Control weeds early

No farmer would argue that allowing weeds to grow with a soybean crop reduces yield and profit potential. But some may argue at exactly what point weeds begin to steal yield. “Anytime a weed emerges and begins growing, it competes with the crop for nutrients, water, sunlight and space,” says Jim Frederick, Syngenta agronomy representative in southern Iowa. “If left unchecked, emerged weeds can cause problems and reduce yields.”

By the time you notice weeds during casual inspections from the field edge, yield reduction has already begun. “Growers today are learning there is a problem with using just postemergence herbicides,” says Mike Owen, Iowa State University Extension weed specialist. “The longer you allow weeds and the crop to co-exist, the greater the loss in yield.”

Key Points

Relying on total postemergence herbicide program for soybeans invites trouble.

Even small weeds compete early with the crop for sunlight, nutrients and moisture.

A preemergence herbicide in program controls weeds early and protects yield.


Some weeds have greater impact on yield than others. Not only do weeds compete for light, nutrients and moisture, but also some have chemical defense mechanisms to edge out the competition. For instance, giant foxtail and Palmer amaranth have the capacity to secrete chemicals into the soil that stunt nearby vegetation. This may even occur despite a grower’s efforts to control weeds with a postemergence herbicide, making preemergence control all the more valuable. Another herbicide-resistant and highly aggressive weed in Iowa is waterhemp, which spreads quickly.

Different tool in toolbox

In addition to protecting yield potential and enabling more flexible application timing for postemergence treatment, you can introduce diversity by using a preemergence application. That will help protect the available herbicide technology. In other words, a preemergence treatment can allow you to use a herbicide that has a different mode of action than the post-emergence product provides.

“If you only use postemergence herbicides, the tools in your weed control arsenal become somewhat limited,” says Frederick. “But if you use preemergence residual herbicides that have multiple modes of action, such as Boundary and Prefix herbicides as part of that toolbox, then it greatly expands your likelihood of effectively controlling weeds.”

Not only does it have immediate repercussions, reliance on only postemergence herbicides exerts major pressure on single modes of action and can hasten the evolution of weed resistance to herbicides. All it takes is for one resistant weed to survive an application for it to reproduce and spread seed throughout the field.

Source: ISU Extension, Syngenta

It’s a matter of timing

When it comes to herbicide effectiveness, application timing is crucial. “Although growers save money on the front end by not adding a preemergence herbicide, it could cost them that savings, and more, in the end. “This is a very clear case of penny-wise, pound-foolish,” warns ISU’s Mike Owen. “Herbicides that are applied at preemergence will pay for themselves by protecting crop yield, and also by providing greater flexibility in the timing of postemergence applications that are applied later.”

Owen adds, “A soil-applied residual herbicide is absolutely critical in corn and soybeans. Besides adding another way to kill weeds and using a different mode of action to do it, the soil-applied herbicide can also get your crop off to a quick, weed-free first few weeks in the field.”

You should use multiple modes of action. This kills any weed biotype beginning to resist a herbicide mode of action. However, you need to be cautious in your choice, advises Owen. Multiple modes may not work for weed resistance that already exists in a field. For example, ALS herbicides will not control waterhemp in Iowa, given most populations of that weed already resist ALS herbicides.


This article published in the May, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.