Cooler temperatures and wetter conditions this spring 2018 have led to slower than normal weed growth and a slower start to field work around Michigan and Ohio. However, now that temperatures have greatly improved, weed growth will flourish and 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 these weeds can lead to interference with planting operations and competition with the emerging crop for space, light, moisture and nutrients that can reduce yield for all crops.

For example, several years ago Michigan State University scientists 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. 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.

One of the other challenges farmers have if they 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 soybeans. Also, if not controlled early, this weed will be more difficult to control in LibertyLink and Roundup Ready 2 Xtend soybeans.

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 MSU’s  “Herbicide-resistant horseweed (marestail) in Michigan” factsheet, or on page 213 of the “2018 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 farmers use in soybeans need to be applied prior to emergence, or else 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 planting soybeans.

Source: MSUE