Switching to soybeans? Be aware of herbicide rotation restrictions

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COLUMBUS – Producers switching from corn to soybeans in fields where corn herbicides have been applied should be aware of corn herbicide rotation restrictions.

Per herbicide labels, restrictions for various preplant/preemergence herbicides are as follows (assuming corn herbicides were applied in April or May):

* Any product containing atrazine or simazine: Do not plant soybeans until next year;

* Axiom, Define, Dual II Magnum, alachlor products, Outlook, Python: Plant soybeans anytime;

* Balance, Epic: Do not plant soybeans until next year;

* Basis: Soybeans can be planted 15 days after application;

* Callisto: Do not plant soybeans until next year;

* Harness, Degree: Do not plant soybeans until next year;

* Hornet: Do not plant soybeans until next year;

* Surpass, TopNotch: Do not plant soybeans until next year.

Atrazine big concern. Atrazine, simazine, and Hornet are probably the most problematic among all of the corn herbicides, when the field absolutely has to be replanted to soybeans.

The residual life of acetochlor (Degree, Harness, TopNotch, Surpass) in soil is relatively short, and it is likely that soybeans planted 30 days or more after application could survive.

Similarly, Aventis sources informed Ohio State that soybeans planted 45 days after Balance or Epic application probably could survive. However, manufacturers assume no liability for injury to soybeans when label restrictions are not followed.

Atrazine and simazine are considered to be persistent herbicides. Soybeans can survive atrazine concentrations in soil of approximately 0.17 to 0.35 ppm (an atrazine rate of 1 lb./A corresponds roughly to 1 ppm).

The concentration that soybeans can survive varies with soil type, because atrazine will be more active on soybeans in low organic matter, coarse-textured soils compared to high organic matter, finer-textured soils.

Atrazine is less persistent in the lower organic matter, coarse-textured soils, though, so it is really impossible to say with any certainty whether soybeans will survive in a given situation, regardless of the amount of rain.

Soil samples. The most effective method for determining the risk of injury to soybeans is to sample the soil and send it to a laboratory to determine the atrazine concentration. This can be accomplished fairly rapidly, but can be somewhat costly. We suggest soil samples be taken from the upper two or three inches of soil, and shipped to the laboratory as soon as possible after sampling.

Source: Mark Loux, Jeff Stachler; Ohio State University

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