Don’t let early-season weeds bully your soybeans.

Start clean and stay clean. It’s a simple weed control message, but when it comes to soybean yield it really does speak volumes.

Research conducted by Dr. Clarence Swanton from the University of Guelph shows that early weed pressure can have a significant impact on soybean yield: the plant can actually sense weeds growing on the soil surface before it emerges and, as a result, will start to change its physiology. When the plant detects weeds during the early growing stage — from the first to third trifoliate (V1 to V3) — it starts to reduce yield potential based on the competition from the surrounding weeds.

We typically refer to this phase as the critical weed-free period. Depending on the amount of weed pressure, growers could lose from 0.5 to 2 bushels/acre per day when weeds are not controlled.

Row spacing can make a difference

There are several cultural and herbicide management strategies that can help keep your soybean fields clean early in the season.

University of Manitoba research scientist Dr. Rob Gulden and M.Sc. student Jon Rosset recently studied ways to actually reduce or shorten the critical weed-free period. Basically, they were looking to reduce that time period when weed pressure is altering the plant and stealing your yield potential.

Their research indicated that narrowing row spacing, maintaining adequate soybean plant densities and variety choice all affected the end of the critical weed-free period to varying degrees.

Of the three cultural practices, Gulden and Rosset found reducing soybean row spacing had the greatest impact on shortening the weed-free period and required fewer in-crop herbicide applications.
The research shows that it simply takes longer for wider rows to close and the extra sunlight gives weeds a better chance to compete.

From a plant density perspective, they concluded that target densities below 180,000 plants/acre may lead to additional in-crop herbicide requirements. 

When it comes to variety selection, the researchers noted that growing a variety that has demonstrated competitive growth in local growing conditions is a good strategy.

Incorporating residual herbicides

Seed and herbicide technology also play a key role in helping growers start clean and stay clean throughout the critical weed-free period. A good way to do this is to start with a pre-emergence burndown that includes a residual herbicide. There are several good options that can be used, depending on your weed spectrum.

If you choose to skip this step and opt for a post-emergent program, the herbicide should be applied early enough for the weeds to be dead by V1. The issue with this plan is that, ideally, your soybean crop doesn’t see weeds until it’s past the V3 stage to maximize yield potential.

Remember, under good growing conditions, a new trifoliate emerges approximately every five to seven days, so you’re counting on your post-emergent application to keep the field clean for up to a month. This will most likely require a second post-emergent application within that timeframe.

Combining cultural management decisions and technology to control weeds before soybean seedlings sense competition is a winning strategy. It really does help create a confident soybean crop that has higher yield potential in the absence of those early season weeds that bully seedlings and steal yields.