Purdue News
Purdue News
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June 21, 2004 Expert: Weather-beaten farmers could soon be weed-whippedWEST LAFAYETTE, Ind. - Farmers who look out over their cropfields and see standing water could be gazing at another unwanted sight once the moisture disappears. Weeds are likely to spring up all over fields that weren't treated with herbicides before a wet weather pattern brought storm after storm to Indiana earlier this month. Even fields treated with herbicides could become weed havens. Growers will have to act fast once fields dry in order to get a handle on the intrusive vegetation, said Bill Johnson, Purdue University Extension weed specialist. Johnson said two factors are behind the weed threat, neither of which farmers could have foreseen. "First of all, we've had an extended corn planting season because of wet weather in April and May," he said. "Second, we've had such high winds this year that many of our cornfields were not sprayed with a pre-emergence herbicide, and farmers exercised good judgment and parked their sprayers on those days. "Unfortunately, we're now in a situation where we need to get herbicides applied to many of these fields in a relatively short period of time, or we may suffer yield losses." Johnson advised farmers to walk their fields as soon as possible to size up weed problems. County Extension educators, co-op agronomists and weed-management guides can help producers determine the best weed-control strategy, he said. "I think weed problems will be particularly severe in cornfields that haven't been sprayed or that received excess rain after soil-applied herbicides were applied," Johnson said. "Now that we have adequate heat and moisture, many of these fields will be growing very rapidly. Most post-emergence grass herbicides for use in corn should not be broadcast-applied when corn is at about the V6 or six-collar stage, or around 24 inches in height." Because corn was planted early in many counties, crops already are chest-high or taller. In those fields, farmers should employ a different method for putting down soil-applied herbicides, Johnson said. "Most of those products can be applied after the V6 stage if they're put on with sprayers that have drop nozzles that direct the spray down between the rows," he said. "But when using drop nozzles it is assumed that you have a height differential between the crop and the weeds. Those applications can be tricky." Producers could have their hands full with a handful of weeds, Johnson said. "Giant ragweed is a problem every year, in both corn and soybean," he said. "Other weeds that like wet conditions are burcucumber and tall waterhemp. Those weeds seem to survive well when we get frequent rainfall or have wet soils for a long period of time and we have an open canopy, which allows sunlight to reach the soil surface so that these weeds can germinate. Their extended germination pattern and love of wet soils make these weeds problematic." Weeds can take as long as three months to germinate during the summer, giving them time to wait out the damaging effects of high water, Johnson said. Corn and soybean hybrids, on the other hand, are bred to germinate much faster. Soybean growers should not face as great a challenge controlling weeds when conditions dry, Johnson said. Since nearly 90 percent of all Indiana soybean acres are planted in Roundup Ready varieties, farmers have more flexibility in applying herbicides. In most cases producers can achieve satisfactory control by increasing the application rate, Johnson said. Farmers with acres remaining to be planted will need to take aggressive steps to clear their fields of weeds, Johnson said. "Aggressive tillage can help, but it also will compact and dry the soil," he said. "A farmer is probably better off spraying a relatively high rate of a glyphosate product. It will work rapidly when the air temperatures are warm, and then a producer can plant in a no-till system. "Many times with particularly large weeds and tillage we end up just sort of knocking the weeds down a little bit and then transplanting them to another area. They end up being a little more difficult to control that way." For more information on weed control following wet soil conditions and a list of herbicides for specific weeds, read "Weather Related Weed Management Items" by Johnson and Glenn Nice, Purdue weed scientist. Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu Source: Bill Johnson, (765) 494-4656, wgjohnso@purdue.edu Ag Communications: (765) 494-2722; Beth Forbes, forbes@purdue.edu Related Web site:
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