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Traits in Agriculture » Herbicide Tolerance

Title: Review Finds GM Herbicide-Resistant Crops Will Bring About Further Loss of Biodiversity
Publication date: October 21, 2014
Posting date: October 21, 2014



Dear Friends and Colleagues


Review Finds GM Herbicide-Resistant Crops Will Bring About Further Loss of Biodiversity


First introduced in the 1990s, 84.6% of all genetically modified (GM) crops worldwide carried herbicide resistance traits (144 mil. ha) by 2012. Herbicide-resistant (HR) crops occupy about 59% of the 170.3 million hectares under GM cultivation globally, with GM crops with stacked traits (basically herbicide and insect resistance) covering 25.6%.


Many studies have been done on HR crop use patterns and impacts. A review of these studies has been jointly conducted by three government agencies from Germany, Switzerland and Austria, to extract the lessons learnt, paying particular attention to the impact on biodiversity.


Scientific studies have established that agricultural intensification and pesticide use are among the main drivers of biodiversity loss. There are concerns that HR crops will help to further intensify farming and may therefore increase pressure on biodiversity. The review finds evidence to conclude that HR crops cannot reduce herbicide use in the longer term, and will in fact be associated with a further loss of biodiversity including whole food webs, wild flora, seeds, and ecosystem functions in agricultural fields. 


Other concerns raised include observations that in regions where HR crops are widely adopted, less crop rotation and crop diversification takes place, with a clear trend towards monoculture; the development of herbicide resistant weeds, resulting in farmers resorting to higher herbicide doses and the use of other herbicides; and the spatial and temporal spread of the HR trait, which is a particular concern in centres of crop origin and regions where interfertile and weedy hybrids occur. In addition, there has been little, if any, contribution of HR crops to increase crop yields.


According to the review, “…herbicide resistant crops are not part of the solution, but part of the problem”. It makes a call to stop the loss of biodiversity, reverse the current development trend in agriculture, and increase biodiversity in agricultural ecosystems with more environmentally friendly practices and less dependence on pesticides.


The Executive Summary of the paper and the link to the full paper are given below.


With best wishes


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- A joint paper of the Federal Agency for Nature Conservation (BfN) Germany, Federal Office for the Environment (FOEN) Switzerland and Environment Agency Austria. Edited by Tappeser, B., Reichenbecher, W., & Teichmann, H. BfN (2014).



Conservation of biodiversity is high on the agenda of international and national environmental policies though not very present in public awareness. The need to protect biodiversity and stop the loss was acknowledged in the Convention on Biological Diversity (CBD), internationally agreed on in 1992, and underscored by relevant decisions since then. It has been known for some time that intensive high input farming is one of the main drivers of ongoing biodiversity losses in agricultural landscapes. An indicator for such losses is the diversity and abundance of weed flora. Transgenic crops resistant to the herbicides glyphosate (accounting for the great majority) and glufosinate have first been cultivated commercially in the nineties of the last century. Since then, a wealth of information has been collected on use patterns and on impacts of herbicide-resistant (HR) crops. There are concerns that HR crops will help to further intensify farming and may therefore increase pressure on biodiversity. The need to study potential environmental consequences of changes in herbicide usage due to transgenic HR plants has recently been underlined by the Council (of Environment Ministers) of the European Union (EU). This paper summarizes the lessons that can be learned from the experience up to now.


Impacts on agricultural practice and agronomy


Like any significant change in crop choice, HR crops can have various impacts on the agricultural practice and agronomy, including weed control, soil tillage, planting, crop rotation, yield, and net income. Glyphosate- and glufosinate-resistance allows previously sensitive crops to survive application of the complementary broad-spectrum systemic herbicide. HR crops facilitate weed control, i.e. they enable the usage of broad-spectrum herbicides and thus an easier choice of products as well as an extension of the time window for spraying, giving the farmer more flexibility. HR crops also allow post-emergence application of herbicides instead of the routine pre-emergence application in conventional crops.


Results on yields of HR crops compared to conventional crops are actually mixed. In general, there has been little, if any contribution of HR crops to increase the yield.


The herbicide usage in HR crop systems and their impacts is difficult to compare to conventional crop management because different herbicides are applied at different rates and the specific environmental impacts may vary. There is some agreement in the literature that with the introduction of HR crops in the US lower amounts of herbicides (as active ingredient per hectare) were applied during the first years (from 1996 onwards), compared to conventional crops. Based on United States Department of Agriculture (USDA) statistics, the trend turned in 2000 and already in 2004 more herbicides were applied to HR crops than to conventional crops. In the following years, the difference rose progressively and led to an estimated amount of 239 million kg additional herbicides in the whole period of 1996-2011, with HR soybean accounting for two thirds of the total increase. Most of the increase was due to the rising dependence on glyphosate. In Argentina, herbicide use, and in particular glyphosate use, increased enormously in line with the introduction of glyphosate-resistant soybean. In case HR crops would be authorized for cultivation in Europe, projections predict that herbicide use would increase significantly in the EU as well.


Mechanical weed control decreased with the introduction of HR varieties. Conservation tillage, often recommended to reduce soil erosion and to save costs and energy, expanded and might even further expand if more HR crops are grown as they are well adapted to tillage systems without or reduced mechanical weed control. In regions where HR crops are widely adopted, less crop rotation and crop diversification takes place. There is a clear trend towards monoculture of HR crops, which enhances disease and pest pressure although, in theory, high weed control levels in HR cropping systems would allow to include crops with higher weed infestation and to broaden crop rotation.


