely only on glyphosate in both corn and beans, I think we’re going to run into a lot of situations where glyphosate-resistant weeds develop very quickly.”

The Indiana Soybean Board and the U.S. Department of Agriculture’s Critical and Emerging Pests Competitive Grants Program provided financial support for the Purdue survey.

Published in the December 7, 2005 issue of Farm World.