Summary:

Herbicide tolerant (HT) crops are not a new phenomenon - in fact, all crops are to some extent tolerant to herbicides. Many herbicides are dose-specific for particular plant groups, for example grasses, and these can be used on broad-leaved crops like potatoes and oilseed rape without causing unacceptable harm. This is the basis for most current agricultural practice in developed countries. Genetic modification, however, has enabled crops to be made resistant to concentrations of 'broad-spectrum' herbicides that have a mode of action that is lethal to all conventional crops and wild plants. Examples of broad-spectrum herbicides to which GM plants have been made resistant are glyphosate and glufosinate-ammonium. These herbicides, particularly glyphosate, are already regularly used by farmers in the UK to 'clean up' weedy stubbles and to desiccate crops such as potatoes and oilseed rape before harvest.

There are three main areas of concern regarding herbicide tolerance as a trait in crop varieties. Firstly, the crop plant in question could become a weed itself or hybridise with wild relatives to produce offspring that are problem weeds. These are commonly referred to as direct effects of the herbicide tolerance trait.

Secondly, genetically modified herbicide tolerant (GMHT) crops are designed to be managed in a different way to 'conventional' crop varieties. This altered management has raised concerns among many groups, including the British statutory conservation agencies, that GMHT crops could be managed in a way that excludes even more biodiversity from farmed areas than is currently the case. If grown over a large area, the result could be a decline in farmland wildlife, particularly those species further up in food webs, like birds, some of which already have dangerously low populations in the UK. These are referred to as indirect effects.

Thirdly, there are practical considerations of how long each specific HT trait would remain agronomically useful. Over-reliance on a limited number of herbicides can lead to 'weed shifts' where weed population dynamics are changed by selection, enabling weeds to avoid the herbicide application by, for example, delayed germination. There can also be evolution of weed physiology towards more resistant species or genotypes. These phenomena are well known in 'conventional' arable fields and can rapidly lead to herbicides becoming ineffective, for example, ALS-inhibiting herbicides in the US mid-west. Although the development of herbicide-resistant weeds is seen primarily as an agronomic issue, it could have important consequences for wildlife if it resulted in increased rate and/or frequency of application, or applications of more environmentally damaging herbicides, to control resistant species.

These three issues are interlinked, for example gene flow leading to herbicide-tolerant volunteers in fields (direct effect) could lead to changed management practices (indirect effect) that have impacts on biodiversity.

This document reviews the current state of scientific understanding of these issues and identifies areas of uncertainty where further research may be needed.

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