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This spring I have looked at many fields with challenging weed issues. Regardless of the weed control program you prefer there are some basic principles that apply to them all. 

If you have heavy winter annual weed pressure and are not using cover crops I would encourage you to consider a fall burndown. Marestail continues to be one of our most challenging weeds in no-till fields.  Fortunately in our part of the world, marestail still emerges like a true winter annual weed which means most of them emerge in the fall and can be relatively easily and economically controlled by a well-timed fall burndown.

A fall burndown might or might not replace your spring burndown depending on timing, weed pressure, effectiveness of control, planting date and other factors. If you are going after marestail in a spring burndown you need to get it when it’s small (preferably under 4”) and use a minimum of a pint of 2,4-D per acre. Once you have planted your soybeans any remaining marestail will likely be there the rest of the year.  We may be able to stunt or burn them, but you have lost the opportunity for complete control.

Waterhemp is a challenge on most soybean acres across Illinois today. Due to the excessively wet summer of 2015 waterhemp had very good seed production and some fields will see heavier weed pressure this year as a result. Waterhemp emerges from mid spring through late summer, so season-long control is challenging. Regardless of the weed control platform you choose recommendations for waterhemp control are going to sound very similar.

Tips for a successful waterhemp control program:

1. Start clean with a good burndown (containing 2,4-D) or full width tillage. Vertical tillage tools will remove some weeds, but that is not what they were designed for.
2. Use a pre-emergence soil residual product that offers effective waterhemp control. This product will need to be applied and activated BEFORE waterhemp emerges.
3. Make your first postemergence application before waterhemp reaches 4” in height. Any product you use to control glyphosate-resistant waterhemp is designed to control small weeds.
4. This early post application should contain a product that offers additional soil residual control of waterhemp to prevent later flushes until the soybean canopy is fully developed.

Generally speaking, if your weed control program is doing a good job of controlling marestail and waterhemp you will probably also achieve good control of other challenging species such as giant ragweed, lambsquarter and velvetleaf.  In areas that have to battle Palmer amaranth (pigweed) it is managed exactly like waterhemp, but the stakes are higher and the penalties for poor control are greater.

Today the reality is that weed control is not as simple or economical as it was 5 – 10 years ago, but you can achieve season-long control of the toughest to control weeds with the right program.

Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto^® he currently supports the Asgrow^® and DEKALB^® brands in seven counties in western Illinois.