Twenty years ago if the term round-up was thrown out what came to mind was a bunch of cowboys horseback gathering cattle.

In more recent years it is the farmers, not cowboys who are talking more about Round-Up.

The Round-Up they are talking about is actually a trade name for the herbicide glyphosate.

“Glyphosate, or Round-Up is a wonderful technology and a very widely used herbicide,” Dallas Peterson, Kansas State University Extension weed scientist told producers at the recent Weed Management School, held in Columbus, Kan.

According to him, glyphosate is great because it controls a lot of weeds. However there is truly such a thing as too much of a good thing.

The technology behind glyphosate as a herbicide was just the beginning.

As seed companies saw its advantages in weed control they then developed corn and soybean varieties that were labeled Roundup Ready.

Roundup Ready seed allows farmers to use glyphosate as a post-emergence herbicide against most broadleaf weeds without killing the crop itself.

The “too much of a good thing,” according to Peterson, comes from the fact that over time a certain amount of weeds have become resistant to glyphosate making it less powerful in weed control.

“There are a number of reasons weeds have become resistant to glyphosate,” he explained. “The biggest reason is the cost. When it was expensive we were forced to use other herbicides. But as the cost came down we used more and more of it allowing weeds to build up resistance.”

Some known glyphosate resistant weeds include:



•Common Ragweed

•Palmer amaranth



•Giant Ragweed



Glyphosate resistant evaluations at Kansas State University include Common waterhemp, Palmer amaranch, Marestail, Giant ragweed and Kochia.

As more and more weeds start becoming resistant to glyphosate Peterson feels it is important look at different modes of action on the farm.

“The best defense against developing glyphosate resistant weeds is to avoid continuous, exclusive use of glyphosate for weed control,” he explained.

Other modes of defense include crop rotation, with non Roundup Ready crops, rotating tank mix herbicides within and across the years and including other control tactics like cultivation.

In addition to using other defense mechanisms Peterson recommended producers implement an Integrated Weed Management program.

“Glyphosate resistance is a much more complicated type of resistance. It eventually gets to a point you can’t control it,” he concluded.

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