Narrow Row Soybeans are a big Advantage for Weed Control

Mark VanGessel, Extension Weed Specialist;

As herbicide-resistance continues to spread, we need to diversify weed control to put less selection pressure for developing resistant populations. We are finding more fields with Palmer amaranth and common ragweed resistant to three modes of action: in many cases leaving no good herbicide options.

Many alternatives to herbicides often require significant changes to production practices. For instance, cultivation is not compatible with no-till production.

Some of the newer tactics like weed electrocution or impact mills on combines are quite effective, but they operate later in the season or at harvest after the weeds have had a chance to compete with the crop and have reduced yields.

Planting into a recently killed cover crop or planting green (killing the cover crop after planting) both significantly reduce the number and size of weed seedlings early in the season. Fewer weeds that emerge mean fewer weeds are exposed to herbicides and less selection pressure for herbicide-resistant weeds. Moreover, for those weed seedlings that emerge, their growth rate is much slower in the presence of a cover crop than growing with bareground, providing a wider window for postemergence treatment. Planting into a field with lots of cover crop residue or planting green, often require some planter modifications, such as sharp, straight coulters (not wavy ones), extra weight, or adjustment to closing wheels. Therefore, if you were not planning on using this approach, it may be too late for this season.

A good crop canopy that shades the ground quickly is very effective, and I am not aware of any weed that grows well under a crop canopy without sunlight. Planting in 15-inch rows instead of 30-inch rows is the best way to achieve this. Soybean leaves will fill in between the rows 7 to 10 days sooner at 15 inches compared to 30 inches and can make a big difference. In my opinion (and assuming you have access to a planter with 15-inch row units), soybeans in 15-inch rows is one of the best strategies to diversify a weed control program.

30 Inch row Soybeans

Figure 1: 30 inch row soybeans

15 inch row soybeans

Figure 2: 15 inch row soybeans

Soybean plots side by side; planted with the same variety on the same day. The only difference is the top photo are soybeans planted in 30-inch rows and bottom photo is 15-inch rows.

A recent article in the UD Agronomy Blog ( January 27, 2023) discusses an increase in yield with 15-inch rows compared to 30-inch rows.

So narrow soybeans is a “low-hanging” fruit to diversify a weed management program for soybeans with limited downside.