State of Science :: Hot Science
Dicamba-Resistant Soybeans to the Rescue?
Herbicide-tolerant soybeans now account for about 90 percent of the U.S. crop. Most of this acreage is planted to Roundup Ready beans genetically engineered to tolerate exposure of glyphosate (Roundup) herbicide.
As predicted by weed scientists when the technology was introduced in 1996, Roundup Ready technology has triggered the emergence of several weed species that are resistant or tolerant to glyphosate. There are more than a dozen newly resistant weeds, many of which are spreading rapidly across the country. In fact, resistance has now placed the future viability of the Roundup Ready system in jeopardy.
In recognition of the seriousness of this problem, the May 25, 2007 issue of Science featured an article on glyphosate resistance entitled "A Growing Threat Down on the Farm" (Service, 2007), along with a research report by a Monsanto-funded team of scientists at the University of Nebraska-Lincoln reporting success in creating dicamba-resistance soybeans (Behrens et al., 2007).
The coverage in Science is remarkably uncritical of many of the questionable claims made by proponents of agricultural biotechnology. The Behrens et al. (2007) research report states in its first paragraph that -
"...biotechnology-derived herbicide-resistant and insect-resistant traits...have contributed to greater productivity per hectare, decreased production costs, greater flexibility and efficiencies in production regimes, reduced pesticide use, and improved farmer health."
Hundreds of university yield trials have shown that herbicide-tolerant soybeans suffer from a moderate to modest yield drag. Roundup Ready soybeans also have proven more vulnerable to a range of plant pathogens and soil borne diseases. Surely there are cases where a farmer increased soybean yields by switching from a failing herbicide-based system to the Roundup Ready system, but such cases clearly have not led to an increase in soybean yields across the millions of acres planted.
Herbicide-tolerant technology has also clearly not reduced costs or increased farmer profits, as shown by dozens of university and USDA surveys and studies. In the first six to eight years of widespread use, the Roundup Ready system lowered soybean farmer's herbicide expenditures but increased seed costs by about the same amounts. Several USDA studies have concluded that RR technology has had no, or a modestly negative impact on per acre profits.
In the last few years, as farmers have added additional glyphosate treatments, plus other herbicides to their systems to deal with newly resistant weeds, their herbicide costs have risen above levels common in the mid-1990s. The problems created by weeds resistant to glyphosate are going to worsen quickly and persist for many years, and will cost soybean farmers hundreds of millions to overcome.
Behrens et al. claim that GM-crops have reduced pesticide use. This is true in the case of Bt-cotton, but clearly not true in the case of herbicide-tolerance. USDA data show that herbicide-tolerant crops did reduce herbicide use modestly for the first few years of widespread use, but then as weed shifts and resistance began to take hold, herbicide use trended upward. Since about 2000, herbicide-tolerant soybeans have increased herbicide use, measured by pounds of active ingredient applied per acre, compared to conventional systems, and the difference is now bound to grow more significant.
One expert quoted in the Science story states that "In 3 to 4 years, it [resistant weeds] will be a major problem" that could cost farmers "billions of dollars...and could have a major environmental consequence..."
The collapse of Roundup Ready technology is nearest in parts of the southeast where farmers grow substantial acreage of Roundup Ready soybeans and Roundup Ready cotton, sometimes in rotation. The heavy and continuous selection pressure on weed populations has created resistant phenotypes, more or less on the schedule predicted by weed scientists back in the mid-1990s.
In order to deal with soybean, corn, and cotton fields infested with glyphosate resistant weeds, university weed scientists, and Monsanto, are advising farmers to apply multiple applications of Roundup, along with two or three additional herbicides targeted at specific resistant weeds. No-till farmers are being advised to use paraquat, along with one or two additional active ingredients, instead of glyphosate for their burndown applications. This intensification of herbicide treatments will likely buy some time, but it will also trigger new and probably more serious resistance problems and an even more costly day of reckoning for soybean and cotton farmers, who will be left with weed-infested fields and only high-cost, but generally less than reliable herbicides.
There may be other problems as well, especially if dicamba-resistant soybeans are approved by the Environmental Protection Agency, and aggressively marketed to farmers as the "solution" to reverse the collapse of Roundup Ready technology. While glyphosate is a relatively safe herbicide that poses little if any risk to most non-target organisms, including the people applying them and living in heavily treated farm communities, the same cannot be said of dicamba.
Dicamba is an old, relatively high-risk herbicide. It is genotoxic (damages the DNA in cells) and has been linked to elevated rates of cancer among farm populations. It is mobile in soils and prone to leaching to groundwater. Dicamba is moderately persistent and will no doubt trigger a host of environmental problems if its use jumps 10-fold or more, as likely if dicamba resistant soybeans are widely adopted. (For an overview of published research on dicamba risks, see the abbreviated bibliography posted below).
Sources: "Dicamba Resistance: Enlarging and Preserving Biotechnology-Based Weed Management Strategies"
Authors: Mark R. Behrens, Nedim Mutlu, Sarbani Chakraborty, Razvan Dumitru, Wen Zhi Jiang, Bradley J. LaVallee, Patricia L. Herman, Thomas E. Clemente, and Donald Weeks.
"A Growing Threat Down on the Farm"
Author: Robert F. Service
Journal: Science, Vol. 316, May 25, 2007