Horseweed and giant ragweed are becoming more difficult to control in glyphosate-resistant soybeans due to repeated use of herbicides with the same site of action, especially glyphosate and ALS-inhibiting herbicides. Greenhouse and field doseresponse studies were conducted to characterize response of giant ragweed and horseweed populations to glyphosate and the combination of glyphosate and cloransulam, respectively. Field studies were conducted to determine the most effective combination and timing of preplant herbicides for the control of multiple-resistant horseweed populations, and to determine whether glyphosate-based herbicide programs could effectively control glyphosate-resistant giant ragweed populations. The GR50 for a multiple-resistant horseweed population treated with cloransulam and glyphosate was 45 g ai/ha and 2120 g ae/ha, respectively. The level of resistance for this biotype was a factor of 31, compared to a sensitive population. In the field, horseweed survived the application of glyphosate plus cloransulam and glyphosate alone at rates up to four times the recommended rate. Combinations of paraquat, metribuzin, plus 2,4-D, and glyphosate (3360 g/ha) plus 2,4-D, controlled emerged multiple-resistant horseweed. The GR50 for glyphosate-resistant giant ragweed populations ranged from 8.3 to 23.9 kg/ha of glyphosate. The level of resistance for these populations ranged iii from a factor of 2.1 to 6.1, compared with a glyphosate-sensitive population. In the field, individual plants within the populations could survive single or multiple applications of glyphosate totaling 2.5 to 3.4 kg/ha. The majority of plants within each glyphosate-resistant giant ragweed population could be controlled when glyphosate (1.7 kg/ha) or fomesafen was applied postemergence, and followed with another application of glyphosate (0.84 kg/ha). Effective control required the preplant use of glyphosate and 2,4-D to control emerged plants, and cloransulam plus flumioxazin to provide partial residual control of later-emerging plants. This research confirms the presence of low-level glyphosate resistance in giant ragweed, and multiple-resistance in horseweed, to glyphosate and ALS-inhibiting herbicides. This is the first confirmation of these resistant cases in the world. Resistant populations can be effectively managed where the herbicide program includes 2,4-D and residual herbicides, applied prior to soybean planting at the appropriate weed growth stage. For giant ragweed, the preplant herbicide treatment must be followed by multiple postemergence applications of glyphosate at maximum rates, or effective alternatives to glyphosate.
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