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Glyphosate-resistant marestail confirmed in Nebraska

University of Nebraska researchers have confirmed the first glyphosate-resistant weed species in Nebraska -- marestail, also know as horseweed.

While Nebraska researchers first suspected resistance had developed almost a year ago, it took tests and growing plants from last year's seeds to confirm it.

Weed resistance usually results from repeated use of the same herbicide. Widespread use of glyphosate-tolerant crops and repeated use of glyphosate herbicide has resulted in selection pressure on weed populations in recent years.

Prior to the introduction of glyphosate-tolerant crops only a few weed species (ryegrass and goosegrass, for example) had developed resistance to glyphosate worldwide. However, the number of glyphosate-resistant weeds tripled in just over eight years of repeated glyphosate use over a large land area (over 50 million acres) due to introduction of Roundup-Ready crops. Current examples of glyphosate-resistant weeds in the United States include waterhemp, lambsquarters, giant ragweed, common ragweed, palmer amaranth and marestail (horseweed).

Identifying resistance
Seeds of three suspect marestail populations were collected in fall 2005 in eastern Nebraska. Greenhouse bioassays were conducted this summer, and dose response curves for glyphosate were defined for each marestail population. Curve comparisons clearly showed glyphosate resistance requiring three to six times the rate needed in a susceptible population. (Resistance level varied with the marestail population).

For example, 90% control of a susceptible population was achieved with 32 ounces of a glyphosate herbicide (three-pound-per-gallon acid equivalent), while the resistant populations needed about 100 ounces per acre and 200 ounces in order to achieve the same level of control.

Using management to counter potential resistance development
We believe that glyphosate- and herbicide-tolerant crops, including those based on glyphosate herbicide, can remain useful components of crop production systems only with proper management. It is easy to fall into a trap of overusing glyphosate, versus combinations of pre-emergence herbicides or tank mix partners, when one Roundup-Ready crop is grown after another.

Therefore, proper use of herbicide tolerant technology, as a component of an integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many concerns about its use or misuse.

University of Nebraska researchers have confirmed the first glyphosate-resistant weed species in Nebraska -- marestail, also know as horseweed.

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