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Learn About Weed Resistance Up Close and Personal

Learn About Weed Resistance Up Close and Personal

Take weed control to a whole new level.

Fire up the Purdue Weed Science Web site and you can access a chart that is the first of its kind for public use. First, find the site at On the right side of the home page there are three distinct buttons. Each one can help you make better decisions for weed control this year.

First, click on the button at the bottom that says 'Herbicide Mode of Action Chart.' What comes up is a very long chart with a color-coded key. However, it's user-friendly and you don't have to be a weed scientist to use it.

Here are various pigweeds that may be showing up soon on your farm.

"We've demonstrated it at meetings and introduced people to it," says Bill Johnson, Purdue University weed control specialist. People can use it to not only know what active ingredients are in the product they are buying, but to learn how those ingredients kill weeds.

The chart came about because of the increasing interest in herbicide resistance, fueled by the increase in herbicide resistant weeds. The pigweed family, including redroot pigweed but now also including waterhemp and Palmer amaranth in parts of Indiana, is stirring up concern about resistance. Another is marestail, also known as horseweed.

Most times in the past people have talked about 'mode of action.' That's the method by which herbicides kill the weeds. Maybe they inhibit a pigment from forming, or interfere with photosynthesis. The goal to help slow down resistance is to hit the weeds with at least two modes of action.

You could have two active ingredients but still have the same mode of action. Now the chart goes one step further. It goes to 'site of action.' Where does the active ingredient with a certain mode of action actually work on the weed. To do the best job of slowing down resistance, you need two herbicides in the same mix or same year that have different sites of action.

The odds that weeds will mutate to form resistance to two different modes of action are slimmer than mutating to one mode of action. The odds of a weed mutating when it's being attacked at two sites of action are even slimmer. That's why it's worth going through the chart.

Find your brand of product, then see which family of chemistry it belongs to. See what the mode and site of action are for the product.

While on the weed science site, try the other two buttons. The 'Select-a-Herbicide' button is a weed control program that lets you put in specific information, and then gives you choices for weed control based on the information you put in. The '2013 weed control guide' button lets you access the entire Ohio State University/Purdue University weed control guide, with information about all herbicides. This is the largest of the three documents.

Back on the 'Herbicide Mode of Action Chart,' note that there is only one chemistry that as of yet does not have any resistant weeds. That's glufosinate, or Liberty. That's why it's being recommended as the best choice for post-emergence control on some tough weeds. However, you must plant LibertyLink crops to be able to spray the herbicide. Spraying it on any other crop will kill the crop.

If you want to practice the 'select-a-Herbicide' program, plug in the following weeds in your list of worst weeds in the field. Use these pictures as an identification guide for the weed.

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