The Promise of GMOs: Herbicides

This is a part of the series The Promise of GMOs. Do GMOs live up to the promises of the biotech industry? In the case of decreasing herbicide use and improving the environmental impact, there is data to back up industry claims.

Reducing herbicide use

Data from the EPA.
Data from the EPA.

BIO’s claims here are that “Biotech is helping to feed the world by: Lowering volumes of agricultural chemicals required by crops-limiting the run-off of these products into the environment [and] Using biotech crops that need fewer applications of pesticides.”

Verdict: Promise met (for herbicides specifically, insecticides will be covered in another post) .

Looking at the most up to date EPA data on agricultural pesticide use, we see that use of herbicides, insecticides, fungicides, and other conventional pesticides (such as miticides) all have a downward trend between 1998 and 2007. The slopes of the linear trendlines are in the legend of the chart to the right. Strangely, other pesticidal substances (such as sulfur, petroleum, and other chemicals used as pesticides) has a strong upward trend, which caused the total to increase. If you were to look at just the total, you wouldn’t get the overall picture.

This dataset from the EPA includes “one from the USDA and others from private pesticide marketing research companies”. I wish there was more recent data, but this is what we have to work with. If you extrapolate into the future without adequate data you might not get the right picture. Brookes and Barfoot have published results from slightly more recent datasets (such as a herbicide use decrease of 75 million lbs from 1996 to 2010) but since part of the dataset is proprietary and I couldn’t look at it, I didn’t feel comfortable using it here.

So we know that herbicide use seems to be decreasing at the same time that GM crops increased. But is that correlation or causation? And is total herbicides even the right way to look at this?

Let’s turn again to the USDA and EPA researchers in their Conservation Tillage, Herbicide Use, and Genetically Engineered Crops in the United States: The Case of Soybeans (paraphrased):

Most researchers measure herbicide use by the total pounds of active ingredients applied. While this is informative, these of these studies can be biased by unobservable conditions prevailing in the year of the study. Also, when different types of active ingredients are grouped together, it covers up the fact that their characteristics (potency, toxicity, etc.) vary widely.

Therefore, any studies that look at volume or mass of herbicides is not telling the whole story. So the small decrease in herbicide use shown above may not be relevant if there was a switch to much more toxic herbicides. They continue (paraphrased):

Some studies suggest that herbicide use on HT soybeans may be slightly higher than herbicide use on conventionally grown soybeans in the United States. However, glyphosate (the herbicide used on most HT crops) is less toxic to humans and not as likely to persist in the environment as the herbicides it replaces. Consequently, increased herbicide use on HT soybeans is not necessarily indicative of worse environmental outcomes.

Data from Fernandez-Cornejo et al, 2012.
Data from Fernandez-Cornejo et al, 2012.

The table to the right summarizes herbicide use on soybeans in 1996, before HT crops were widely adopted, and in 2006, after HT crops were widely adopted. The data is again from Conservation Tillage, Herbicide Use, and Genetically Engineered Crops in the United States: The Case of Soybeans.

For the column headings, Use = % of this pesticide compared to the total of all pesticides; Rate = pounds of active ingredient applied per acre of soybeans; Toxicity = chronic toxicity score (higher is safer); and Half life = number of days it takes for 1/2 of the herbicide active ingredient to degrade in the soil.

I added the EIQ (Environmental Impact Quotient, lower is safer) for each herbicide because I’m not familiar with the chronic toxicity score. EIQ is a numerical representation of the risks a pesticide poses for the environment, consumers, and farm workers. The EIQ for 2,4-D is an average of the EIQs for 5 different formulations of the herbicide.

In 1996, soybean farmers were using a variety of pesticides, and at least 68% of herbicides used had toxicity scores that were worse than glyphosate. In 2006, the majority of farmers had switched to glyphosate, the least toxic of the bunch, making up 85% of herbicide use.

The total amount of herbicides is trending down and the total environmental impact due to herbicides has decreased. While we can’t be sure if HT crops were the cause of the total decrease in herbicide use, we can be reasonably certain the change in herbicide types was due to HT crops. So, the verdict for BIO’s claim is promise met. However BIO’s use of “volumes” in their claim specifically is a poor measure for pesticides, it makes much more sense to look at the types of herbicides.

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Anastasia is Policy Director of Biology Fortified, Inc. and the Co-Executive Editor of the Biofortified Blog. She has a PhD in genetics with a minor in sustainable agriculture from Iowa State University. Her favorite produce is artichokes!
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