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Weed-control lessons from a wild seasonWeather and application challenges in 2008 make fall-applied herbicides an important tool for next season. If challenging weather made your weed-control window less than ideal this season, you're probably still suffering from the effects of weed escapes. Weedy fields are causing yield losses this season, but that's not the worst of it. Weed seeds left behind are likely to hurt your crop next season, say weed scientists. Taking action this fall is your best strategy for controlling weeds in 2009. The 2008 growing season proved why growers can't rely on postemergence weed management alone, says Bill Johnson, Purdue University associate professor, weed science. "In too many cases, the window for post application was too short and weeds, including lambsquarters, ragweed and waterhemp, got out of control. I'm sure there will be significant yield losses due to weed competition." More mid-season weed escapes mean more harvest difficulties and more weed seeds, adds Helen Flanigan, field development representative, DuPont Crop Protection. "There will be a larger weed seed bank out there after this season, and growers will likely see more problems with winter annuals. Using a residual herbicide is critical this fall to get the jump on next season's weeds." Growers first need to determine which weeds are the biggest threat to their crops, says Johnson. "If a soil-applied residual herbicide will be effective against that first flush of weeds in the spring, make sure you're using one." Herbicides with residual control are now used on about 60 percent of corn acres, and only 30 percent of soybean acres, he estimates. "If I could convince growers of just one thing when it comes to weed control, it would be to add a herbicide with residual control. That simple step will reduce early-season weed competition and help reduce weed exposure to glyphosate, which should lower the odds of developing resistant weeds." Residual control a must Glyphosate-resistant waterhemp and marestail have appeared in northwestern Missouri, where Greg Wade farms several thousand acres with sons Nicholas and Brice. The Stewartsville, Mo.-based grower says they haven't had weed-resistance problems in their fields and have a strategy to avoid them. "The biggest weed-control management shift we've made over the last few years is making sure we use herbicides that give us more modes of action." Using a herbicide that provides burndown and residual control of targeted winter and spring annuals on fall corn stubble going into soybeans the last four years has added that additional mode of action and it solved an early-spring weed challenge. "We don't like competition of any kind for our crops," he explains. "Fall application gives us a clean field every spring--no marestail and no lambsquarters." His fields usually stay clean well into June. Some years, just one postemergence herbicide application is needed. "This year we had good control until June 1, when we saw waterhemp break through," says Wade. "Considering how wet things were this spring, I was pleased. We had to make two post applications, finishing the last one in early August, but we got good control." The Wades use a sulfonylurea herbicide containing chlorimuron for postemergent application, tank-mixing with glyphosate, mainly to provide two modes of action. "We haven't had any problem with resistant weeds in our fields and want to make sure that it stays that way." All growers need to be more proactive, says Johnson. "I've been hearing from growers about more glyphosate-resistant marestail this year, but my colleagues and I have been out beating the drum on that problem since it appeared in 2002. Too many growers don't realize the value of diversified weed control until they've got a resistance problem." Value in using multiple modes Today's herbicide economics actually favor greater diversity, says Johnson. "A single-herbicide system is easy and attractive when it's inexpensive, but glyphosate isn't $10 a gallon anymore. Residual herbicides are very affordable additions to a sound weed-control program." "Products such as DuPont Canopy EX herbicide for soybeans and DuPont Basis herbicide for corn deliver contact control for weeds that grow after harvest, but also provide important residual control of winter annuals, well into spring," says Flanigan. "Treating any weed escapes as well as winter annuals in fall, before they become a problem, reduces the risk of planting delays in the spring. These herbicides work well in no-till and reduced-tillage fields, and with fewer spring weeds, fields dry out and warm up faster. "University studies have demonstrated the effectiveness of using a fall-application of residual herbicides to control tough weeds, including dandelion, pennycress, henbit, downy brome and chickweed," she says. "Add to that the fact that a fall application helps reduce your busy spring workload, and it's a win-win weed-control solution." 11/17/08 Date: 11/7/08
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