by Jordan Wilkerson
figures by Brian Chow
Summary: In the history of agriculture, no technology has been adopted so quickly and completely as genetically engineered crops. Particularly useful crops are ones that have an engineered resistance to herbicides. These crops have alluring benefits: reduced crop damage when herbicides are sprayed, easier weed management, and even the potential for environmental benefits. So what’s the problem? Herbicide-resistant weeds. The benefits gleaned from these crops begin to disappear as these superweeds gain prominence on farmlands across world. However, to fully appreciate the current predicament, it is necessary to understand what led to the difficult problem of superweeds. And it starts with the most common herbicide used in agriculture: Roundup.
Roundup and the crops that resist it
Chances are you’ve heard of Roundup, a common weed killer used by farmers and homeowners alike. Roundup is the brand name for the herbicide, glyphosate. Glyphosate works by preventing plants from being able to make the proteins they need to survive. Since virtually all plants make these essential proteins the same way, glyphosate affects nearly all plants. For this reason, glyphosate is deemed a “broad-spectrum” herbicide .
Of course, this is a double-edged sword. While Roundup is a great weed killer, its broad-spectrum effects make it a decent crop killer, too. That’s no good. If our crops aren’t resistant to Roundup, can we figure out a way to make them resistant? In fact, the agriculture company Monsanto did just that.
In 1996, Monsanto introduced the Roundup Ready soybean, a genetically engineered crop resistant to glyphosate. In the few years after, Roundup Ready cotton, maize, and various other crops also made their debut. While almost all plants are susceptible to glyphosate’s grip, the beautiful thing about genetic engineering is that the genes need not be from similar organisms . In this case, the gene allowing resistance to glyphosate was taken from a type of bacteria called Agrobacteria . With the introduction of this organism’s relevant gene into the desired plant genetics, a Roundup Ready crop is born (Figure 1).
Figure 1. Roundup contains glyphosate, which is toxic to standard plants that contain its target protein. The plants containing this protein are destroyed upon exposure to glyphosate, indicated by the red X. Roundup Ready crops have been engineered to contain a gene from Agrobacteria, making them immune to the herbicide.
While companies are developing these brand new plants, one question that naturally arises is this: do Roundup Ready crops directly affect the environment? Based on the large amount of scientific evidence so far, the short answer is no. There are two ways in which scientists and the public speculated these crops could have an effect on their surrounding environment .
One is the possibility that Roundup Ready crops may crossbreed with weeds to produce glyphosate-resistant weeds. This is slightly possible for some crops such as oilseed rape and sugar beet, but in most cases, the weeds present are not sexually compatible with the crops present, so crossbreeding cannot occur .
The other possibility is that dead, Roundup Ready plant matter could be different from normal plant matter. This could be a problem if this difference results in plant tissue that is toxic to organisms in the surrounding ecosystem. However, evidence strongly suggests that the two plant types have very similar composition, and their fast breakdown renders the final products that enter the environment basically identical .
A win for farmers and the environment
Assuming that farmers only use Roundup at the recommended rates in their weed management techniques, results show that farming practices associated with Roundup Ready crops actually have a lower environmental impact.
One reason for this is that they are able to reduce their pesticide use and replace more harmful pesticides with glyphosate . The harmfulness of a pesticide is assessed by exposing organisms to higher and higher concentrations of the pesticide until it appears to inhibit their ability to survive. Glyphosate is considered less harmful because much higher concentrations are required to hurt aquatic animals and plants than with other herbicides . When herbicides are considered relatively non-toxic to ecosystems, this means that the concentrations that could enter the environment from the recommended application of the herbicide is usually lower than the minimum concentration for that chemical to be toxic.
Another reason Roundup Ready crops theoretically result in a new environmental benefit is that farmers no longer have to till their cropland. While tilling has other benefits, its primary purpose is to mechanically destroy weeds present on the land before the crops are planted. However, this comes with a price. Tilling the land loosens the soil, which causes more of it to run off into nearby water bodies when there’s rain or even a strong wind. Soil on a farm plot is different from natural soil in that it contains much higher concentrations of fertilizer and residual pesticides .
With Roundup Ready crops, farmers can spray glyphosate on their land instead of plowing their entire area to get rid of the weeds. Furthermore, since their crops are resistant to glyphosate, farmers can replace outdated, more harmful herbicides with glyphosate, which breaks down quickly and is not very toxic to aquatic organisms . Thus, runoff is reduced overall, and the small amount that may occur has less harmful effects.
