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ST. LOUIS (DTN) -- Soil-applied herbicides have become a linchpin in weed control programs aimed at fending off herbicide resistance. However, weather conditions are complicating those plans this spring and growers are being urged to check for weed escapes.
Both wet and dry conditions can trip up soil-applied herbicides used for pre-emergence weed control, Michigan State University weed scientist Christy Sprague told DTN.
"If you're going out to scout and see how the crop is coming up, some might notice weeds are coming up at the same time," she said. Many soybean growers could require follow-up post-emergence sprays sooner than expected and corn growers should consider a range of delayed soil-applied herbicide options, as well as post-emergence products, she added.
TOO DRY TO WORK
Spring rainfalls have kept many Midwestern states in good moisture conditions, but farther north, south and east, some growers are struggling with dry soils. Agronomists and weed scientists from Minnesota, North Dakota, Pennsylvania and Tennessee have called attention to the effects dry conditions can have on soil-applied herbicides over the past week.
"You need rainfall for incorporation of herbicides," Sprague explained. "The moisture moves herbicides into the germination zone of those weeds as they're coming up." In general, at least half an inch to an inch of rain is needed within a week or two of application to allow soil-applied herbicides to do their job, she said.
In the South, a long bout of dry conditions is already taking a toll on weed control, University of Tennessee weed scientist Larry Steckel told growers in a university newsletter. Palmer amaranth seedlings up to 2 inches tall are already growing rapidly alongside corn and soybean seedlings in many fields, he noted. This weed's ability to grow up to 3 inches per day intensifies the need for early season weed management and subsequent scouting.
In Minnesota and North Dakota, growers may still have time for rainfall before shallow-seeded broadleaf weeds like waterhemp and lambsquarters begin to emerge, said Tom Peters, an extension agronomist for the University of Minnesota and North Dakota State University. Deeper-germinating weeds like giant ragweed can escape past un-activated soil-applied herbicides and require post-emergence applications later, he told growers in both university newsletters.
TOO WET TO WORK
Like Goldilocks, soil-residual herbicides prefer a happy medium of moisture. Too much rain too fast can be equally problematic, Sprague noted.
"In super-heavy rainfalls, the rain can move herbicides past the weed germination zone," Sprague explained. Also, in cool, wet conditions, crop growth can stall, which leaves emerging seedlings more vulnerable to soil-applied herbicides, she said. "The herbicide gets taken up when the plant is not actively growing, so it can't metabolize the herbicide, and you see crop injury," she explained.
Long, wet springs may have also forced some growers to miss the application window for pre-emergence herbicides, Sprague added.
MAKING A PLAN
For growers who missed the application window for soil-applied herbicides or who got one in, but are finding weed escapes, corn fields will be more forgiving, Sprague noted.
Typically, soil-applied herbicides shouldn't be applied any later than three days after planting. However, a number of pre-emergence corn herbicides are also labeled for application after emergence.
See this Michigan State University weed control guide, which organizes these delayed soil-herbicide options in a chart on page 51: http://bit.ly/…. Here, growers can also find a comprehensive list of post-emergence corn herbicides for weeds that snuck past a pre-emergence application.