Make sure weeds have been cleaned up prior to planting. Left alone, they might take a bigger bite out of yield potential than you suspect, and research bears that out.
Several years ago, we made comparisons over six locations, examining 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.
The numbers tell the story:
- 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 also eliminates several winter annual weeds that may potentially host destructive insects and soybean cyst nematode. That clean field could be the result of a burndown application or tillage.
If we don’t control weeds prior to planting, these weeds continue to grow, and they can be harder to control later. This is especially a problem with glyphosate- and ALS-resistant horseweed (marestail).
With Horseweed, No Fallback Options
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.