Waterhemp is a pigweed with high genetic diversity. This dioecious pigweed, meaning male and female flowers are on separate plants, has a high propensity to adapt to control tactics and has evolved resistance to herbicides from as many as six or seven different herbicide groups.

Out of 75 waterhemp populations collected from corn and soybean fields in Iowa in fall 2019, almost 25% had four-way multiple resistance to groups 2 (ALS inhibitors), 5 (atrazine), 9 (glyphosate) and 14 (PPO inhibitors). A diverse herbicide program with multiple, effective sites of action is needed to reduce overreliance on any single herbicide and slow further development of herbicide resistance.

The Iowa State University Weed Science program offers results from recent weed control trials for waterhemp management in soybean.

There is a little difference among pre-products (Figure 1) when appropriate rates are used. Based on Figures 2 and 3, with an effective pre- followed by an effective post-program (two-pass layered residual, with multiple sites-of-action herbicides), there were no significant differences in waterhemp control (less than 90%; red dotted line) in soybean.

For instance, when Verdict pre at 5 ounces per acre (70% control) was followed by Liberty (glufosinate) plus Group 15 post, waterhemp control increased to 90% or more. A slight difference in percent control observed between the Group 15 herbicides (Outlook, Dual II, versus Zidua with Roundup plus Liberty post) in Figure 2 could possibly be a rate response.

There is an increased selection pressure for herbicide Group 15 used in both corn and soybean. Waterhemp populations with resistance to Group 15 have been reported in the Midwest (not in Iowa yet). Since herbicides from this group are the only ones labeled for use in post residual programs in soybean, it is recommended to use alternative sites of action including herbicide groups 3, 5 and 14 in conjunction (tank mix or premix) with Group 15 in pre or burndown programs in the spring (Figures 1, 2 and 3).

Use of 2,4-D choline (Enlist One) or glufosinate (Liberty) applied alone post provided inadequate control (less than 90%) of waterhemp (Figure 3). In contrast, 2,4-D choline and glufosinate (Interline) applied as a tank mixture provided superior end-of-season control of waterhemp in Enlist E3 soybean.

This strategy (multiple sites of action) is one of the best management practices to delay resistance and preserve the utility of 2,4-D choline and glufosinate for waterhemp control. While these programs evaluated a limited number of available postemergence herbicides, the same principles apply to dicamba-based herbicide programs for Xtend (tolerance to glyphosate and dicamba) and XtendFlex (tolerance to glyphosate, glufosinate and dicamba) soybeans available for 2021.

Waterhemp has an extended emergence period (May through August in Iowa). Waterhemp grows more rapidly than most weeds prevalent in Iowa soybean fields — up to 1 to 1¼ inches per day. Therefore, timing of herbicide applications is critical.

Furthermore, a high percentage of waterhemp plants can emerge late in the season (at or after V5 to V6 stage of soybean). Plants that emerge or survive under the soybean canopy most likely don't contribute to crop yield reductions; however, those plants can add several thousand seeds (waterhemp being a prolific seed producer) to the soil seed bank. This warrants the need for a season-long management of waterhemp to prevent any seed bank additions using a diversified approach.

Jha is an associate professor and extension weed specialist with the agronomy department at ISU.

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