Possible concerns about Roundup Ready crops
These benefits only occur if spraying Roundup is the only weed management technique required. As early as the 1960s, scientists have known that the tactic of only spraying one pesticide has strong potential to push weeds to develop resistance to it. Resistance to an herbicide occurs when one weed has a mutation that allows it to survive a Roundup spraying. If this resistant weed reproduces, it can pass its mutation to its offspring. Soon, a cluster of superweeds can quickly spread across acres of cropland, rendering Roundup largely ineffective .
One strategy that would reduce the probability of weeds developing resistance is to periodically use another herbicide that kills weeds through a different route. Therefore, resistant weeds would have to establish two mutations to survive the herbicidal spray [6,7]. However, this strategy would periodically eliminate the Roundup Ready crop, too, making it not much better than using non-resistant crops.
Another approach that is even more environmentally friendly is to change what crop is grown on the land each year. Different crops are planted at different times, deplete different nutrients, and root into the soil differently. This makes the environment much less stable and therefore less habitable for weeds to prosper . However, this technique requires much more work of farmers.
Soon after the introduction of Roundup Ready crops, several research articles were published concluding that there is no benefit to using a portfolio of weed management strategies such as crop rotations and herbicide rotations mentioned above, one of which was cited by Monsanto in an advertorial they published . Therefore, most farmers believed that employing much less convenient weed management just seemed unnecessary.
Rise of superweeds, return to old farming practices
Unfortunately, these studies had one major problem: the croplands they created for their experiments were rather small . Industrial croplands, on the other hand, can be on the order of a million acres. Therefore, even though developing resistance to Roundup is not probable for any individual plant, there are a massive number of weeds growing that have the potential to resist its poison: this significantly increases the probability of at least some Roundup resistant weeds developing [7,8].
We do not have to go into detail about probabilities to assess whether superweeds will form – we already have confirmation that they have. Twenty-four cases of glyphosate-resistant weeds have been reported around the world, 14 of which are in the United States . Farmers are now back to tilling their farmlands and spraying more toxic herbicides in addition to Roundup in an attempt to control the superweeds spreading across their farmlands .
Additionally, because many of the superweeds can still be killed by glyphosate if it is sprayed in higher doses, many farmers are spraying more glyphosate and other herbicides to combat the weeds. The attraction is that this is much less labor intensive than plowing and handpicking weeds out of the soil . Consequently, a report drawing from US Department of Agriculture data on pesticide use estimated that an additional 383 million pounds of herbicides have been used than if Roundup Ready crops were never introduced . This increased use of glyphosate heightens the likelihood of higher concentrations of the chemical running off into nearby ecosystems. At these elevated concentrations, glyphosate may be capable of causing environmental damage.
Furthermore, the practices of tilling and increased herbicide use are similar to what they were 20 years ago (with even more reported herbicide use). This is both an environmental problem and a financial problem for farmers who must now revert to spending more on herbicides and labor costs to till the land .
While Roundup Ready crops themselves have not caused environmental damage, they are certainly responsible for the Roundup-intensive weed management practices that have accompanied them. The environmental benefits – reduced tilling and reduced use of more toxic herbicides – are fading because the weeds Roundup was supposed to control have sprung up in revolt.
More complex weed management strategies than the ones mentioned here can be employed to help reverse this, and they’re already being used by some farmers . These and other techniques mentioned previously may be worthwhile for realizing the benefits of adopting Roundup Ready technology on a larger scale. However, it is up to farmers to decide whether to invest in complex weed management practices. They are less convenient, but can help reduce risk of resistance in the future. This was a hard decision when Roundup was so effective. However, the advent of glyphosate-resistant weeds has shown that just spraying Roundup is not sufficient – not for farmers and not for our environment.
Jordan Wilkerson is a third year graduate student in the Department of Chemistry at Harvard University.
This article is part of the August 2015 Special Edition, Genetically Modified Organisms and Our Food.
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- Selection Pressure, Shifting Populations, and Herbicide Resistance and Tolerance. University of California, Agriculture and Natural Resources. http://anrcatalog.ucdavis.edu/pdf/8493.pdf
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Note that a previous version of this article stated that the estimate of 383 million pounds of increased pesticide use came directly from the USDA. This has been corrected.