Annual Grass Control in 2022

By John Wallace and Dwight Lingenfelter 

Glyphosate shortages in the 2022 growing season will require rethinking several aspects of herbicide program strategies. Annual grass control in corn and soybean has been relatively easy in the Roundup-Ready era because glyphosate has excellent activity on grass species, even when targeting emerged grasses at larger than recommended heights with later postemergence applications. If glyphosate remains in short supply, alternative strategies to maintain grass control will be needed. To provide greater management flexibility this season, consider optimizing use of soil-applied products that have grass activity.

Annual grass control with residual herbicides.
While several herbicide groups provide some residual activity on grass species, seedling shoot inhibiting herbicides (Group 15; dimethenamid, acetochlor, S-metolachlor, pyroxasulfone) or pendimethalin, a seedling root inhibitor (Group 3), should be considered for foundation residual control of annual grasses. These products provide control of annual grasses that commonly emerge early in the season, such as foxtail species. Later emerging grasses like crabgrass and fall panicum are also controlled by most of these products, but the level of control will vary depending on differences in the length of residual activity between products. Importantly, these products also provide control of some small-seeded broadleaf species, including pigweed species. Additional detail on differences in weed control spectrum among products can be found in the Penn State Agronomy Guide ( Table 2.2-7  , pg 258; Table 2.4-7  , pg 393).

Several factors should also be considered when selecting from these options (Table 1). First, Group 3 and 15 herbicides differ in the amount of precipitation needed for activation in the soil. These products do not control emerged grass or broadleaf weeds, so the lack of an activating rainfall prior to weed emergence can result in poor early season weed control. Previous field studies suggest that 0.5" is likely sufficient for activating dimethenamid, acetochlor, and S-metolachlor. Reduced levels of weed control due to incomplete activation are more likely at this precipitation level with pendimethalin. Second, these products differ in the length of residual weed control activity. Soil and environmental conditions will influence the length of residual activity in any given year, but on average, we expect S-metolachlor and pyroxasulfone to provide the greatest length of residual control for annual grass species. Finally, several Group 15 products are included in postemergence pre-mix products or can be tank-mixed to provide 'overlapping' residual activity in both corn and soybean. Given that Group 3 and 15 products are only active on emerging weeds, they are typically best-positioned as a component of a pre-plant or early-postemergence application. Keep in mind that unlike products such as triazines or isoxaflutole (e.g., Balance Pro), these Group 3 and 15 herbicides do not have "reach-back or recharge" activity that can provide some control shortly after the weeds have emerged. Thus, it is necessary for them to be applied and activated before weed seed germination. But in 2022, an overlapping residual grass herbicide is a viable strategy for controlling later emerging grasses if glyphosate is in short supply.

Table 1. Herbicides that provide soil-applied residual activity of annual grasses.

Several generic products are now available that contain S-metolachlor, acetochlor, and pendimethalin, and many pre-mix products labeled for preemergence or early-postemergence contain a Group 15 herbicide. In some cases, pre-mix products may be formulated with a lower rate of the Group 15 herbicide compared to standard labeled rates when they are applied alone, which may reduce annual grass control. Consider consulting tables in the Agronomy Guide that compare rates of pre-mixed components before selecting from pre-mix options ( Table 2.2-2  , pg 246; Table 2.4-2  , pg 383). Common pre-mix products that contain Group 15 herbicides are provided in Table 2.

Table 2. Examples of pre-mix preemergence products that contain SOA 15 herbicides.