MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

No-till soybean production in Michigan relies almost exclusively on glyphosate for preplant and postemergence weed control. While glyphosate continues to provide excellent control of most weeds, timely applications before planting and after soybean emergence are necessary to ensure the best growing conditions for your soybean crop. The following recommendations are based on MSU research funded by the Michigan Soybean Promotion Committee (MSPC).

Don’t wait to make your burndown herbicide application

One way to give your soybean crop an early advantage is to make burndown herbicide applications before or near the time of planting. In MSU trials, preplant burndown applications of glyphosate + 2,4-D ester1, made at least seven days prior to planting, provided excellent control of most weeds present prior to planting. Burndown applications of glyphosate that were delayed until soybeans were at the VC (unifoliate) to V2 (2 trifoliates) growth stages resulted in an average yield loss of 8.3 bushels/acre (Table 1). Waiting until soybeans were at the V3 to V5 growth stages resulted in a 9.2 bushel/acre loss. In addition to protecting yield by reducing early-season weed competition, starting the growing season with a clean field will also eliminate several winter annual weeds that may potentially serve as hosts for destructive insects and soybean cyst nematode.

¹ 2,4-D ester at 1 pt/A must be applied at least 7 days prior to planting soybean.

Table 1. Delaying burndown herbicide applications will reduce soybean yields. Soybean yield is averaged over six sites.

^a Roundup WeatherMax (22 fl oz/A) + 2,4-D ester (1 pt/A) + AMS 
       (17 lb/100 gal) was applied.
^b Roundup WeatherMax (22 fl oz/A) + AMS (17 lb/100 gal) was applied after soybean emergence.

Residual herbicides can improve early-season and postemergence weed control

Though postemergence application timings for glyphosate are generally flexible in terms of weed height, making the application when weeds are an appropriate size remains a major factor for ensuring satisfactory control. Beyond starting the growing season with a clean field, including an herbicide with residual activity with your burndown application will reduce in-season weed growth and extend early-season weed control. Residual herbicides can improve control and reduce the growth of problematic weeds, such as horseweed (marestail), common ragweed, giant ragweed, and common lambsquarters. As a result of reduced weed growth, the time needed between a burndown herbicide application and a postemergence glyphosate treatment may be extended. This is especially important when weather conditions may prevent timely postemergence applications. There are several different residual herbicides that can be included with a burndown application. For a complete listing of soybean herbicides with residual weed control consult Table 2G in the MSU Extension publication E-434 Weed Control Guide for Field Crops.

Residual herbicides can pay for themselves

There is no added application expense when a residual herbicide is included in a burndown herbicide application. Depending on market prices, weed pressure, and herbicide cost, the extra expense of adding a residual herbicide to a burndown application may pay for itself with as little as a 1 bushel/acre increase in yield. In fact, the economic returns from the addition of any of six the different residual herbicides that we examined over two years at three locations were not different from a burndown application of glyphosate + 2,4-D ester alone. This was true regardless of soybean commodity prices from $5 to $15/bushel. When choosing residual herbicides, the decision should be based on the types of weeds that are in the field and weed pressures to maximize economic return.

A pro-active approach to glyphosate-resistant weeds

One of the most important benefits of including a residual herbicide in a weed control program is the opportunity to use herbicides with additional modes of action besides glyphosate. While glyphosate continues to provide excellent control of most weeds, the spread of glyphosate-resistant weeds throughout Michigan and the Midwest is becoming an increasing concern.

In 2007, the first population of glyphosate-resistant horseweed in Michigan was confirmed in Mason County. As glyphosate-resistant weeds become more widespread, use of residual herbicides will give growers another option for weed control in no-till soybean production to improve the stewardship of glyphosate and glyphosate-resistant cropping systems.