In the 1990s, Monsanto began selling genetically modified corn and soybeans, crops that were resistant to the herbicide/weed killer Roundup (glyphosate). The idea was that farmers could now grow these crops and kill the weeds that compete with them by spraying with glyphosate. Normally, the herbicide would also kill the corn and soybeans. This would not happen with the new genetically modified varieties.

In less than two decades, Roundup resistant weeds have become a major problem.

According to Monsanto, these "Roundup Ready" crops have been a huge success. But not everyone agrees. One problem that has sprung up is the increasing resistance of weeds to glyphosate. In less than two decades, Roundup resistant weeds have become a major problem.

Because of this, another large chemical company, Dow Chemical, has applied to the USDA to begin marketing a new type of genetically modified corn that is resistant to another herbicide, 2,4-D. The corn, called Enlist, contains a gene from a soil bacterium that causes it to make a protein that breaks down 2,4-D so that it is no longer toxic to the corn. Dow Agrosciences, a unit of Dow Chemical Company, hopes to have the product approved this year and released by the 2013 crop.

Not everybody shares Dow's hopes for approval. Many think that if the USDA approves Enlist, it will simply be ignoring the lessons of the past 16 years.

What Agent Orange Has to Do with Corn

The compound 2,4-D (2,4-Dichlorophenoxyacetic acid) was one of the two major ingredients in Agent Orange, a defoliant used extensively during the Vietnam War that has been linked to birth defects, cancers and other medical problems in both U.S. soldiers and Vietnamese. This has led some opponents of Enlist to label it "Agent Orange Corn." The medical problems caused by Agent Orange are thought to stem not from 2,4-D, but from the defoliant's other major ingredient, 2,4,5-T and from contamination with dioxins.

The compound 2,4-D was one of the two major ingredients in Agent Orange, a defoliant used extensively during the Vietnam War. This has led some opponents of Enlist to label it "Agent Orange Corn."

But 2,4-D poses its own health concerns The International Agency for Research on Cancer (IARC) classifies it as a possible human carcinogen (Class 2B), along with all other chlorophenoxy herbicides. But the U.S. Environmental Protection Agency says that there is not enough evidence to call 2,4-D a human carcinogen. 2,4-D exposure has also been linked to an increased risk of Parkinson's disease.

Aside from health concerns, many people do not think that increasing reliance on weed killers is a sensible way to deal with increasingly resistant weeds.

An Overreliance on Herbicides Breeds Resistant Weeds

People have been using both glyphosate and 2,4-D to kill weeds on their lawns for decades. But Monsanto was the first company to genetically modify crops so that they could survive a dosing with the weed killer glyphosate, a chemical that is marketed by Monsanto under the name Roundup. They called these crops Roundup Ready.

Monsanto introduced Roundup Ready soybeans in 1996. This was followed by Roundup Ready canola and cotton in 1997, corn in 1998 and alfalfa and sugarbeets in 2011. In 2010, estimates were that Roundup Ready crops account for about 90 percent of the soybeans and 70 percent of the corn and cotton grown in the United States.

According to the article, introduction of crops like Enlist corn and the increase in herbicide spraying they will cause are likely to increase multiple herbicide resistance in weeds, as increased antibiotic use has increased multiply-drug resistant bacteria.

This has led to substantially increased use of the herbicide glyphosate. And because constant exposure to an herbicide puts selective pressure on weeds to acquire resistance to it, this has spawned a large variety of troublesome weeds that are now thoroughly resistant to glyphosate. In essence, glyphosate's useful days may soon be over.

Genetically modified crops and seeds are patented. This limits the amount and types of research that independent scientists are permitted to perform on them, essentially giving the companies who own the patents the right to choose what is being researched. But scientists have been able to monitor some of the changes in herbicide-resistant weeds that have occurred in the past 16 years since Monsanto began marketing Roundup Ready crops. And what they've found is disturbing.
The Rise of "Super Weeds"

Just as the heavy use of antibiotics contributed to the rise of drug resistant bacteria such as MRSA, an overdependence on herbicides is producing herbicide-resistant "super weeds."

These include the weed Palmer amaranth (pigweed), a particularly strong and aggressive weed that has already overrun many southern cotton fields. Pigweed can grow three inches a day and reach seven feet tall or higher, choking out crops. It's also so sturdy that it can damage harvesting equipment.

The first resistant weed species to pose a serious threat to agriculture was spotted in a Delaware soybean field in 2000. Since then, the problem has spread, with resistant species in at least 22 states infesting millions of acres, mostly soybeans, cotton and corn.

Some farmers have likened increased dependence on herbicides to an endless treadmill. Yet there are currently 20 genetically engineered crops awaiting approval and 13 of them are intended to be resistant to one or more herbicide.

In addition, Roundup Ready seeds are sterile, forcing farmers to buy new seeds from Monsanto every year, instead of reusing their best seeds as farmers have traditionally done.

If it only took 16 years of Roundup Ready crops to lead to unacceptable levels of glyphosate-resistant weeds, why would anyone expect Dow's 2,4-D resistant corn to be anything more than another short-term fix?

A Long-Term Approach to Weed Management

Many would prefer a more balanced approach to weed management.

In an article published in the January 2012 issue of Bioscience, Dave Mortensen, a Penn State weed ecologist, and his colleagues make the case that only an integrated approach to weed control that includes crop rotation, cover crops, judicious use of tillage, and targeted use of herbicides is likely to successfully minimize the emergence of resistant weeds. In support, they point to 28 species of weeds that have already evolved resistance to synthetic herbicides.

According to the article, introduction of crops like Enlist corn and the increase in herbicide spraying they will cause are likely to increase multiple herbicide resistance in weeds, as increased antibiotic use has increased multiply-drug resistant bacteria. Due to its potency and ability to drift onto land where it has not been sprayed, increased use of 2,4‐D will threaten wild plants, as well as pollinators and other beneficial organisms that depend on them. It will also threaten herbicide-sensitive crops like tomatoes, grapes and potatoes.

Mortensen and colleagues see Enlist as a short-term fix to the weed problem, much as Roundup Ready crops were. The question is whether the USDA will agree with them and reject Dow's application to market Enlist corn. A broader question, considered next week, is whether genetic modifications are safe or not.