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Coping with herbicide-resistant weeds, after decades of the easy, sure control afforded by glyphosate (Roundup), will require a multi-pronged approach, Mississippi growers were told at the 2011 Delta Ag Expo. As glyphosate-resistant pigweed (Palmer amaranth) spreads in the Mid-South, and resistant Italian ryegrass continues to be documented, producers will need to include alternate chemistries — many of which are decades-old active ingredients — in their weed control programs, a panel of specialists noted.
Only a matter of time for resistant weeds
“Currently, we have 10 counties in Mississippi, bordering the river, with glyphosate-resistant pigweed. If you don’t have them, it’s only a matter of time until you will. In 2010, we had some resistant pigweeds in areas near the coast, a long way from the Delta. Most of the coastal counties of Alabama have resistant pigweed, and it’s probably only a matter of time until it spreads to the Mississippi hills.”
“Residual herbicides now represent a key line of defense against resistant pigweed. How long they will be effective depends on how carefully we manage them.
“Resistance is a moving target and can vary from one year to the next. In some cases, control is textbook; in others, it depends on weather and other factors.
“Remember: residuals need water for activation. If you don’t get rain, they won’t work. If you look at historical weather data, there usually will be rainfall to activate residual herbicides applied in April. After that, it becomes chancy.
“If you have the ability to irrigate, you can facilitate activation. Overhead irrigation trumps furrow irrigation for herbicide activation.”
Dodds: “With residual herbicides, be aware of label restrictions regarding the number of days from application to planting. Counting starts on the date you get the specified amount of rainfall — not the day the material was applied.”
Eubank: “Treflan, which goes back many years, is still in our recommendations. But remember, it’s a single mode of action — you can’t use just that one material — and if you don’t incorporate it properly, it’s not going to work. “Nothing is going to give you 100 percent control of pigweed, just because of the sheer numbers of seed produced.