So many people, including me, have talked about how glyphosate tolerant crops allowed replacement of more toxic herbicides. That was certainly true for a while, but that trend seems to be over, according to this “largest ever” study of the environmental impacts of GE crops: Genetically engineered crops and pesticide use in U.S. maize and soybeans (open access). You can also view the press release: Largest-ever study reveals environmental impact of genetically modified crops.
To put things in context, let’s look at adoption of GE crops. There has been a steady trend of increasing adoption and then market saturation. Nothing new there, but it does set the stage for the conclusions of the study: 89% of US corn (aka maize) and cotton and 94% of US soybeans are herbicide tolerant.
Some previous studies have looked at herbicide “pounds of active ingredient” without considering the toxicity of the herbicides. The researchers of this paper factored in EIQ (Environmental Impact Quotient) which considers toxicity of pesticides. EIQ is not a perfect method of assessing pesticides, but is much more useful than pounds.
The first conclusion that sticks out in this study is a confirmation that insect resistant GE crops have been and continue to be positive for the environment. This has been shown again and again. This image from the paper shows a clear decreasing trend of insecticide use in corn, though that trend may be reversing in 2011, the latest years in this dataset.
The herbicide story is a bit more complicated. This set of images (adapted from the paper) shows total herbicide use in soybeans and maize, with the black lines showing active ingredients weighted by EIQ. For soybeans (left) the total EIQ of herbicides used decreased slightly then started increasing in 2006. For maize (right) the total EIQ of herbicides used decreased slightly then leveled off.
This set of images has the same information as the pair above, but provides a visual contrast between total glyphosate (red, top) and other herbicide use (blue, bottom). For soybeans (left) non-glyphosate herbicides seem to be staying constant or even increasing slightly as use of glyphosate increases from 2006 to 2011.For maize (right) there’s a decreasing trend in non-glyphosate herbicides that continues into 2011, the most recent year in this dataset.
One mitigating factor could be number of acres. There is an increasing number of acres of soybean planted from 1998-2011. However, there was a dip in planted acres of soybeans in 2007 and the herbicides used didn’t dip correspondingly. The planted acres of corn also increased during this time, but the total amount of herbicides used on corn has remained pretty steady, indicating that farmers are growing more corn with less herbicides.
Another issue barely touched on by these researchers is tillage. Herbicide tolerant GE crops have helped some farmers to adopt no-till or low-till farming (tilling is used to kill weeds, so herbicide can replace tillage in some situations). And reducing tillage definitely has a positive impact on the environment. However, no-till adoption is lower than you’d expect. According to the 2015 USDA report Conservation-Practice Adoption Rates Vary Widely by Crop and Region, no-till/strip-till was used in 31% of corn, 46% of soybeans, 33% of cotton, and 43% of wheat in 2010 and 2011. If herbicide tolerance traits were very effective in reducing tillage, we should expect higher numbers in corn and cotton, and we should expect increasing trends in no-till use, not decreasing.
There is a non-GE herbicide tolerance trait called Clearfield in wheat (also in canola, sunflower, rice, and lentil), so that may account for the high percentage and increasing trend of reduced till wheat, but I wasn’t able to find how much of US wheat is Clearfield. It would be interesting to see what the environmental impacts have been of the Clearfield trait, and compare impacts of non-GE herbicide tolerance to GE herbicide tolerance. I suspect the impacts would be similar, though we’d have to consider that the Clearfield system works with imazamox herbicide, which has a slightly higher EIQ (19.52 for imazamox compared to 15.33 for glyphosate).
Anyway, let’s look at the main findings of the paper: “For all soybean herbicides, corn herbicides, and maize insecticides, the farmworker and consumer components were lower on account of GE variety adoption. For the ecology component, maize herbicides and insecticides were improved by GE adoption, but for soybean herbicides, GE adoption had a detrimental effect “
In other words, GE has overall had a positive effect, but in the case of GE herbicide tolerant soybeans, it has actually had a negative environmental impact. Not because of glyphosate, but because of other herbicides used with glyphosate. Now, if GE herbicide tolerant soy had never existed, more toxic herbicides would have been used anyway so overall there has still been a net positive. But based on this paper, claims about environmental benefits of GE herbicide tolerant soybeans are are overstated, especially when you consider that the claims about tillage may be exaggerated as well.
The authors of this study propose that the increases in non-glyphosate herbicide use in soybean is due in part to glyphosate resistant weeds. It may only be a matter of time before non-glyphosate herbicide use increases in maize as well. Glyphosate is too important of an agricultural tool to lose, so we need to take steps to reduce weed resistance. Ironically, the best way to do that may be spraying more herbicides.
To end this post on a positive note, let’s look again at the chart showing insecticide use in corn. While isolated examples of insect resistance to Bt corn have been found, in the US at least, that resistance is not widespread enough to warrant increasing use of insecticides, at least up to 2011, the last year found in this dataset.