Glyphosate-resistant weeds – such as these pigweed plants in a soybean field – have led to a 28% hike in herbicide use on GM soybeans compared with non-GM. Report: Claire Robinson

According to new research from University of Virginia economist Federico Ciliberto, widespread adoption of GM crops has decreased the use of insecticides, but increased the use of weed-killing herbicides as weeds become more resistant.

Ciliberto led the largest study of genetically modified crops and pesticide use to date, alongside Edward D. Perry of Kansas State University, David A. Hennessy of Michigan State University and GianCarlo Moschini of Iowa State University. The four economists studied annual data from more than 5,000 soybean and 5,000 maize farmers in the US from 1998 to 2011, far exceeding previous studies that have been limited to one or two years of data.

“The fact that we have 14 years of farm-level data from farmers all over the US makes this study very special,” Ciliberto said. “We have repeated observations of the same farmers and can see when they adopted genetically modified seeds and how that changed their use of chemicals.”

Since 2008, GM crops have accounted for more than 80 percent of maize and soybean crops planted in the US. Maize seeds are modified with two genes: one (in Bt maize) kills insects that eat the seed and one allows the seed to tolerate glyphosate, a herbicide commonly used in weed killers like Roundup. Soybeans are modified with just one glyphosate-resistant gene.

Lower insecticide use finding must be put into context

The study found that maize farmers who used GM Bt insect-resistant seeds used significantly less insecticide – about 11.2 percent less – than farmers who did not use GM maize.

Unlike some other studies which ignore insecticidal seed treatments in calculating insecticide use on GM crops, this study did factor them in. The researchers found that since the adoption of GM Bt maize, insecticidal seed treatments have increased as insecticide sprays applied to the growing crop have decreased. However, seed treatments are used in relatively low volumes compared with insecticide sprays, so the overall effect is a decrease in chemical insecticide use by GM maize growers compared with non-GM maize growers.

Even so, in my view, it is not certain that GM Bt maize has a less harmful environmental impact than non-GM maize. Reasons are as follows:

1. Insecticidal seed treatments are highly potent even at small doses. International scientific concern has arisen regarding the toxic effects of a class of seed treatment chemicals, neonicotinoids, on bees and other pollinators, leading to restrictions on their use in Europe.

2. Bt insecticidal maize is in itself a pesticide, generally containing far more pesticide active ingredient than is displaced in the form of chemical insecticides that do not need to be sprayed. Bt crops have been found to have harmful and toxic effects on non-target species and mammals.

3. The new study stops in 2011, but pests are gradually evolving resistance to GM Bt crops, meaning that chemical insecticide use is likely to increase further. Interestingly, the last data point, collected in 2011, shows an uptick in insecticide use (both as seed treatments and sprayed-on) in maize, compared with the previous year (Figure 1B). Data for the next few years need to be analyzed to see if the upward trend continues.

Massive increase in herbicide use with GM crops

The new study found that GM maize farmers used 1.3 percent less herbicide over the 13-year period. But soybean crops saw a significant increase in herbicide use, with adopters of GM crops using 28 percent more herbicides than non-adopters.

Ciliberto attributes this increase to the proliferation of glyphosate-resistant weeds.

“In the beginning, there was a reduction in herbicide use, but over time the use of chemicals increased because farmers were having to add new chemicals as weeds developed a resistance to glyphosate,” Ciliberto said.

Maize farmers, he said, have not yet had to address the same level of resistance, in part because they did not adopt genetically modified crops as quickly as their counterparts in the soy industry. However, the study did find evidence that both maize and soybean farmers increased herbicide use during the last five years of the study, indicating that weed resistance is a growing problem for both groups.

From 2006 to 2011, the percentage of hectares sprayed with only glyphosate shrunk from more than 70 percent to 41 percent for soybean farmers and from more than 40 percent to 19 percent for maize farmers. The decrease resulted from farmers having to resort to other chemicals as glyphosate-resistant weeds became more common.

“Evidence suggests that weeds are becoming more resistant and farmers are having to use additional chemicals, and more of them,” Ciliberto said.

Despite the decrease in insecticide use, continued growth in herbicide use poses a significant environmental problem as large doses of the chemicals can harm biodiversity and increase water and air pollution.

Ciliberto and his colleagues measured the overall environmental impact of the changes in chemical use that have resulted from the adoption of genetically modified crops, using a measure called the environmental impact quotient, or EIQ, to account for chemicals’ impact on farmworkers, consumers and the environment. Comparing adopters to non-adopters, they found little change in the impact on farmworkers and consumers. However, the adoption of genetically modified soybeans correlated with a negative impact on the environment as increased herbicide use also increased contamination of local ecosystems.

Overall, Ciliberto said he was surprised by the extent to which herbicide use had increased and concerned about the potential environmental impact.

“I did not expect to see such a strong pattern,” he said.

GM crop herbicide trends “ominous”

Dr Chuck Benbrook, author of a previous study on pesticide use on GM crops over the first sixteen years of use, commented on the new findings:

“Their empirical findings regarding the impacts of GE crops on pesticide use in the US are close to mine, lending confidence in publicly available NASS [USDA National Agricultural Statistics] data. Their analysis stops at 2011, around the time herbicide use took off in response to the spread of glyphosate resistant weeds.

“The increase in pounds applied - the most common metric - since 2011 is real but modest, because many farmers have dropped a third glyphosate application at ~0.75 pound of active ingredient per acre, and replaced it with 2-3 active ingredients, usually in a pre-mix, with a combined rate of around 0.2 +/- 0.1 pound of active ingredient.

“When measured by acre-treatments (number of distinct, single applications of any pesticide), the trends look very ominous for farmers and the environment.”


Source: University of Virginia