Palmer amaranth and ivyleaf morningglory management in glyphosate-resistant cotton
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Palmer amaranth (Amaranthus palmeri S. Wats.) is the most common weed on the Texas High Plains. Residual herbicides are typically used in conjunction with glyphosate to control this weed, but weed shifts have been observed following continued use in a “glyphosate” system. Weeds such as ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.] are now problematic in parts of this region. GlyTol® + LibertyLink® (GL) cotton offers opportunities to manage weeds such as ivyleaf morningglory while maintaining effective control of Palmer amaranth. However, there are concerns about antagonism between glyphosate and glufosinate when tank-mixed. Field trials were conducted in Lubbock, TX in 2010 and 2011 to evaluate tank-mix combinations of glyphosate and glufosinate in GL cotton for control of Palmer amaranth and ivyleaf morningglory. Field trials included glyphosate and glufosinate applied at varying tank-mix rates (1X:1X, 1X:0.75X, 1X:0.5X, 1X:0.25X and 1X:0X for each herbicide), proportional tank-mix rates (1X:0X, 0.75X:0.25X, 0.5X:0.5X, 0.25X:0.75X, and 0X:1X), and in sequential applications of both herbicides in an overall weed management system. Greenhouse studies were conducted in 2011 to quantify antagonistic or synergistic effects. Treatments included an untreated control; glyphosate at 0.84, 0.63, 0.42, and 0.21 kg ae ha-1; glufosinate at 0.58, 0.44, 0.29, and 0.15 kg ai ha-1; and all tank-mix combinations of each herbicide rate. Dry weights were converted to percent growth values for each rate of the two herbicides alone and these values were used to calculate expected responses of tank-mix combinations using Colby’s Method. Expected values were compared to observed percent growth values using an augmented mixed-model method. Results of field studies indicated that tank-mixes of glyphosate and glufosinate were less effective at controlling Palmer amaranth than glyphosate applied alone. The addition of any rate of glufosinate to a 1X rate of glyphosate reduced Palmer amaranth control compared to glyphosate alone. Greenhouse studies confirmed antagonism seen in the field. Tank mixes of glyphosate and glufosinate were as effective at controlling ivyleaf morningglory as glufosinate alone. The addition of any rate of glyphosate to a 1X rate of glufosinate did not affect control of ivyleaf morningglory. Greenhouse studies suggested low levels of antagonism with some tank-mix treatments and no antagonism in other treatments. The data from field and greenhouse studies on ivyleaf morningglory suggest some antagonism with glyphosate/glufosinate tank-mixes, but not to the degree seen in Palmer amaranth. These results indicate that sequential applications of these two herbicides are a better option for Palmer amaranth and ivyleaf morningglory weed management.