Seeing Postemergence Problems in Soybeans

Weeds are tough enough to control today. Throw in weather-related issues and it’s a recipe for disaster.

This is the year we can all learn from our experiences with so-called “resistant/tolerant weeds.”  I have been experimenting with many different combinations of chemicals to find the most effective suppression, because I do not think we will ever have full control of these most troublesome weeds.

First and foremost, we need to begin targeting each weed individually, instead of a blanket chemical program across all acres. This means we need to map and record each field and identify the weeds that are present and problematic.

Second, each weed has different and varying life cycles. For example, waterhemp emerges later in the season and then continues to germinate all season long, whereas marestail germinates in the fall and then is there from the beginning of the season, like our winter annuals. Both weeds need to be treated differently. Waterhemp needs to have a residual applied in the early spring and then follow up with a second residual later. Marestail needs to have a fall applied application of dicamba/2,4-D along with a possible residual at that time, too. Then it needs a second application of a residual in the spring.

Third, the weeds need to be monitored closely so the second application is timely and the weeds themselves don’t get too tall.  This is probably the hardest part of keeping these weeds under control.  The best control is when weeds are under four inches in height.  Otherwise, control is extremely varied.

Here are photos of some of the postemergence mishaps that have occurred this season.

Due to the season, some residuals were applied later than usual and only included glyphosate without 2,4-D added to the mix.

Another mishap was an application of no residual early and then an application of glyphosate and a light rate of a PPO inhibitor. The waterhemp were left standing and becoming too tall to control.

Here is a photo of varied weed control on volunteer corn, probably due to an incorrect rate and not enough of the correct adjuvant needed for complete control.

And last is a photo of weeds that are just too tall to control. Remember, the height limit for effective control is 4 inches. Some of these marestail measured 10 to 14 inches. This field was sprayed with a combination of glyphosate and an ALS inhibitor.

We are still spraying soybeans in a second pass in northern Illinois, with more combinations being tried. I will try to review them all in my August blog.

Dawn Kielsmeier is an agronomy sales specialist with Pearl City Elevator in Baileyville, Ill. She has a B.S. in dairy science and an M.S. in agronomy, both from the University of Illinois and has been a CCA since 1993. She is a 2017 Illinois Soybean Association CCA Soy Envoy.