Attack Weeds with Multiple Controls to Prevent Resistance

Giant Ragweed

by Greg LaBarge, Fulton County

Currently northwest Ohio has background populations of weeds resistant to glyphosate and ALS products. The weeds of concern are maretails, giant ragweed and common ragweed. To keep these levels just as background levels of resistance and not widespread problems we need to put a sound weed control program in place.

There are not a lot of new chemistries with new modes of action we can deploy for the future. A recent article by University of Illinois Extension weed specialist Aaron Hager noted that “No new herbicide active ingredients are currently available for the 2011 growing season, and said he expects this “drought” of no commercialization of herbicides with unique sites of action to continue into the foreseeable future. More…” So we need to use a good mix of products and modes to keep our current lineup of products effective into the future.

Dr Mark Loux, Ohio State University Extension Weed Specialist, provides a four step system approach to weed control:

1) Start Weed Free in the spring.

Use tillage or burndown herbicide treatments in the spring to eliminate existing weeds. Burndowns should include 2,4-D whenever possible for problem weeds like marestail.

2) Use residual herbicides at preemergence application timing.

Apply with the burndown if near planting. Products selected should control lambsquarter and marestail plus have activity on giant ragweed and common ragweed.

3) Apply a first Post application on small weeds in the 4-6 inch range.

These small weeds are easier to control and if they get much large cause irreversible yield loss. In soybeans, weed reaching just 9 inches tall will reduce yields 6% and 12 inch weeds cause 10% yield losses. Corn is more sensitive to weed competition with 6 inches weeds causing 6% yield loss, 9 inch weeds 9% and 12 inches weeds 22% yield losses. If a field has heavy pressure from giant ragweed or other problem weeds use herbicides with residual axtivity.

4) Apply a second Post application 3 weeks later if needed based on emerging weeds.

If fields have dandelion or other winter annuals in the fall use a fall herbicide treatment. Winter annuals are not generally competitive with crops but the cover can keep the ground cold in the spring plus some winter annuals support soybean cyst nematode development.

A excellent electronic reference is the 2011 Weed Control Guide for Ohio and Indiana.
Bound copies of the publication can be purchased at your local Ohio State University Extension Office for $13.50, call ahead to see if they have copies on hand or online  by clicking here.

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