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Figure 1. Kochia seedling.
Kochia and waterhemp are two of the most important weeds in South Dakota soybean and corn fields (Figures 1 & 2). They are annual broadleaf weeds that produce large quantities of seed with high germinability. Kochia is an early germinating weed, beginning in early March and continuing into May.1,2 Waterhemp begins to germinate in mid-May, with peak emergence in early July. Both species have a high degree of genetic diversity, resulting in numerous biotypes with varying degrees of sensitivity to herbicides.
Kochia and waterhemp require integrated management in corn and soybeans that begins with tillage or a burndown herbicide application (glyphosate plus dicamba). Populations of kochia and waterhemp in South Dakota are mixtures of tolerant and resistant biotypes. Monsanto recommends tank-mixing multiple herbicides with effective sites of action for kochia or waterhemp, and that preplant or preemergence (PRE) residual herbicides be the foundation of a corn or soybean weed management plan. A PRE herbicide application will help reduce early season weed competition, add additional sites of action to a resistance management program and expand the post-emergence (POST) application window. Sequential applications of multiple site of action herbicides and tank mixtures are the best options for waterhemp and kochia management. Early post-emergence (POST) applications may require a burndown and residual herbicide combination and should be made prior to weeds reaching 4 inches in height. Work with your agronomist and dealer to determine the appropriate herbicides to use for your weed situations.
Most research studies have found that crop yield potential is protected if weeds are controlled before they reach a height of four inches in corn or soybean.3 Waterhemp plants compensate for small seed size with a relatively higher growth rate than most weeds or crops, upwards of one inch per day during the season.4 Full season waterhemp competition resulted in a 44% reduction in yield potential in 30-inch or 37% reduction in 7.5-inch row soybean.4 Corn yield potential was reduced up to 15% with heavy infestations of waterhemp.
A residual herbicide program can help delay waterhemp emergence. Iowa State University research has shown that delaying waterhemp emergence beyond the V5 stage of soybean growth resulted in only 10% reduction in yield potential or one percent in corn if waterhemp was controlled prior to six inches in height.4 Post-emergence herbicides used in corn and soybeans should be applied before waterhemp exceeds 4 inches in height. Crop competition with waterhemp or kochia is essential for effective management and herbicide effectiveness.
1 Hartzler, B. and Buhler, D. 1997. Weed emergence research. Iowa State University. 2 Werle, R. 2016. Start now to manage early weeds in your soybean fields. University of Nebraska Cropwatch. 3 Hartzler, R. 2000. Early season weed competition. Iowa State University Integrated Crop Management. 4 Nordby, D., Hartzler, B., and Bradley, K. 2007. Biology and management of waterhemp. GWC-13. Iowa State University. 160413155041