March 9, 2007

Weed resistance to herbicides began developing when man started using chemicals for weed control. Well-documented literature suggests that over 150 weed species are resistant to one or more of 15 herbicide families (see the Weed Science Society of America Web site). In most cases, weed resistance developed from the repeated use of the same herbicide for weed control.

The widespread use of glyphosate-tolerant crops and repeated use of glyphosate-based herbicides is now creating practical concerns due to single selection pressure on weed populations which had already resulted in glyphosate resistance. There were only a few weed species (eg. ryegrass and goosegrass) known to have developed resistance to glyphosate worldwide prior to introduction of glyphosate-tolerant crops. The number of glyphosate-resistant weeds increased sharply in just over eight years of repeated glyphosate use over a large land area worldwide (more than 200 million acres) due to introduction of Roundup-Ready crops. Worldwide 11 weed species now have developed resistance to glyphosate (http://www.weedscience.org/glyphosate.gif).

In the U.S., we have six glyphosate-resistant weeds: waterhemp, ryegrass, giant ragweed, common ragweed, palmer amaranth and marestail (horseweed). Marestail (horseweed) is the first weed species exhibiting resistance to glyphosate in Nebraska. UNL research found a glyphosate resistance level ranging from 3-6X normal, depending on the population tested. For example, 90% control of a susceptible population was achieved with 32 oz/ac of glyphosate (3 lbs/gal acid equivalent, as 1X rate), while the resistant populations needed about 100 oz/ac (3X rate) and 200 oz (6X rate) to achieve the same level of control. Glyphosate and herbicide-tolerant crops can remain useful components of the crop production system only with proper management. When one Roundup-Ready crop is grown after another, it's easy to overuse glyphosate rather integrating control options, such as pre-emergence herbicides or tank mix partners.

Proper use of herbicide-tolerant technology, as a component of an integrated weed management program, is key to preserving the long-term benefits of this technology while avoiding many of the concerns about its use or misuse. For more information about glyphosate resistance contact your local UNL extension office or my office at 402-584-3808.

Stevan Knezevic
Extension Weed Specialist
Haskell Ag Lab, Northeast REC