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Do GMOs increase herbicide use?

Do GMOs increase herbicide use?

Submitted by: MrDavenportSCMS


Expert response from Graham Brookes

Agricultural Economist, PG Economics Ltd, UK

Monday, 31/07/2017 13:51

In order the answer this question, it is important to first be clear about what a GMO is and secondly to discuss the complex issues relating to herbicide use.


What is a GMO?

It is assumed that this question refers to genetically modified crops. GM crop technology has been widely used since the mid-1990s and in 2015 were planted on about 175 million hectares worldwide.   The main GMO traits convey:


  •          Tolerance to specific herbicides (notably to glyphosate) in maize, cotton, canola (spring oilseed rape), soybean, sugar beet and alfalfa. This GM herbicide tolerant (GM HT) technology allows for the ‘over the top’ spraying of GM HT crops with these herbicides, that target weeds but do not harm the crop itself – the aim of the technology is to provide farmers with better (and usually cheaper) weed control than is possible with conventional crops;
  •          Resistance to specific insect pests of maize, cotton, soybeans and brinjal. This GM insect resistance (GM IR), or ‘Bt’ technology offers farmers resistance in the plants to major pests of these crops – the aim of the technology is to control these pests more effectively than is possible in conventional crops, where the most common form of control used is insecticides. 


The nature of these two main types of GMO trait means that GM IR crop technology has no impact at all on herbicide use – its main pesticide-related impact is associated with insecticide use. GM HT crops are therefore the only type of GMO that impacts on herbicide use.


What are herbicides used for?

Weeds cause major problems for farmers – they compete with crops for water and nutrients in the soil and can have a significant negative effect on crop yields.  Farmers spend a lot of time and money trying to control weeds and the main method used is to apply herbicides. These need to be carefully targeted at, and applied to weeds so as not to damage or kill the crops themselves. GM HT crops make weed control easier, cheaper and more effective than is possible with conventional cropping largely because the risk of damage to the crop from herbicides is effectively removed.


Is increased herbicide use bad?

Not necessarily. The issues are complex and context is required. 


A key context is agriculture’s impact on the environment. Firstly, all forms of agriculture, GMO, conventional or organic are not natural and therefore all farmers are engaged with a ‘fight with nature’ to secure their production. This means that pests and weeds need to be controlled if humans are to feed themselves. The balance is to secure and maximize production as efficiently as possible, whilst at the same time, minimize the negative impact on the environment of the weed and pest control methods used.  


As indicated above, herbicides are the main form of weed control used in agriculture.  Therefore, if agricultural productivity is to be maximized, with the least environmental impact, herbicide use needs to have the smallest environmental ‘footprint’ possible. This does, however, not necessarily mean less herbicide = less environmental footprint or more herbicides = a bigger environmental footprint because the amount of herbicide applied to a crop is not necessarily a good measure of the impact on the environment because the toxicity of each herbicide is not directly related to the amount (weight) applied. For example, the environmental impact of applying a kilogram of dioxin to land is far more toxic than applying a kilogram of salt.


Secondly, pests and weeds develop resistance to all forms of control humans try to impose on them. This is an inevitable consequence for all forms of agriculture, whether GM, conventional or organic forms of production are used. This means that farmers have to change and adapt their pest and weed control practices to minimize resistance development and maintain effective levels of pest and weed control.  A common feature of this resistance management is the application of a mix of control measures, which may include using a wider range or mix of herbicides or insecticides.     


Thirdly, in looking at trends in usage of herbicides on crops, it is important consider what the alternative would be – this means that it may be possible to see a trend of increased use in herbicides that uses one production system that would be even higher if an alternative and different production was used. 

These contextual factors are important to understanding changes in herbicide use with GM HT crops.


Has the use of GM HT crops increased use of herbicides and what does this mean for the environment?

A key impact of GM HT crop use has been a change in the profile of herbicides typically used.  In general, a fairly broad range of, mostly selective herbicides that target grass weeds or broad-leaved weeds have been replaced by one or two ‘broad-spectrum’ or general herbicides (mostly glyphosate - that target both grass and broad-leaved weeds) used in conjunction with one or two other (complementary) herbicides. This has resulted in:

·         Aggregate reductions in both the volume of herbicides used (in terms of weight of active ingredient applied) and the associated environmental impact (in terms of toxicity) when compared to usage on conventional (non-GM) crops in some countries, indicating net improvements to the environment;

·         In other countries, the average amount of herbicide active ingredient applied to GM HT crops represents a net increase relative to usage on the conventional crop alternative. However, even though the amount of active ingredient use has increased, in terms of the associated environmental impact, the environmental profile of the herbicides used with the GM HT crop has commonly been better than its conventional equivalent;

·         Where GM HT crops have been widely grown (eg, the USA), incidences of weed resistance to glyphosate have occurred. This can be attributed to how glyphosate was originally used with GM HT crops, where because of its high effectiveness in controlling weeds, it was often used as the sole method of weed control. This approach to weed control contributed to the evolution of weed populations becoming resistant to glyphosate. As a result, growers of GM HT crops have been and, are increasingly being advised to include other herbicides (with different and complementary modes of action to glyphosate) in combination with glyphosate and in some cases to adopt cultural practices (eg, mechanical weed control) in more integrated weed management systems.  At the national level, these changes have influenced the mix, total amount and overall profile of herbicides applied to GM HT crops in the last 10 years. Compared to a decade ago, the amount of herbicide active ingredient applied and number of herbicides used with GM HT crops in many regions has increased, and the associated environmental profile, deteriorated. This increase in herbicide use relative to several years ago is often cited by anti GM technology proponents as an environmental failing of the technology. However, what such authors fail to acknowledge is that the amount of herbicide used on conventional crops has also increased relative to several years ago and that compared to the conventional alternative, the environmental profile of GM HT crop use has continued to provide an improved environmental profile compared to the conventional alternative. It should also be noted that many of the herbicides used in conventional production systems had significant resistance issues themselves in the mid-1990s. This was, for example, one of the reasons why GM HT soybeans were rapidly adopted in the late 1990s, as glyphosate provided good control of these weeds.



Brookes G and Barfoot P (2017) Environmental impacts of GM crop use 1996-2015: impacts on pesticide use and carbon emissions, GM Crops and Food Journal 2017 8,2, p117-147


Graham Brookes is an agricultural economist with the UK-based analysts’ PG Economics ( He has been studying the impact of crop biotechnology for nearly 20 years and is author of 24 peer reviewed papers on the economic and environmental impact of crop biotechnology