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GMOs and Why Weeds Matter

By Dan Flynn and Suzanna Schreck

I am not about to jump into the often heated debate over the merits and dangers of genetically modified organisms (GMOs), however I am intrigued by a new study by the National Research Council that finds both benefits and risks in the long-term use of genetically modified crops.[1] Specifically, I note the key role weeds have in shaping the way we grow our food.  In U.S. agribusiness, growing food means largely growing soybeans and corn, the twin towers of commercial agriculture and a basic constituent of much of what most of us eat.

To grow crops on a massive scale means control of diseases, insects, and critically, weeds.  Since weeds compete for the same nutrients and water that crops do, eliminating them boosts yields.  The leading weed control chemical today is glysophate, better known as Monsanto's Roundup®.  The virtues of Roundup® are summarized in this statement from weed scientists who are part of the Glysophate, Weeds, and Crops group.[2]

Glyphosate (Roundup® and other products) is a valuable herbicide for corn and soybean growers. When applied post emergence to Roundup Ready® soybean varieties and corn hybrids, glyphosate provides broad-spectrum, low-cost weed control with excellent crop safety. It is better than many other herbicides at controlling larger weeds, has no soil activity (allowing for flexible crop rotations), and has low environmental and human health risks. In several respects, glyphosate and Roundup Ready® crops have simplified weed management. Even before Roundup Ready® crops were introduced, glyphosate was (and continues to be) a valuable herbicide in no-till cropping systems, and saves soil, fuel, and labor. No other single herbicide has provided these benefits to U.S. corn and soybean growers.

Glysophate-based herbicides all work by inhibiting a critical plant enzyme--EPSP synthase[3]--needed to make proteins essential to plant growth.  Since most plants share this enzyme, glysophate is effective across a broad spectrum of plants.

The chart below depicts the rapid rise of herbicide-tolerant (HT, i.e. Roundup Ready®) crops in the US.  Bt is Bacillus thuringiensis, the naturally occurring soil-borne bacterium that is fatal to some insect larvae and now also a standard part of many genetically modified crops.

The enormous success of Roundup Ready® crops has, of course, meant a concomitant increase in the use of Roundup as a frontline agricultural herbicide.  As seen below, the use of Roundup (glysophate) took off in the 1990s and is still climbing.  This has proven to be a brilliant business model for Monstanto, which supplies the GMO seed and the companion herbicide while taking market share from other competing herbicides.  The widespread use of Roundup® is due to the acknowledged benefits of glyphosate: it is less toxic and persists in the environment for a shorter time than most of its older competitors.

But as the National Research Council report notes, one of the risks of the continued widespread use of glyphosate is the seemingly inevitable rise of glyphosate-tolerant weeds.  This is already happening.  Since the late 1990s various glyphosate-resistant broadleaf weeds have been reported across the U.S. and in other parts of the world.[4] Additionally, an increasing number of week species have become resistant to ALS-inhibitors (acetolactate synthase), a group of herbicides that kill weeds by preventing the plants from producing essential amino acids that are needed for proper growth and development.  As weeds become resistant, their progeny also carry genes resistant to herbicides.  Unless control measures are targeted and immediate, the spread of herbicide-resistant weeds grows.

What this means for the genetically modified crops that form the backbone of U.S. agriculture remains to be seen.  But it is certainly true that part of the answer will include divergence from the routine and homogeneous use of Roundup® and a change in basic weed management practices.  For those opposed to GMOs, the triumph of "weed resistance" will provide fodder for commentaries on the dangers of tinkering with plant genes even as the benefits are documented.

If Nature has its way, it may just be the weeds that alter the course of genetically modified crops and agribusiness in the U.S.  Or as the noted writer and lepidopterist Robert M. Pyle has aptly put it: "But make no mistake: the weeds will win; nature bats last."

References

1.  Impact of Genetically Engineered Crops on Farm Sustainibility in the United States.  Committee on the Impact of Biotechnology on Farm-Level Economics and Sustainability; National Research Council 2010, 318 pages.  To purchase: http://www.nap.edu/catalog.php?record_id=12804.  

2.  See Facts about Glysophate Resistant Weeds on the web at:  www.ces.purdue.edu/extmedia/GWC/GWC-1.pdf.  The authors are part of a group of university based scientists from primary agricultural states. The group's website is www.glysophateweedscrops.org.

3.  5-enolpyruvylshikimate-3-phosphate.  If you are really up on your biochemistry, see http://gpries.myweb.uga.edu/bcmb8010/.    

4.  The first glyphosate resistant weed was rigid ryegrass (Lolium rigidum) in Australia in 1996.  The distinction in the US appears to go to Horseweed or Marestail (Conyza Canadensis) an annual weed, formerly of little import that is now becoming a significant problem.

Full post as published by Food Safety News on April 26, 2010 (boomark / email).