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Early-Season Weed Control is Important

By Christy Sprague
Winter annual weeds and newly emerged summer annuals are starting to flourish. As temperatures start to increase, competition for field operations will occur. Most growers 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.
Several years ago we conducted research over six locations that examined pre-plant burndown applications made at least seven days prior to planting compared with delayed applications of glyphosate at VC (unifoliate) to V1 (one trifoliate) and V3 soybean. Average soybean yield loss was 8.3 bushels per acre if applications were delayed until VC/V1 soybean (Fig. 1). Waiting until soybeans were at the V3 growth stages resulted in a 9.2 bushel per acre loss.
Figure 1. Soybean yield due to delayed burndown herbicide applications. Burndown applications were made seven days prior to planting at unifoliate to V1 (1-trifoliate) soybean and V3 (3-trifoliate) soybean.
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 the 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 post-emergence herbicides for control in Roundup Ready or non-GMO soybean. Also, if not controlled early, this weed will be more difficult to control in LibertyLink, LibertyLink GT27 and Roundup Ready 2 Xtend 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’s “Herbicide-resistant horseweed (marestail) in Michigan” fact sheet or on page 217 of the “2019 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 “2019 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 2019 Weed Control Guide.