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Start right with residual

With current advantageous grain prices and our ability to produce impressive yields, we have the potential to generate some great gross margins this year if the weather cooperates and we implement sound crop and pest management. Let’s focus on one key management strategy in this article: using a residual herbicide to complement your post-program in corn and soybean fields.

I can guess what some of you are probably thinking: You just don’t have time to get a residual on, so you’ll spray everything postemergence.

And why make two trips anyway to control weeds when there are postemergence herbicides that can handle them in one post shot, especially in glyphosate- and glufosinate-resistant crops?

I agree, time is always at a premium during planting season. But in the long run, using a residual to protect your crops will make you more profitable.

First, early-season weeds compete with crops for water and sunlight, and can consume valuable fertilizer intended for our crops. With the fertilizer price spikes of the last few years, I want to be sure the crop gets our high-dollar fertilizer, not the weeds. Second, and perhaps more important long term, herbicide-resistant weeds are increasing in the Midwest. As she almost always does, Mother Nature found a way to beat our convenient “post-emergence-only” systems.

Most notably in Iowa, common waterhemp and giant ragweed have documented glyphosate resistance, and we are pretty sure marestail will be added to that list. We also often fight weeds that are not technically “resistant,” but are just plain tough to control with one posttreatment. We had a pretty hard time dealing with these weeds even before they were resistant to our big gun, glyphosate. Using residual herbicides with different modes of action will help to contain these hard-to-manage weeds.

More important than ever

I can still hear some arguing: Why worsen the spring rush applying residuals when, most of the time they’re used, we still clean up with postemergence products anyway?

For those of you still unconvinced, here are two more reasons why residual herbicides are more important than ever:

Saving time. Postemerge spray season is now probably more stressful than the preemerge season used to be. Using a residual herbicide, even at the lower “set-up” rates promoted in herbicide-resistant crops, will buy you time. Expanding the postemerge spray window by a couple weeks can further help you with risk management.

Mother Nature is pretty stingy with good spraying days. With residual herbicides reducing weed pressure, the risk of yield loss is greatly reduced while you wait out windy and rainy days to get the post-herbicides applied.

Better yields. Weeds steal vital resources from crops. Residual herbicides reduce weed pressure, which reduces the resources lost to weeds.

When to use residual herbicide

Does every field need a residual herbicide? Probably not. Consider the following criteria to help decide which fields to target with a residual application:

History of weed escapes. If the field has had high weed pressure the past few years, it is a prime candidate for a residual herbicide.

Herbicide-resistant weeds. If you have them or suspect you do, remember the value of multiple applications and multiple modes of action to keep them in check. Residual herbicides are a cornerstone of managing resistant weeds.

Problem weeds. Many weeds, even if not herbicide-resistant, are still hard to control. While this list could be endless, waterhemp, lambsquarter, velvetleaf and marestail come to mind. The right residual herbicide will offer valuable help with your problem weeds.

With the great control postemergence products provide, the preemergence products don’t have to perform perfectly. We can often get by with reduced rates of application as long as the product has good activity on the dominant weeds.

On the other hand, with the competitive prices of many residual herbicides, both name brand and generic, an argument can be made for not reducing rates.

My thoughts are if you contend with any of the three challenges I’ve listed here, move to at least a three-quarter rate of the residuals, and if your local agronomist makes a compelling case for a full rate, be sure to hear him out.

If you don’t have the time and manpower to apply your own residual herbicides, consider a custom applicator. These professionals can cover a lot of ground with herbicides while you are planting, and they will spray at a very competitive price.

Also, be sure to keep in mind that residuals don’t necessarily have to go on prior to planting; there is a small window with many of the residuals after planting, depending on the particular herbicideproduct and what your carrier is.

Good luck this spring, be safe and think hard about getting residual herbicides on in the next couple weeks.

McGrath is partnership program manager of ISU’s Corn and Soybean Initiative.

This article published in the April, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.