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Weed Control in Soybeans

Commercialization of glyphosate-resistant (Roundup Ready) soybeans introduced a new era in soybean weed control. While this technology made postemergence broad-spectrum weed control with a single non-residual herbicide a reality in some situations, other situations require a tank-mix partner or a two-pass weed control program.

With conventional tillage, it is recommended that glyphosate (Roundup PowerMax, Durango DMA, Touchdown Total, etc.) be applied to glyphosate-resistant soybeans 24 to 30 days after planting. While this works in most situations, less than desirable common lambsquarters control has been reported in some instances. Normal application rates of glyphosate in the field or greenhouse have consistently controlled lambsquarters that is no more than 3 inches tall. Lambsquarters that is greater than 3 inches tall may require increased rates of glyphosate or a tank mix of Harmony SG or of Synchrony XP with glyphosate. Table 6.7.2 provides suggestions on how to improve lambsquarters control with either a one pass or two pass program.

When planting no-tillage glyphosate-resistant soybeans, a burndown herbicide should be used before or at the time of planting if significant weed cover is present. Several options that enhance burndown, provide residual weed control, or control dandelions in no-tillage soybeans can be found in Table 6.7.3. A postemergence application of glyphosate should be made 24 to 30 days after planting.

Because soybeans are sensitive to *atrazine and *†Princep residues, care should be taken the year before planting soybeans to ensure that herbicide carryover will not affect their establishment.

Herbicide resistance management involves the use of crop rotation and cultivation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will help manage existing herbicide resistant weed populations and delay development of new resistant weed populations.

To effectively utilize herbicides with different sites of action, everyone involved in decisions about weed management must have the site of action classification readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619). A group number is given to all herbicides with the same site of action. These “GROUP NUMBERS” are included in the “Chemical weed control tables” in each crop section.

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