Genetically Modified Crops

5. What effects could genetically modified crops have on the environment?

  • 5.1 What direct effects could genetically modified plants have on the environment?
  • 5.2 What indirect effects could genetically modified plants have on the environment?
  • 5.3 How should these environmental effects be assessed?

5.1 What direct effects could genetically modified plants have on the environment?

Maize farmer
Farmer in a field of maize in Bolivia Source: FAO

Agriculture of any type - subsistence, organic or intensive - affects the environment, so it is expected that the use of new genetic techniques in agriculture will also affect the environment.

Genetic engineering may accelerate the damaging effects of agriculture, have the same impact as conventional agriculture, or contribute to more sustainable agricultural practices and the conservation of natural resources, including biodiversity.

Although scientific opinion is divided over these risks, it is agreed that environmental impacts need to be assessed on a case-by-case basis. They recommend ecological monitoring to detect any unexpected events once the plants are grown in the environment. More...

5.1.1 Horizontal gene flow refers to a gene transfer, usually through pollen, from cultivated species to their wild relatives (and vice-versa). This may happen with either conventional or genetically modified plants.

However, many of the world's major food plants are not native to the areas where they are grown and thus lack close wild relatives that would be needed for gene flow to occur. For example, potatoes (which originate in South America) and maize (originating in Mexico) have no wild relatives in Europe. In such cases, horizontal gene flow to wild relatives is impossible. In the USA, cotton and maize have no wild relatives, whereas sunflowers, squash, and radishes do, making the latter possible candidates for gene flow.

In general, gene flow between cultivated plants and their wild relatives is not considered an environmental problem unless it leads to undesirable consequences. Moreover, gene flow from cultivated crops to wild relatives is expected to create hybrids with characteristics that are advantageous in agricultural environments, but that would not thrive in the wild. In the UK, for instance, no hybrid between a crop and a wild relative has ever become invasive.

Future genetically modified plants may be designed to prevent gene flow to other plants. This is important for the co-existence of GM and conventional crops, and may be particularly important for genetically modified plants producing substances of medical or industrial interest. Management strategies to control gene flow include avoiding the planting of genetically modified crops where wild relatives are present, or using buffer zones to isolate genetically modified varieties from conventional or organic varieties. More...

5.1.2 lants which carry a specific “Btgene produce a toxin that kills insect pests that feed upon them, but is harmless to humans and other species which are not considered insect pests. Bt is used as a natural insecticide in organic agriculture.

A controversy arose about whether pollen from Bt plants could harm beneficial species (such as the monarch butterfly). However, a series of follow-up studies concluded that under field conditions the risk of harm to monarch butterfly caterpillars from Bt maize pollen is very small, particularly in comparison with other threats such as conventional pesticides and drought.

In the field, no significant adverse effects on non-target wildlife nor long term effects of higher Bt concentrations in soil have so far been observed. Yet scientists disagree on how much evidence is needed to demonstrate that growing Bt crops is sustainable in the long term.

Therefore, scientists call for continued monitoring for such effects, and for comparing effects of the Bt gene on the crops with the effect of other current agricultural practices such as chemical pesticide use. More...

5.2 What indirect effects could genetically modified plants have on the environment?

Pesticide rice, Laos
Farmer spraying pesticide on rice seedlings in Laos Source: FAO

5.2.1 Scientists agree that the use of conventional agricultural pesticides and herbicides has damaged habitats for farmland birds, wild plants and insects and has seriously reduced their numbers.

Genetically modified crops may have indirect environmental effects as a result of changes in agricultural or environmental practices associated with the new varieties.

However, it remains controversial whether the net effect of these changes will be positive or negative for the environment, so more comparative analysis of new technologies and current farming practices is still required.

The following paragraphs explore possible environmental benefits: More...

5.2.2 Using genetically modified crops which are insect resistant because they carry the Bt gene has reduced insecticide use on maize, cotton, and soybean. The environmental benefits include less contamination of water supplies and less damage to non-target insects. In turn, this may be beneficial to biodiversity, in comparison to conventional crops that receive regular broad-spectrum pesticide applications. Health benefits to farm workers due to reduced chemical pesticide spraying have been documented in China. More...

5.2.3 The adoption of genetically modified crops which are tolerant to certain less toxic forms of herbicides, has resulted in a marked shift towards the use of these less toxic forms though total herbicide use has increased. Scientists agree that herbicide-tolerant crops are encouraging low-till agriculture (which limits the use of plows), with resulting benefits on soil conservation. On the other hand greater use of herbicides - even less toxic herbicides - could further erode habitats for farmland birds and other species.

Extensive farm-scale evaluations of the impacts of genetically modified herbicide-resistant crops in the United Kingdom concluded that commercialization of these crops would have a range of impacts on weed vegetation, with consequent effects on the herbivores, pollinators and other populations that feed on it. The observed effects on biodiversity varied for different genetically modified species, with negative effects in sugar beets, positive effects for maize and no effects for oilseed rape. Scientists acknowledge that evidence is insufficient to predict the long-term impacts of such genetically modified crops. More...

5.2.4 The extensive use of herbicides and insect resistant crops could result in the emergence of resistant weeds and insects. This has often occurred as a consequence of conventional herbicide and insecticide spraying. Several weed species have developed resistance to specific herbicides which are extensively used in combination with herbicide-resistant genetically modified crops. Insect-resistant Bt-crops similarly could lead to the emergence of Bt-resistant insects. The extent and possible severity of impacts of resistant weeds and insects are subject to continuing scientific investigation. More...

5.2.5 New genetically modified crops are being developed that can withstand environmental stresses such as drought, salinity, or the presence of aluminum in the environment. They may permit cultivation of soils that are presently of low productivity for agriculture. Scientists agree that these crops may be either beneficial or harmful for society, depending on the crop, the characteristic, and the environment. More...

5.3 How should these environmental effects be assessed?

The broad consensus is that genetically modified plants should be evaluated using science-based assessment procedures, on a case-by-case basis depending on the species, characteristic, and agricultural ecosystems. The use of genetically modified plants should be compared to other agricultural practices and technology options, in particular to conventional agriculture, which has already had a profound effect on the environment.

Procedures and international guidelines for the assessment of GMOs are well developed for food safety but not for environmental impacts. For instance the FAO/WHO Codex Alimentarius Commission provides an international forum for developing food safety guidelines.

In the absence of international guidelines, environmental impact assessments differ in:

  • the interpretation of data and of what constitutes an environmental risk or harm,
  • the basis for comparison used: comparing the use of genetically modified crops either with conventional agricultural or with non-cultivated environments,
  • the extent to which small-scale laboratory and field trials are valuable and can be used to extrapolate large-scale effects.

The scientific community recommends more research and better monitoring regarding post-release effects of genetically modified crops. More...

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