Greenhouse studies conducted by University of Nebraska weed scientists have confirmed glyphosate resistance in multiple giant ragweed (Ambrosia trifida) populations.

Resistant giant ragweed

Giant ragweed (Ambrosia trifida) 5 weeks after glyphosate application. (Photo by Lowell Sandell)

Giant ragweed seeds were collected from suspected populations in Butler, Nemaha, Richardson, and Washington counties in eastern Nebraska in fall 2010. Greenhouse bioassays were conducted, and glyphosate dose-response curves were defined for each giant ragweed population.

Visual weed-control ratings and plant biomass reduction-curve comparisons clearly show a glyphosate resistance level ranging from two to six fold, depending on the population and plant size.

For example, 90% control of a susceptible population was achieved with 32 oz of glyphosate (3 lb/gal acid equivalent), while the resistant populations needed approximately 100 oz/ac and 200 oz/ac at 4- and 8-inch plant heights respectively, in order to achieve the same level of control.

The Development of Herbicide Resistance

Herbicide resistance usually results from repeated use of the same herbicide. Widespread adoption of glyphosate tolerant crops in the Midwest (primarily corn and soybean), coupled with an over-reliance on glyphosate-based herbicides, has resulted in the evolution of glyphosate resistant weed populations.

The selection pressure exerted on weed populations by increased glyphosate use in glyphosate-tolerant crops over the last 10 to 15 years is unprecedented in the era of herbicide weed control. Prior to the introduction of glyphosate-tolerant crops, only a few weed species were resistant worldwide.

The number of weed species with reported glyphosate resistant populations has reached almost 20 worldwide, and 12 in the U.S., due to repeated glyphosate use over a large land area (more than 300 million acres just in the U.S.).

Weed species with glyphosate resistant populations in the U.S. include common waterhemp, giant ragweed, common ragweed, kochia, palmer amaranth, marestail (horseweed), hairy fleabane, junglerice, goosegrass, Johnsongrass, Italian ryegrass, and annual bluegrass (source: International Survey of Herbicide Resistant Weeds).

Diversify to Reduce the Potential for Resistance

While these giant ragweed populations are currently in small pockets, this further illustrates the need for everyone making weed management decisions  to adopt a more diversified approach to weed control. We believe that herbicide tolerant crops, including those based on glyphosate, can remain a useful component of our crop production systems, but only with proper management.

Glyphosate’s low cost, effectiveness, crop safety, and ease of use make it easy to become over reliant on it rather than use using a diversity of preemergence and postemergence tank mix partners when glyphosate tolerant crops are grown in succession.

Properly using herbicide-tolerant-crop technologies as a component of an integrated weed management program is the key to preserving the long-term benefits of these technologies, while avoiding many of the concerns associated with their use or misuse.