However, crop rotations in HR systems may change due to volunteer problems. HR volunteers which survive pre-seeding herbicides can cause undesirable effects in less competitive crops and require that different or further herbicides are applied.


Reasons for farmers to adopt HR systems are, besides simplification of weed control, reduced production risks, the currently low herbicide prices and expected lower costs of HR systems (e.g. in combination with conservation tillage and other production factors such as less labor and fuel consumption). It is not increased yields that are cited in first place as reason for HR crop adoption. Overall, the higher flexibility rather than the crop yield and the final economic success (costs vs. returns) are the decisive factors for adopting HR systems.


Changes in weed susceptibility


In general, increased dependence on herbicides for weed control leads to a shift in weed species composition. Less sensitive species and populations will survive sprayings and subsequently grow and spread, whereas more sensitive species disappear. Although glyphosate was not considered to be a high-risk herbicide with regard to the development of resistances, at least 24 glyphosate-resistant weed species, comprising more than 150 populations, have been found. Today, they infest millions of hectares of HR crops and conventional crops. Some of the resistant weed biotypes are cross-resistant to other herbicides. Glyphosate-resistant weeds can withstand up to 19-fold the dose tolerated by ordinary sensitive weeds and exhibit a great diversity of molecular and genetic resistance mechanisms. Recently, two weed species resistant to glufosinate have been described as well.


Weed scientists recommended for years farmers should implement an integrated weed management approach, comprising a combination of a number of weed management methods ranging from crop rotation, herbicide rotation and mechanical weeding to cover crops, inter- cropping and mulching. But continuous glyphosate-resistant cropping became common in the Americas, and farmers often simply resorted to higher herbicide doses and other herbicides. Increasingly, companies develop and sell transgenic crops with stacked HR traits, which combine glyphosate-resistance with resistance to glufosinate and/or resistance to other herbicides such as synthetic auxins like 2,4-D or ALS (acetolactate synthase)-inhibiting herbicides. However, a number of hard to control weeds is already resistant to synthetic auxins and even more so to ALS-inhibitors. In addition, merely rotating herbicides may exacerbate resistance problems by selecting for more generalist resistance mechanisms in weeds.


In particular, crops with characteristics such as shattering and seed persistence, e.g. oilseed rape, are likely to emerge as volunteers. Seed spill can also occur outside the fields and along transport routes potentially leading to HR feral plants. Oilseed rape volunteers with resistance to glyphosate and glufosinate have already been detected in fields where HR crops have not been planted previously. Oilseed rape plants with multiple herbicide resistance genes not commercially sold have also been found, providing evidence of novel transgene combinations in the wild. Thus, the HR trait can spread both spatially and temporally. More HR plants might show up, if outcrossing from HR crops into the same or related species occurs. The transfer of HR genes to wild relatives should be particularly taken into account and avoided in centers of crop origin and regions where interfertile and weedy hybrids occur.


Impacts on biodiversity


Farmland biodiversity is an important characteristic when assessing sustainability of agricultural practices and is of major international concern. The environmental impacts of a particular HR crop are difficult to assess as they depend on a range of factors that vary from region to region. These factors encompass the whole agricultural management, e.g. the complementary herbicide compared to the conventional herbicides, the dose, time and frequency of herbicide applications, and additional management features of the HR crop and of other crops in rotation with it. The receiving environment plays an important role.


Growing HR crops is associated with the use of broad-spectrum herbicides that have long been perceived as less hazardous. For herbicides, specific legal frameworks regulating the approval procedures and assessment criteria are established. While glufosinate, due to its reproductive toxicity, is expected to be phased out in the EU in 2017, glyphosate is presently evaluated for renewed approval in the EU. Due to the adoption of HR crops almost twenty years ago, glyphosate is today by far the herbicide most widely used in the world. In light of the great number of glyphosate-resistant crops that are authorized or in the pipeline, glyphosate will likely remain one of the most used herbicides for the next decade.


Data collected within the last years indicate that glyphosate and glyphosate-based herbicides, apart from being toxic to plants, can also be toxic to other life forms. There are adverse effects on mammals, some invertebrates, aquatic species and the soil microflora. Glyphosate-based herbicides are particularly toxic to amphibians. Glyphosate impacts plants also by binding minerals which can lead to an undersupply of necessary micronutrients and thereby decrease their disease resistance.


Growing HR crops facilitate the operation of reduced/no-tillage systems and consequently may support their expansion. Long-term experiences with reduced tillage indicate that weed populations shift to perennial and grass species and the diversity and abundance of broad-leaf plants may decrease further when reduced/no-tillage systems are combined with HR crops.

The Farm Scale Evaluations have provided ample evidence that, compared to conventional farming, weeds are removed more efficiently in HR systems, leading to a further reduction of flora and fauna biodiversity and abundance in farmland. This includes direct effects, such as depletion of the weed seedbank and low weed density and diversity, as well as indirect effects, e.g. impacts on animals feeding on weeds and on predators of these animals. Thus, farmland birds may be particularly affected. The significant reduction in monarch butterfly populations in the US has been linked to the widespread cultivation of HR crops in the Midwest which drastically reduced the population of milkweed, the feeding plant of monarch larvae.


As agricultural intensification and pesticide use are among the main drivers of biodiversity loss, agreement on farming practices is required, that are more environmentally friendly and less dependent on pesticides. According to the experience in countries adopting HR crops, where herbicide use was increased instead of reduced, it is highly questionable whether HR systems comply with measures to stop the loss of biodiversity on farmland or can be managed in a sustainable way without further adverse impacts on biodiversity. From a nature protection perspective HR crops seem to be no option for a sustainable agriculture focussing also on protecting biodiversity.


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