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Herbicide carryover a concern?

Yea, I know, I’m one of the guys who have been pounding the rock, promoting the use of residual herbicides to increase productivity and profitability, and help manage herbicide resistance. Now I bring you this topic: potential carryover of some of these same residual herbicides. Go ahead and say it; I can take it. As the guys on Monday Night Countdown say, “C’mon, man!”

In reality, things aren’t as bad as they seem. Yes, 2011’s weather conspired to bring us the potential for carryover in 2012, but as we drill down into the topic, you’ll see that we can manage the issue, and it will likely be very situational and, hopefully, on a limited number of acres.

Let’s start with some basic concepts I learned from our Iowa State University Weed Science team, Mike Owen and Bob Hartzler. Several herbicides commonly used in Iowa corn and soybean crops have sufficient persistence for herbicide residues to remain in the soil into the following spring. The less-than-cooperative weather we had for the 2011 growing season increased the odds of this happening.

The wet spring delayed many herbicide applications, so the time frame for breakdown was shortened. Our next challenge — the shortage of rainfall across much of Iowa during the growing season — may result in higher concentrations of these herbicides in 2012 than normally encountered. As you can guess, this combination increases the possibility of herbicide injury to rotational crops.

Several factors influence the longevity of herbicides, including soil characteristics, date of application, environmental conditions and application rate (keep in mind that with the advent of herbicide-resistant crops, we often reduce the rate of the preplant and preemergence herbicides, greatly reducing carryover risk).

The longevity of triazine and sulfonylurea herbicides increases greatly when soil pH exceeds 7.0; that’s why injury from these herbicides is usually closely associated with pH changes across the field. The later a herbicide is applied in the growing season, the greater potential for damaging herbicide residues to remain in the soil.

Minimize risk of carryover

The potential for carryover injury to rotation crops is influenced not only by the amount of herbicide present in the soil, but also on the susceptibility of the rotational crop and early-season growing conditions. Because of these interacting factors, it is difficult to accurately predict the likelihood of injury occurring in a specific field.

A farmer can reduce risks of problems in fields suspected of having carryover potential by reducing other stresses that might weaken the crop during establishment.

Selecting corn hybrids and soybean varieties with good early-season vigor, selecting herbicides with high margins of crop safety, and avoiding very early planting dates can reduce the risk of carry-over problems.

What about tillage? Increased tillage has not consistently reduced problems associated with low levels of herbicide residues in the soil, particularly systemic products such as the ALS inhibitors.

Avoid the use of herbicides that have the same mode of action as the herbicide suspected to pose a carryover risk. The amount of herbicide remaining in the soil from last year’s application may be insufficient to pose a threat to a rotational crop.

However, if a second herbicide with the same mode of action is applied to the field, the additive effect of last year’s herbicide residues and the current herbicide may overwhelm the crop’s defense system.

From my ag chemical dealer experiences, I can share a few more insights. A proactive strategy we talk about is to do a “bioassay” or a soil chemical analysis if there is a lot of concern for the safety of a rotational crop. Talk to your local agronomist for details on how these tests are done if they are needed, since we could write an entire column about these tests.

If we have carryover issues, many times we see the symptoms more profoundly in “overlap” areas; so if you do see a few areas in your fields that concern your next growing season, double check and see if overlap during application was a possibility. While those overlap areas may suffer some significant crop injury, good scouting could reveal that the problems may be minimal in the rest of the field.

Precision spraying systems have reduced the overlap areas, so our hope is that these areas will be limited in size where those technologies were used. You should “geotag,” or flag, these areas and watch the crop progress. This will give you a good read on the recovery of crops and a helpful point of reference for the rest of the field.

While we don’t expect widespread problems from herbicide carryover in 2012 in Iowa, the shortage of rainfall and late herbicide applications during 2011 may result in problems for some growers.

A discussion with your local ISU Extension field agronomist and your local ag retailer about what residual products were used last year and your plans for 2012 will help guide your crop management to minimize carryover challenges.

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

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

All rights reserved. Copyright Farm Progress Cos. 2011.