Herbicides that inhibit acetolactate synthase (ALS) have been widely used due to their relatively low use rates, limited environmental impact, low mammalian toxicity, wide crop selectivity, and high efficacy. The rapid adoption and persistent use of these herbicides, which include imidazolinone and sulfonylurea herbicides, has selected for resistant weeds. Currently there are 43 monocots and 20 dicots reported worldwide to be resistant to ALS-inhibiting herbicides. Within the Midwestern states, there are 16 species resistant to ALS-inhibiting herbicides.
Giant foxtail with suspected resistance to ALS inhibitors was identified in Wisconsin (WI) and Illinois (IL) in 1999 and in Minnesota (MN) in 1997. These populations were identified in fields with a history of ALS inhibitor use in both corn and soybean. In most cases, an imidazolinone herbicide such as Pursuit was applied for weed management in soybean and a sulfonylurea herbicide such as Accent was applied for weed management in corn. We confirmed and quantified giant foxtail resistance to these herbicides in greenhouse and laboratory experiments.
In the greenhouse, giant foxtail plants from WI with suspected resistance were 15-and 20-fold resistant to Pursuit and Accent, respectively, compared to susceptible plants. Similarly, giant foxtail plants from MN were 14- and 19-fold resistant to Pursuit and Accent, respectively. In contrast, giant foxtail plants from IL were 13- and only 7-fold resistant to Pursuit and Accent, respectively. These results indicated that WI, MN, and IL plants had similar levels of cross-resistance to Pursuit, whereas the IL plants had lower levels of cross-resistance to Accent compared to those from WI and MN.
ALS enzyme assays in the laboratory indicated that resistance of WI, IL, and MN plants was associated with an insensitive ALS enzyme. These results suggested that the WI, MN, and IL plants had similar levels of cross-resistance to Pursuit at the enzyme level, whereas the IL plants had lower levels of cross-resistance at the enzyme level to Accent compared to WI and MN plants.
These results confirm that giant foxtail cross-resistance to Pursuit and Accent has occurred in three geographic locations. The field history in these three locations is similar. The fields were in corn-soybean rotations, and ALS-inhibiting herbicides were applied on an annual basis. The occurrence of giant foxtail resistance has several important implications for management, especially because this weed is competitive in both corn and soybean. Integrated weed management practices that include alternative herbicide chemistries, cultural methods, and mechanical methods need to be implemented to manage these resistant giant foxtail populations, and to delay development of additional resistance problems. For more information on herbicide resistant weeds, visit the International Survey of Herbicide-Resistant Weeds web site athttp://www.weedscience.com.
* Graduate Research Assistant