This is a complex subject, and therefore, providing an answer requires discussion of a number of factors, such as the nature of the GM technology, what it is replacing, what might be reasonably used as an alternative nowadays, after many years of using GM technology, and lastly, the environmental impact associated with the pesticide-use changes.
Firstly, there is the context and type of GM crop technology being used. This currently falls into two main types: insect-resistant crops, which are specifically designed to make a crop resistant to a specific pest or pests and can be found in widespread use in corn and cotton crops around the world, and herbicide-tolerant crops, where the GM technology allows crops to tolerate application of specific herbicides (notably glyphosate) for improved weed control, and which can be found in widespread use in the crops of soybeans, corn, canola, sugar beet and cotton.
The GM insect-resistant (GM IR) technology provides a form of protection against pests and often replaces insecticides as a form of control. In corn and cotton, the use of GM insect-resistant technology has resulted in major reductions in the usage of insecticides that have been traditionally used to control the pests the GM technology now controls. For example, between 1996 and 2011, the use of insecticides on these crops in the countries using the technology has fallen by nearly 240 million kg of insecticide active ingredient.
The GM herbicide-tolerant (GM HT) technology allows farmers to simplify and improve their weed control through the use of one or two herbicides that are effective against a broad range of weeds, instead of having to often rely on the use of a larger number of herbicides that are more selective in their ability to control weeds. In other words, the adoption of this GM technology has resulted in a change in the profile of herbicides used in many countries. In some, mostly developing countries, the GM HT technology has also enabled farmers to significantly improve their weed control by replacing hand weeding, which is unpopular and difficult to find people willing to do. Not surprisingly, the impact of adoption of this technology on herbicide usage varies by crop, country and time. Using the United States as an example, in the early years of adoption across all crops, GM HT technology use resulted in significant aggregate reductions in the volume of (weight of active ingredient) herbicides used in crops such as corn and canola. However, there were differences among the crops, and in some, such as soybeans, the average amount of herbicide active ingredient applied remained largely unaltered, or increased in the case of sugar beet.
Since the mid-2000s, in the main crops of corn, cotton and soybeans in the United States, the average amount of herbicide applied to crops has tended to increase. The main reason for this has been increasing incidence of weed species becoming resistance to the main herbicide used with GM HT crops, glyphosate, and increasing recognition among farmers, coupled with both public- and private-sector weed scientist recommendations, that weed-management programs should diversify, and not rely on a single herbicide for total weed control. Farmers have therefore increasingly incorporated one or two other herbicides, in addition to glyphosate, into their weed-management programs.
The development of weed species resistant to herbicides should, however, be placed in context. Nearly all weed species have the potential to develop resistance to herbicides, and there are hundreds of resistant weed species confirmed in the International Survey of Herbicide Resistant Weeds. Reports of herbicide-resistant weeds predate the use of GM HT crops by decades. The development of weeds resistant to herbicides is therefore a problem faced by all farmers, not just those using GM HT technology. In fact, GM HT technology offered a solution to controlling some weeds that had developed resistance to mainstream herbicides used in soybeans in the mid-1990s. The use of herbicides on conventional (non-GM) arable crops in the United States is equally affected by weed-resistance issues, and herbicide use patterns on conventional crops have followed the upward trends that have occurred in GM HT crops.
Secondly, any examination of the impact of GM crop technology should also consider the alternative if GM technology were not used. Past practices for weed or pest control from the days before GM technology was first used are unlikely to reflect what current farmers would likely use, because of the development of new pesticides and other control methods, the withdrawal of some old pesticides, changes in farm practices and experience and a desire amongst farmers to maintain or improve levels of weed or pest control, rather than accept poorer levels of control that may have occurred in the past. Any reasonable assessment of what the "alternative" pattern of pesticide use on crops would be in the absence of GM crops should therefore take these factors into consideration, and a common approach used to do this is to consult weed or pest control scientists and advisors as to what they think are likely alternative pest or weed control programs that would currently be applied if GM technology was no longer used. This is an approach I have used in numerous studies in peer-reviewed journals of pesticide-use change with GM crops (an example reference is provided at the end). In summary, the key findings of this research shows that the conventional alternative to GM crops invariably result in higher levels of pesticide being used relative to the current levels with GM crops. This means that while, for example, total herbicide use with GM HT crops in the United States has increased in recent years, it would have likely risen by even greater amounts if conventional (non-GM) technology had been used instead.
Lastly, any consideration of pesticide use change impacts with GM crops should assess the associated environmental impact. While the amount of pesticide applied to a crop is one way of trying to measure the environmental impact of pesticide use, this is not a good measure of environmental impact, because the toxicity and risk of each pesticide is not directly related to the amount (weight) applied. For example, the environmental impact of applying 1 kg of dioxin to a crop or land is far more toxic than applying 1 kg of salt. There exist alternative (and better) measures, which have been used by a number of authors of peer-reviewed papers, to assess the environmental impact of pesticide use change with GM crops, rather than simply looking at changes in the volume of active ingredient applied to crops. In analysis I have been involved in undertaking for several years on pesticide use change impact with GM crops, we analyzed both active-ingredient use changes and utilized the indicator known as the environmental impact quotient (EIQ) to assess the broader impact on the environment (plus impact on animal and human health). The EIQ distills the various environmental and health impacts of individual pesticides in different GM and conventional production systems into a single “field value per hectare” and draws on key toxicity and environmental exposure data related to individual products. Developed at Cornell University in the 1990s, it provides a better measure to contrast and compare the impact of various pesticides on the environment and human health than weight of active ingredient alone. It is, however, an indicator only (primarily of toxicity) and does not take into account all environmental issues and impacts.
Our latest analysis, covering the period 1996–2011 (see reference at the end) shows that GM traits have contributed to a significant reduction in the environmental impact associated with insecticide and herbicide use on the areas devoted to GM crops. Since 1996, the use of pesticides on the GM crop area was reduced by 473.7 million kg of active ingredient (an 8.9 percent reduction), and the environmental impact associated with herbicide and insecticide use on these crops, as measured by the EIQ indicator, fell by 18.3 percent.
In absolute terms, the largest environmental gain has been associated with the adoption of GM insect-resistant (IR) technology. GM IR cotton has contributed a 24.8 percent reduction in the volume of active ingredient used and a 27.3 percent reduction in the EIQ indicator (1996–2011), due to the significant reduction in insecticide use that the technology has allowed, in what has traditionally been an intensive user of insecticides. Similarly, the use of GM IR technology in corn has led to important reductions in insecticide use, with associated environmental benefits.
The volume of herbicides used in GM corn crops also decreased by 193 million kg (1996–2011), a 10.1 percent reduction, while the overall environmental impact associated with herbicide use on these crops decreased by a significantly larger 12.5 percent. This highlights the switch in herbicides used with most GM herbicide-tolerant (HT) crops to active ingredients with a more environmentally benign profile than the ones generally used on conventional crops.
Important environmental gains have also arisen in the soybean and canola sectors. In the soybean sector, herbicide use decreased by 12.5 million kg (1996–2011), and the associated environmental impact of herbicide use on this crop area decreased, due to a switch to more environmentally benign herbicides (-15.5 percent). In the canola sector, farmers reduced herbicide use by14.8 million kg (a 17.3 percent reduction), and the associated environmental impact of herbicide use on this crop area fell by 27.1 percent (due to a switch to more environmentally benign herbicides).