Executive Summary continued -
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Possible development
of herbicide resistant weeds, weed shifts and pest resistance
Whilst it is beyond the remit of this study to examine the incidence of
these potential problems (which are not GM crop-specific and relate to
agriculture in general), there are possible implications for farm
profitability. Our review of the evidence to date shows the following:
- The development of weed resistance to
the main herbicides used in herbicide tolerant crops (glyphosate and
glufosinate), and problems with volunteers has probably not had any
significant impact on the economics of using herbicide tolerant crops to
date.
- In the longer term, some degree of
reduced effectiveness of glyphosate and glufosinate against weeds may
develop, along with some possible instances of weeds with resistance to
more than one herbicide.
- To the extent to which weed or pest
resistance may occur, this will add cost to farmers who are required to
use additional levels of glyphosate/glufosinate or include low dose
applications of other herbicides in their weed control programmes . For
example in Australia, where instances of glyphosate resistant weeds have
been found, farmers increasingly use other herbicides like trifluralin
as a pre-sowing treatment instead of glyphosate. This may therefore
reduce, marginally, the average level of cost saving and profit gains
cited in the most recent studies of GM herbicide tolerant crops.
- Similar problems of weed or pest
resistance build up to herbicides/insecticides used on conventional
arable crops can also be expected to develop, leading to similar
problems and solutions for conventional crop producers (ie, the issue of
weed resistance to herbicides is not a GM specific issue). Any
assessment of the possible benefits and costs of GM crops should
recognise these points because to only examine the possible impact of
weed/pest resistance build up in relation to GM crops would not be
comparing ‘like for like’ with the alternative production systems.
- The net impact on profitability of
weed/pest resistance, weed shifts and volunteer problems is likely to be
fairly small. Current commercial practice in conventional agriculture is
to use tank mixes of herbicides (and possibly other products) to deal
with difficult weeds/pests. Where farmers are faced with the build up of
weed resistance to one herbicide, the solution is to add a different
herbicide into an existing tank mix that is effective against a
particular weed. Additional spray runs are rarely needed and therefore
the overall impact on variable costs of production is very low (+1 to
+2% for the additional herbicide). This issue is examined further in
section 4;
- Farmers decide to adopt new technology
based largely on their perception (and eventual experience) of the level
of benefit for them. With time and repeated use of a specific piece of
technology (eg, a particular herbicide, or seed), the effectiveness of
the seed, herbicide etc declines, reducing the level of benefit derived.
Eventually the technology is replaced, itself by newer technology (eg, a
new seed containing a different GM herbicide tolerant trait, or a new
herbicide that may have broad spectrum applications like glyphosate, or
targets the weeds that glyphosate is less effective against) and the
cycle of adoption/rejection of technology continues.
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Executive Summary
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