As farmers are pulling planters into the fields this spring, Craig Abell and Bruce Battles, crop specialists for Syngenta, who cover Southern Iowa and Northern Iowa, respectively, are fielding a number of questions about weed control. One thing about herbicides that these two agronomists are emphasizing to farmers in this earlier than normal spring is that "not all residual herbicides are created equal."

* Some herbicides have longer-lasting residual than others. There's a big difference in two to three weeks residual versus six to eight weeks residual.

How Long Will Your Residual Herbicide Last?

* Some herbicides have greater application flexibility than others. Some can only be applied pre-emergence or up to V2 growth stage of corn, while others can be applied up to 12-inch corn or 30-inch corn. Choosing herbicides with maximum application flexibility is critical, so that farmers are prepared in the event Mother Nature throws a curveball.

"One thing is for sure, the mild spring is accelerating the entire growing season including the growth of weeds," says Abell. "It will be imperative that growers choose the longest-lasting residual herbicides with the greatest application flexibility to get the most bang for their buck."

Farmers are asking a lot of questions about weed management this spring

Here are some other questions the Syngenta crop specialists are getting from farmers. Full rates vs. reduced rates of application—which should I apply? That's an important question. Also, should I make a two-pass application or a one-pass application? What about using overlapping residuals (using a pre-emergence herbicide with residual followed by a post-emergence herbicide with residual)? And how important is it to use herbicides with multiple modes of action? Abell and Battles offer the following answers and recommendations.

* Winter annual weeds need to be controlled by burndown herbicides to prevent competition with the crop for moisture and nutrients. If these weeds aren't killed, they will also potentially interfere with planting and crop establishment. Killing these weeds with a burndown herbicide will also prevent weed seed production and reduce sites in fields for insects like black cutworm moths to deposit their eggs, which eventually hatch into cutworms.

* Horseweed (marestail) is a common winter annual that should be controlled as early as possible, but before it exceeds 4 to 6 inches in height. Glyphosate-resistant horseweed is common throughout the Midwest so alternative herbicides will be required.

* Pre-plant tillage operations can help control winter annuals, but this also promotes seed germination of summer annuals. If summer annuals have emerged before planting, a residual herbicide that also has burndown activity should be considered.

* Weeds will grow more rapidly in a warm spring like this one we're having in 2012, compared to a "normal" spring. Growers should use full rates of pre-emergence herbicides (versus foundation rates) for longer-lasting residual control. Two-pass weed management systems should be considered. Use a herbicide with residual activity for both preemerence and for post emergence applications. Use herbicides with multiple modes of action to control both grasses and broadleaf weeds.