Cooler temperatures and wetter conditions this spring have led to slower-than-normal weed growth and a delayed start to field work around Michigan. However, as temperatures start to increase, weed growth will flourish and competition for field operations will occur.
Most will want to begin planting as soon as possible, but it is important to make sure weeds are managed prior to planting. Not controlling these weeds can lead to interference with planting and competition with the emerging crop for space, light, moisture and nutrients that can reduce yields.
For example, several years ago we conducted research 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 (1 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.
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.
Horseweed Escapes Can Be A Huge Headache
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 and Roundup Ready 2 Xtend soybean.
There are several steps to follow when managing herbicide-resistant horseweed that include using effective burndown applications and using good soil-applied residual herbicides. These steps are outlined in Michigan State University’s Herbicide-resistant horseweed (marestail) in Michigan factsheet.
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 2O of the 2018 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.