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Glyphosate-resistant weeds

May 8, 2009
From Wayne Schoper, Brown/Nicollet Extension Educator

Glyphosate (Roundup) herbicide has been around for over 30 years and has proven to be an excellent choice for control of some very tough weeds. We used to have a lot of problems with perennial weeds such as Canadian thistle and Quackgrass. However, we don't see a lot of these weeds in row crop situations anymore once we bring glyphosate herbicide into the weed control program. Glyphosate is a non-selective herbicide, meaning it controls everything that it touches. In the early days, we would spray either before the crop was planted or as soon as the crop was harvested. Then in the early 1990's, scientists developed the technology to insert a gene that was resistant to glyphosate herbicide and the race was on to adopt this herbicide program for all of our crops that we plant in this area. In fact, most of the soybeans and a lot of the corn planted in southern Minnesota is "Roundup Ready" meaning that you can use glyphosate herbicide sprayed over the top and control all of the weeds in the field.

However, a new situation has developed. Glyphosate-resistant biotypes of giant ragweed and common waterhemp along with common ragweed and horseweed have been confirmed in Minnesota. To see the International Survey of Resistant Weeds go to their website at: There you will find a world-wide listing of herbicide resistant weeds including the latest additions of some biotypes that are glyphosate resistant.

Giant Ragweed:

Article Photos

Wayne Schoper

Glyphosate-resistant giant ragweed was confirmed in McLeod County last year on approximately 40 acres. Field management records indicate that glyphosate was used at least once each year at this site since 1998, with the first four years in a continuous soybean rotation. This led to the development of herbicide resistance in a few weeds that then spread out from there. In addition to Minnesota, glyphosate-resistant giant ragweed has also been confirmed in Arkansas, Tennessee, Ohio and Indiana.

Common Waterhemp:

Glyphosate-resistant common waterhemp was confirmed in Renville County on approximately 50 acres in 2007. There was no evidence of continuous glyphosate use at this site but there was evidence that the seed could have been transferred in by field equipment. Missouri, Illinois and Kansas also have reported this weed problem. It is important to note that the Missouri biotype is resistant to three different modes of action, including Raptor and Pursuit (ALS inhibitors) and Cobra and Reflex herbicides (PPO inhibitors). Illinois has reported similar biotypes and it is highly likely that the Minnesota biotypes are also ALS resistant and the integration of glyphosate resistance will only complicate future management plans.

Management Recommendations:

Diversification with other herbicide modes of action via preemergence herbicides, postemergence tank mix partners or rotation to Liberty Link corn are some suggested chemical diversification strategies that can slow the rate of glyphosate resistance and reduce economic risk to the grower. It is important to note that for chemical diversification to work the alternative herbicide must provide highly effective control of the targeted weed species.

In summary, chemical diversification can provide consistent economic performance to the grower who uses Roundup Ready Technology and can help to reduce the probability of glyphosate-resistant weeds diminishing the economic value of this technology.

Points to consider include:

- Utilize other modes of herbicide action through use of a preemergence herbicide or a tank-mix partner.

- Consider alternating Roundup ready crops with Liberty Link technology or a conventional hybrid or variety.

- As the growing season moves into high gear don't forget to scout your fields approximately 10 to 14 days after your first glyphosate application to detect weed escapes.

- If the weeds survived, try to determine why. Was the failure due to misapplication, poor weather, poor timing, or later weed flushes? Get on these problems early while there is still some time in the current growing season to address them.



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