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Watch out for herbicide carryover

Posted Thursday, August 23, 2012, at 12:03 PM

Herbicide residual is a good thing in year with adequate rainfall. Weed scientist are now warning that the dry conditions may also increase the risk of herbicide carryover in subsequent crops.

The most immediate risks include damage to a fall-seeded crop like wheat, cover crops or a forage grass crop. The drought not only reduced the effectiveness of soil-residual herbicides, but the lack of rainfall also slowed the rate of herbicide degradation.

Unfortunately , once an herbicide is applied, there is little that can b e done to shorten the time it remains active in the soil. It is a function of time and moisture and both are running short.

Without the proper rainfall this past spring, the herbicide molecules became absorbed or bound to soil colloids and the chemical and microbial processes responsible for degradation didn't get to do their job. The biggest unknown factor is how much rainfall it will take to reverse the situation.

The risk of carryover injury will vary from field to field and even within fields. The two greatest factors determining carryover injury risk are persistence of t he herbicide (half-life) and herbicide application rate. Application date, soil characteristics (organic matter, texture, pH), rainfall totals, sensitivity of the rotational crop and growing conditions next spring all influence carryover injury potential.

Shallow tillage can help distribute herbicide more evenly across a field, but will do little to speed herbicide degradation rate in extremely dry soil.

Planting the same crop that was planted in 2012 would eliminate the potential for crop injury but does create the possibilities for other problems such as diseases and insects.

In corn fields, triazine herbicides are of the greatest concern to succeeding wheat crops. This includes atrazine or any of the many prepackaged herbicide mixtures that contain atrazine as one of the active ingredients (Bicep II Magnum, Degree Xtra, Guardsman Max, Harness Extra, Lumax, Lexar, etc.). Atrazine or any of the atrazine-containing products DO NOT allow wheat or forage grasses to be planted in the fall following a spring applications.

In soybean fields, the likelihood of carryover injury to wheat is lower but still possible in a year with little rainfall. There are generally fewer residuals herbicides used in wheat, but as a result of glyphosate resistant waterhemp populations more residual chemicals are being used.

The herbicide fomesafen, which is the active ingredient in Flexstar, Flexstar GT, Rhythm, and Prefix, has now become a very common post-emergence herbicide of choice in soybean. Fomesafen-containing products have a 4-month wheat replant internal and in areas that have received little to no rainfall following application, fomesafen carryover injury to wheat and other forage grasses can be a concern this fall.

The best practice is to follow the label of the herbicide that was applied. In fields were there is a concern, a soil bioassay. Gather several soil sample across the field that is of concern then mix them together and place the soil in some kind of pot or greenhouse flat.

Plant your wheat or forage seed in these pots and wait for the seedlings to germinate in order to observe any signs of herbicide damage. You also need to plant the seeds in soil from a location where there is no concern with herbicide injury for comparison.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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