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Now’s a good time to manage winter annuals
If your fields are being overrun by marestail and other winter annuals, warm days with sunshine like we had earlier this week in most areas of the Midwest are good days to control them with a bundown herbicide.
“We have many effective products that could be used specifically to control the existing vegetation, as well as more that have soil residual activity,” says Aaron Hager, a University of Illinois (U of I) Extension weed specialist. This can protect against emerging weeds for several weeks.
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Still, farmers battling winter annuals like marestail aren’t out of the woods.
“Because of the biology of marestail and known (glyphosate) resistance issues, we can expect more variability in burndown efficacy in a one-shot approach, especially as the plants get larger as we progress throughout the early part of the spring,” says Hager. “There tend to be more instances of burndown applications that are not effective as plants grow larger.”
Why Controlling Winter Annuals is Important
Winter annuals may not garner the attention of weeds like waterhemp and Palmer amaranth, but they still can cause trouble. Common chickweed, for example, can form a dense mat that shades the soil and delays field drying. Winter annuals also provide a haven for black cutworm to lay eggs and the resulting larvae to damage corn after it emerges.
“Some species of winter annuals form a green bridge for soil pathogens like soybean cyst nematode,” says Hager. “The earlier we control them, the better off we are during the growing season.”
Under wet conditions, it can be tempting to plant first and then control winter annuals and other early emerging weeds. However, that’s risky, as adverse weather can nix a timely spraying operation, Hager says.
Tillage with tools like a tandem disk or field cultivator can work, particularly with small vegetation, says Hager. “But as we realized during the 2019 growing season, as (weed) populations gain in size, a one-pass tillage operation might not be effective in getting control,” he says.
A two-pass burndown strategy is another alternative. This works best when split between fall and spring. Unfortunately, fall is long past.
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Hager points out one alternative now may be a burndown mix that includes a low rate of a residual product, such as a combination of 2,4-D (Group 4) and glyphosate (Group 9) with 4 to 8 ounces per acre of dry metribuzin (like Tricor, Group 5) or saflufenacil (Sharpen, Group 14). If needed, a follow-up burndown could then occur close to planting, such as paraquat (Group 22) or glufosinate (Group 10) teamed with Authority (premixes contain Groups 2, 5, and 14), or Valor (Group 14).
“Many populations of marestail are glyphosate resistant, so consider alternative products,” reminds Hager.
Dicamba is a good burndown herbicide and can increase efficacy. U of I trials show dicamba has better efficacy as a marestail burndown than does a mix of 2,4-D and glyphosate for marestail ranging in height from 2 to 14 inches.
Dicamba and Early Planting
This week’s break in the weather has prompted some farmers to plant soybeans earlier than normal. This has yield benefits, but it also comes with a weed management caveat for farmers planting dicamba-tolerant soybeans. In this system, dicamba cannot be applied more than 45 days after planting. Thus, dicamba for April 10 planted soybeans could not be applied after May 25.
“This means there is a great portion of the growing season in front of us,” he says. Thus, farmers would have to tap another herbicide or herbicide mix to control late-emerging weeds like waterhemp or Palmer amaranth or even marestail.
“In some areas of Illinois, marestail is acting more like a summer annual,” says Hager.