Roundup (Glyphosate) herbicide has been around for over thirty years. It was first discovered in the 1950's when a chemical company was conducting research for new detergent products. It came out in the 1970's as a new herbicide that would kill most plants including corn and soybeans. During the early 1990's scientists developed soybean varieties that could tolerate applications of Roundup herbicide. This opened the door to development of Roundup Ready technology that has seen the development of corn, alfalfa and sugar beets among many others that have been genetically altered to tolerate Roundup while the herbicide controlled all of the weeds in the field. This meant no more cultivating or use of many other commonly used herbicides. By the year 2010, probably 90% of all soybeans and 80-90% of all corn utilizes Roundup Ready technology. This has meant very clean fields and less time spent in the field.
However, in recent years we have seen Glyphosate resistant weeds start to develop. Over the past two years extension staff has conducted a survey of soybean fields at harvest to determine the extent and distribution of herbicide-resistant weeds throughout south central and western Minnesota. The majority of the weed samples that have been collected are giant ragweed and waterhemp. In 2009, they observed an increase in the number of fields with giant ragweed that survived treatment and were present at harvest. North Dakota State University has completed an initial screening of these plants and has discovered that in south central Minnesota, 15 fields that were tested with 13 fields testing positive for herbicide resistance. These fields will undergo further testing this summer to determine the extent of glyphosate resistance.
What can farmers do about this problem? We need to be using herbicides with different modes of action in our herbicide programs. There is a recent publication out from the University of Minnesota and North Dakota State University entitled "Pre and Post Herbicide Diversification Options for Glyphosate-Resistant Corn and Soybeans" This publication can be found at appliedweeds.cfans.umn.edu/index.html or in paper format at your local extension office. The tables in the publication list various herbicide options for difficult to control weeds such as waterhemp, lambsquarters and giant ragweed.
In summary, herbicide 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 which can diminish the economic value of this technology. Points to consider:
Utilize other modes of action through use of a pre-emergence herbicide or a tank-mix partner.
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 weeds survived, try to determine why. Was the failure caused by 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.
Keep in mind that many postemergence herbicides have crop size restrictions.
If your glyphosate applications have failed to control the same weed species, in the same area of your fields for several years, you may have a weed resistance problem. The sooner that you react on a potential problem the better.
Other weed species that are difficult to consistently control in glyphosate dominated cropping systems are: common lambsquarters, common ragweed and in no-till fields- horseweed.
Next year consider alternating Roundup Ready crops with Liberty-Link or a conventional hybrid or variety.
For more details on the biology and management of these weed species go to the Glyphosate Weeds and Crops website at www.glyphosateweedscrops.org/