ST. LOUIS, Mo. — There is no silver bullet when it comes to combating glyphosate-resistant weeds, Southern Illinois University weed scientist Bryan Young said at the National No-Tillage Conference last Thursday.

But producers can follow a multi-pronged strategy that Young summarized under the acronym DIRT, to manage resistant weeds in 2012.

Resistant weeds are continually challenging growers. Young’s surveys of producers in southern Illinois showed 12 percent of respondents reporting glyphosate-resistant waterhemp in 2009, and 20 percent in 2010. Last year, that latter percentage doubled.

Glyphosate-resistant marestail (horseweed) is also widely reported in Illinois, said Young. Resistant Palmer amaranth has also moved north to Illinois after proving a widespread challenge for Southern producers.

Resistant pigweed has been most problematic in the southern U.S., where glyphosate-resistant Johnsongrass and ryegrass are also reported. At least 25 glyphosate-resistant weed species have been reported in 27 states.

Young said field differences in glyphosate-resistance make it often impossible to provide the same weed management advice to neighboring farms. “For sure, (advice) is not going to apply to somebody in North Dakota, versus somebody in Arkansas,” he told no-tillers from around the globe.

But growers facing glyphosate resistance in any region, Young said, may use his recommended DIRT management strategy. This strategic “thought process” follows weed control strategies grouped under the headings Diversify, Integrate, Rate and Timing.

“This is an acronym about killing weeds,” said Young.

First, producers need to diversify their weed management tactics to combat glyphosate resistance. This means not relying on any single application, mode of action or active ingredient for weed control, for the emergence of glyphosate resistance has illustrated the risks of relying on any single control. “Multiple applications will help improve the robustness of your weed control,” said Young.

Integrate at least two effective modes of action into the spray program, said Young. “Glyphosate-resistant weeds, in some ways, are just like any other old herbicide-resistant weed,” he said. “In other ways, they’re different.”

The difference, he said, is producers cannot “open a new jug” of herbicide that is more effective than the previous herbicide to which weeds are now resistant. “We don’t have another herbicide that is going to be better than glyphosate,” he said.  That makes it essential to use more than one mode of action to take out problem weeds.

For the “Rate” in Young’s acronym, he referred to using full application rates. This is critical, he said, to avoid partial or insufficient kill that can lead to future herbicide resistance.

“Use the full rate; I think with today’s commodity prices it’s well-justified,” he recommended.

Timing is the last part of Young’s general strategy. “I think we need to make sure we’re applying the residual herbicides close enough to the peak emergence period of these problematic weed species,” he said.

For example, he said a residual herbicide application in the fall could not be relied upon for waterhemp control in June. Post-emergent herbicide applications at shorter weed heights are also essential, he said.

Combining the four DIRT strategies can result in future productivity. “A good job planning can reduce your cost per acre, increase yields and improve weed control,” said Young. “If you want to have better control of your herbicide bill, plan wisely – don’t react to the weeds.”

The Weed Science Society of America also recommends Best Management Practices for managing resistant weeds, such as field scouting and cleaning out combines after harvesting fields where resistant weeds are present.

An additional practice for resistant weed control was unlikely to be as relevant in a room full of no-tillers, noted Young: Mechanical cultivation.