In 2012, 187 waterhemp seed samples were collected from soybean fields across the state of Missouri to determine the distribution and extent of herbicide resistance to six different herbicide modes of action. We screened each of these populations to labeled use rates (1X rates) of these herbicides once plants reached 4-inches in height.
Results indicate that practically all waterhemp populations are resistant to group 2 (ALS-inhibiting) herbicides like chlorimuron (Classic). Atrazine (group 5) and glyphosate (group 9) resistance was present in 51 and 58% of the populations tested, respectively. Resistance to group 14 (PPO-inhibiting) herbicides like lactofen (Cobra) was observed in 11% of the populations while resistance to group 27 (HPPD-inhibiting) herbicides like mesotrione (Callisto) was present in 14% of the populations. Perhaps even more concerning is the fact that 84% of the populations tested were resistant to at least 2 different herbicide modes of action, 39% were 3-way resistant, 11% were 4-way resistant, and one population was resistant to 5 different herbicide modes of action.
As you plan your soybean weed management program for the season, it is important to consider these results and understand the extent of multiple herbicide resistances in waterhemp in Missouri. In order to manage and mitigate herbicide-resistant waterhemp in your fields, you must integrate all available cultural and chemical control tactics available with the ultimate goal of eliminating waterhemp from your fields and preventing seed production altogether. This means incorporating cultural control practices like narrow row spacings and optimum soybean plant populations along with a herbicide program that contains multiple herbicide modes of action that are effective on waterhemp.
As a result of the multiple resistances present in Missouri waterhemp, some of the most effective herbicide options left for waterhemp are pre-emergence, residual applications of the group 14 (PPO-inhibiting) and group 15 (long-chain fatty acid inhibiting) herbicides and/or “overlapping” or “layered” applications of these herbicides. But we will save a discussion of the overlapping residual herbicide program for a future article. For a more complete explanation of herbicide classification and list of herbicides belonging to these groups, see here.