Protect Soybean Yield from Weeds

By Scout Nelson

Eric Yu and David Nicolai, both UMN Extension Crops Educators, highlighted the importance of strong pre-emergence herbicide programs as soybean planting begins across Minnesota. These products remain one of the most effective tools for controlling weeds early and protecting soybean yield potential.

Many soybean fields in recent years have relied heavily on post-emergence herbicide programs. However, this approach is becoming riskier as herbicide-resistant weeds continue to spread. Resistance to glyphosate and other post-emergence products has reduced control options in many fields. The recent confirmation of glufosinate-resistant waterhemp in Minnesota increases concern for soybean growers.

Research shows that soybeans should remain weed-free from emergence through the V3 to V4 growth stage to avoid yield loss. This early period is the critical window for weed control. Weeds that emerge later may still produce seed and increase future infestations. Because of this, a “start clean, stay clean” approach remains important.

Waterhemp is one of the most difficult weeds in Minnesota soybean systems. It emerges over a long period, creating several flushes during the growing season. Depending only on post-emergence sprays often means chasing weeds that may already be too large or resistant. Pre-emergence herbicides help by controlling early flushes and reducing overall pressure.

Layered residual programs can also help. This means adding another residual herbicide later in the season to extend weed control. If using only a pre-emergence residual program, growers should use full labeled rates and products that target major weed species.

Proper timing is important. Pre-emergence herbicides can be applied before planting, at planting, or shortly after planting before crop emergence. Rainfall is needed to activate many products, and dry conditions may delay performance.

Growers should begin with clean fields because pre-emergence herbicides do not control large established weeds. Using multiple modes of action broadens control and helps slow resistance development.

Some products may cause temporary soybean injury during cool and wet spring conditions. However, soybeans often recover well. In most cases, the benefit of early weed control outweighs the risk of short-term crop response.

Photo Credit: istock-ds