Dicamba and 2,4-D are certainly not new herbicides and weed species responses are mostly well known. Controlling many of our problem weeds such as morningglory, horseweed, tropical spiderwort, wild radish, and primrose can be achieved very effectively with both of these herbicides when applied at proper use rates and application timings.
Of course, the big question surrounds Palmer amaranth. Neither 2,4-D nor dicamba are very effective in controlling this plant species. See the figure below summarizing data from GA, NC, and TN. Mixtures of 2,4-D or dicamba with Roundup will, of course, improve control of non-glyphosate resistant Palmer amaranth while mixtures with Liberty should improve control compared to either product applied alone. Results from this data offers several valuable points. First, large Palmer is almost never controlled so applications (regardless of herbicide mix) need to be made when the largest Palmer in the field is 3 inches. Second, a single postemergence application will not control the pest adequately further stressing the importance of why sound diversified programs must be implemented.
Although neither dicamba nor 2,4-D are all that effective on Palmer amaranth, a programs approach can be very effective. Georgia cotton research suggests the new auxin systems have the potential to provide slightly more flexibility with a little less hand weeding compared to standard programs in fields heavily infested with Palmer amaranth. However, research clearly notes two points: 1) herbicide tank mixes are needed for in-season auxin applications (many needed tank mixes are not currently approved by the EPA; check with extension agent or industry rep for latest approvals) and 2) in fields where Palmer amaranth populations are not overwhelming, standard Liberty- and Roundup-based programs are as effective as the auxin systems. For specific weed management programs, visit the home page of www.gaweed.com.
The greatest challenge for most Georgia growers in selecting the ideal weed management program is weighing the risk of potentially having slightly better weed control with a program that offers greater potential for off-target movement issues. A program that improves weed control and reduces inputs that can be managed without negative consequences sounds wonderful; however, a management program that saves a few dollars per acre on weed control but ends me up in court or having to deal with an upset neighbor and the Department of Agriculture……..not as enjoyable!!!