A recent paper published in the journal Weed Science by Ted Webster and Lynn Sosnoskie with USDA-ARS in Tifton, Ga., lays the responsibility for glyphosate resistance on an “overall lack of stewardship.”

“Glyphosate susceptibility in the most frequently occurring and troublesome weed species is a common resource that is rapidly being lost in Georgia because of an overall lack of stewardship,” states the paper, adding that with the occurrence of glyphosate-resistant Palmer amaranth in Arkansas, Mississippi, North Carolina, South Carolina and Tennessee, “it is likely these states will follow the same path as Georgia and similarly endanger glyphosate susceptibility in Palmer amaranth.”

A greater understanding of herbicide resistance, according to the article, will allow for more complete information to be provided to growers on improved stewardship and weed management systems. “As development of new technologies continues for crop production (e.g., dicamba- or 2,4-D-resistant cotton cultivars, or other new technologies), it will be more important to wisely use these advances and be better stewards over these traits, instead of compromising them as we have done with glyphosate.”

University of Georgia Extension Weed Scientist Stanley Culpepper recently stated that glyphosate-resistant Palmer amaranth pigweed was confirmed in 14 more counties in 2009, bringing the total to 50 counties. By the end of this year or early next year, he predicts it might be found in every agronomic county in the state.

The introduction of glyphosate resistance into crops through genetic modification has revolutionized crop production, says Webster and Soskie. “Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species”

Glyphosate-tolerant cultivars were introduced in the United States in 1997, states the paper, and in just 11 years, producers in Arkansas, Georgia, Louisiana, Mississippi, Missouri, North Carolina, South Carolina and Tennessee used these cultivars and used glyphosate for postemergence weed control on more than 95 percent of the acres.

A common characteristic of glyphosate-tolerant and glyphosate-resistant weeds is that they developed in systems with little or no diversity in weed control practices, say the authors. “In an effort to manage glyphosate-resistant Palmer amaranth and minimize the selection pressure and occurrence of additional glyphosate-tolerant and glyphosate-resistant weed species, growers have begun to incorporate other weed control practices into their production systems, including multiple tank-mixtures and cultivation. The application of herbicides with different modes of action in mixtures is likely to have a large impact in delaying the development of herbicide resistance.”

However, the paper continues, agricultural practices aimed at delaying or preventing the development of herbicide resistance are not viewed as being economical in the short-term, and are not readily used by all growers.

“Weed control performance and cost are often of greater importance to growers than site of action when selecting a herbicide (Beckie 2006). Because herbicide resistance can spread quickly, indiscriminate use of glyphosate may result in a loss of weed susceptibility for a growers, a tragedy of the commons (Gould 1995).”

Awareness of a problem such as herbicide resistance does not always lead to proactive measures, states the article. “The message to growers in Georgia has been that failure to manage glyphosate-resistant Palmer amaranth not only causes problems on their farms, but on their neighbors’ farms as well.”

There are at least two factors, say Webster and Sosnoskie, that may be hindering the adoption of practices that contribute to glyphosate stewardship: 1) the belief that a new technology will be developed to solve the resistance (and tolerance) problems and 2) the belief that resistance management strategies will be futile.

The authors note, however, that there has not been a new herbicide mode action introduced commercially since 1998.

While there are many questions that can be addressed through research in the understanding of herbicide resistance and weed traits, the following list would be beneficial in providing improved stewardship of future technologies, state Webster and Sosnoskie:

1) What is the most effective means of mitigating the risk of the development of herbicide-resistant and herbicide-tolerant weeds?

2) Will herbicide mixtures that include glyphosate (or other herbicides of interest) minimize the risk of resistant and tolerant weeds, or will glyphosate use need to be limited to once a season (or less)?

3) Are there factors (e.g., cultural practices, physical weed control or biological control) that can improve control and stewardship of current herbicide options?

4) How do herbicide resistance traits move and how mobile are they across the landscape?

5) Are there basic factors that can be used to access the risk of a species to develop resistance so that we can target our programs to reduce the risk of selecting for resistance?

6) What types of incentives could be implemented to improve herbicide stewardship and promote herbicide susceptibility in weed populations?

Answers to the complex question of how to avoid repeating mistakes will likely require the entire agricultural community — growers, consultants, county agents, public scientists and the agricultural industry — to generate and apply knowledge of weed science (both basic and applied), evolutionary biology, economics and sociology, states the article.

e-mail: phollis@farmpress.com