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Herbicide resistance is a growing problem
By Jennifer Carrico The development of herbicides has made management easier in crop production; however, it has also led to problems with some weeds developing resistance to these weed killers. Iowa State University Extension Weed Specialist Mike Owen said the changes in Iowa agriculture over the past 30 years have led to different ways of managing crop production. From aggressive tillage in the 1970s to the introduction of acetolactate synthase inhibiting herbicides in the 1980s and then the application of imidazolinone and sulfonyl urea herbicides on nearly all of the crop land, Owen said farmers have seen challenges because of some of these practices. "Despite warnings that this production practice would result in significant problems, commercial agriculture continued with the unsustainable practice of using one type of herbicide exclusively, and the inevitable resistant weed problem evolved as predicted," he said. Owen said the problem has become that some of the weeds that have become ALS inhibitor resistant have also become glyphosate resistant as well. "By the time the new technologies brought us glyphosate-resistant crops, ALS resistance was widespread and much of the utility of these important products had been lost," he added. Currently only glyphosate is used on more than 10 percent of the soybean fields, but a great number of fields have issues that need to be dealt with in order to help gain maximum production. Glyphosate and ALS-inhibiting resistant weeds in the upper Midwest include common waterhemp, common and giant ragweed, horseweed, Palmer amaranth, pigweed and Johnsongrass. While these are the weeds that are most commonly resistant to glyphosate and other herbicides, there are still many weeds that react to these herbicides. "While we eliminate the weeds we can with the common herbicides, the others that are resistant become a bigger problem," he said. Iowa State University professor of agronomy Matt Liebman said it is important to know what weeds are problems in your fields and what modes of action can be used against those weeds. "Just as the seed companies are working stacking of resistance to herbicides among certain hybrids, weeds have started to adapt to these conditions too and stack their own resistances naturally," said Liebman. If farmers think they can use another product when ALS inhibitors or glyphosate don't work against a weed, that may not be the case. Australian ryegrass has developed resistance to eight different herbicides. Management tactics "We may need to use multiple management tactics to suppress these weeds," he said. "And farmers are going to have to really know their fields and what works in each of them." Liebman said some of these management tactics may include cultivating, mowing crop stubble, reducing competition by using a higher seed concentration, and microbial decay of weed seeds to prevent their growing the next year. "Rotation of crops has been thought of as one of the best ways for weed reduction all the way back to the 1930s," he said. "By rotating crops, farmers also rotate herbicides and weed problems aren't nearly as bad." Studies done at Iowa State's Marsden Farm in Boone County indicated that small grains and perennial forage legumes added to conventional row crop systems help to maintain crop yields, minimize requirements for synthetic fertilizers and herbicides, reduce production costs, and maintain or increase net returns. Liebman said three different rotations were used. One was a two-year rotation of corn and soybeans, managed conventionally. The second was a three-year rotation of corn, soybeans and an oat/red clover combination. The clover wasn't harvested and manure was applied to the legume the year before it was planted to corn. The third was a four-year rotation of corn, soybeans, oats plus alfalfa combination and alfalfa hay. Three to four cuttings of hay were taken and manure was applied on the legume the year before it was planted to corn. "The results of this study indicate that diversified crop rotation systems can produce high yields of corn and soybean, suppress weeds effectively, and improve soil quality, while substantially reducing requirements for synthetic N fertilizer, herbicides, and fossil energy inputs," he said. "Despite historically high market incentives for corn and soybean throughout the period of study (2007-2010), the three-year rotation system was 6 percent more profitable than the conventionally managed two-year system. The four-year system was slightly (3 percent) less profitable than the conventional two-year system." Liebman said this experiment shows that farmers would be able to manage their crops agronomically and economically if environmental regulations forced them to reduce their reliance on synthetic fertilizers and pesticides. These results may also be a way to manage weeds that become herbicide resistant. "Weed suppression has been highly effective in all the systems we studied," he said. "One of the keys to fewer weeds is lowering the weed seed banks." Seed predators such as the prairie field mouse and the cricket are important and can remove 60 to 90 percent of the weed seeds in a field if they have a viable population. Liebman suggests delaying fall tillage in order to increase the amount of seed predation. Owen agreed that farmers must become more efficient in managing crops in order to better manage the weed problems. He said multiple modes of action are helpful in controlling weeds, but must be used redundantly, must be efficacious on the targeted weeds, and should ideally have similar physicochemical characteristics to cut down on selection pressure. "You can start with one mode of action and follow up with another. For instance, using a soil applied herbicide and another herbicide post-emergence. Rotating modes of action can also help delay the resistance," Owen said. He stressed the importance of considering mechanical, cultural and herbicidal tactics when managing risk each year. Different hybrid and different herbicides should be used to help manage different situations. He also suggested the use of cultivation in the areas that need it. "The greater the diversity, the greater the profitability and sustainability," he said. "We continually have new technology available that should help our crops grow better, but if that technology is misused, then it is not effective. Sometimes using older practices such as rotation can help prevent some of the new problems we have encountered." Jennifer Carrico can be reached by phone at 515-833-2120, or by email at jcarrico@hpj.com.
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