Purdue News

March 30, 2001 In weed wars, some unwanted vegetation never says die WEST LAFAYETTE, Ind. – Farmers still hold the upper hand in the annual war against weeds, but weeds are winning more battles. Several troublesome weeds are refusing to surrender to herbicides. In recent years, Purdue University weed experts have identified herbicide-resistant populations of pigweed, lambsquarters, jimsonweed and giant ragweed in Indiana. Now they've found herbicide-resistant shattercane. For Hoosier producers, controlling the pesky vegetation may mean shifting to other weed-fighting products and practices or rotating crops, said Case Medlin, Purdue Cooperative Extension Service weed specialist. "Herbicide-resistant weeds have become an important issue to consider when making weed management decisions," Medlin said. "Herbicide-resistant weeds can, and have, developed from natural weed populations." Herbicide-tolerant weeds generally are found in fields where corn has been grown in consecutive years, Medlin said. There also appears to be greater herbicide immunity in areas treated with the same products each season. "The potential for weed resistance to develop increases with the continuous use of herbicides that have the same mode of action – that is, similar chemistry," Medlin said. "Therefore, herbicide use should be monitored and production practices implemented to prevent and reduce the potential for using the same herbicide chemistries year after year." Resistance is not limited to a single herbicide or in corn, Medlin said. Some weed species are developing resistance to different products within the same herbicide family. This cross-resistance occurs frequently in corn and soybean fields regularly treated with aceto-lactase synthase-inhibitor herbicides, commonly known as ALS inhibitors. ALS inhibitors kill weeds by preventing weeds from producing essential amino acids. Weeds stop growing and slowly die – turning either a golden or flat green color, depending on the species. This winter, Purdue weed specialists documented the state's first ALS-resistant shattercane in southern Indiana. "Currently, we do not know if this shattercane population is cross-resistant to the other ALS-inhibiting herbicides, but there is a potential," Medlin said. "If you have this problem, or shattercane resistance to ALS herbicides develops in your field, one alternative control measure is to use the grass herbicides in a conventional soybean system." Medlin said farmers have other weed control weapons at their disposal. He recommends producers: • Scout fields regularly, identify weeds and respond quickly. • Apply herbicides intended for weeds present in the field, and use the products only when necessary. • Rotate crops to disrupt weed cycles. • Apply herbicides of different chemistries as a tank mixture or sequential application during the same season for the same weed. • Combine herbicide treatments with cultivation in fields with minimal soil erosion concerns. • Clean tillage and harvest equipment to keep weeds from moving to other fields. "But more importantly, avoid reliance on the same herbicide, or ones with a similar chemical action, for consecutive years," Medlin said. For more information and an updated herbicide list, read the article "Herbicide Resistant Weed Problems Increasing in Indiana: Add Shattercane to the List," by Medlin and Purdue weed specialist Tom Bauman. The article appears in the current issue of Purdue's Pest & Crop Newsletter and can be downloaded online. Source: Case Medlin, (765) 494-0599; medlin@btny.purdue.edu Writer: Steve Leer, (765) 494-8415; sleer@aes.purdue.edu Purdue News Service: (765) 494-2096; purduenews@purdue.edu Related Web site: Purdue Department of Botany & Plant Pathology Weed Science Extension To the Purdue News and Photos Page