Colder temperatures have really hindered weed growth over the past couple weeks. However, there are several winter annual weeds present that will start to flourish once temperatures start to warm. As temperatures increase, competition for field operations will occur. Most will want to start planting as soon as possible, but it is important to make sure weeds are managed prior to planting. Not controlling weeds can interfere with planting and compete with the emerging crop for light, water, nutrients and space that can reduce crop yield.

For example, several years ago we conducted research over six locations that examined preplant burndown applications made at least seven days prior to planting compared with delayed applications of glyphosate at VC (unifoliate) to V1 (1 trifoliate) and V3 soybean. Average soybean yield loss was 8.3 bushels per acre if applications were delayed until VC/V1 soybean. Waiting until soybeans were at the V3 growth stages resulted in a 9.2 bushel per acre loss (Figure 1).

In addition to protecting yield by reducing early-season weed competition, starting the growing season with a clean field either with a burndown application or tillage will also eliminate several winter annual weeds that may potentially serve as hosts for destructive insects and soybean cyst nematode.

One of other challenges we have if we don’t control weeds prior to planting is that as these weeds continue to grow, they can be harder to control. This is especially a problem in the case of herbicide (glyphosate and ALS)-resistant horseweed (marestail). If resistant horseweed is not managed prior to planting, there are no postemergence herbicides for control in Roundup Ready or non-GMO soybean. Also, if not controlled early, this weed will also be more difficult to control in LibertyLink, LibertyLink GT27, Roundup Ready 2 Xtend and Enlist E3 soybean.

There are several steps to follow when managing herbicide-resistant horseweed that include using effective burndown applications and good soil-applied residual herbicides. These steps are outlined in Michigan State University Extension's “Herbicide-resistant horseweed (marestail) in Michigan” fact sheet or on page 220 of the “2020 Weed Control Guide for Field Crops” (MSU Extension bulletin E-434).

Additionally, keep in mind many of the burndown herbicides and effective soil-applied residual herbicides that we use in soybean need to be applied prior to soybean emergence or severe crop injury can occur.

A complete listing of burndown herbicide programs and their effectiveness ratings can be found in the no-till soybean section, Table 2P of the “2020 Weed Control Guide for Field Crops." Remember, treatments that contain 1 pint per acre of 2,4-D ester need to be applied a minimum of seven days before soybean planting.

Corn is also very susceptible to early-season weed competition. Starting with a weed-free seedbed with tillage or an effective burndown herbicide program helps protects corn from yield loss later in the season. Soil-applied (PRE) residual herbicides are also important to an overall weed control program in corn. However, sometimes corn planting operations can get ahead of the sprayer and there are several soil-applied (PRE residual) herbicide options that can be used once corn has emerged. A complete listing of these herbicides can be found in Table 1H of the 2020 Weed Control Guide.