Chemistry Reference
In-Depth Information
Ecologically, such a large reduction of weed species represents a major change
in farmland ecosystems and may be expected to have knock-on effects on other
species. Certain problems have come to light with the investigation of the status of
birds on farmland. In one study, the Game Conservancy investigated the reasons for
a severe and continuing decline of the grey partridge (
Perdix perdix
) on farmland in
Britain. The study commenced in the late 1960s and established that the decline was
closely related to increased chick mortality (Potts 1986, Potts 2000; also Chapter 12
of Walker et al. 2006). The high chick mortality was largely explained by a short-
age of their insect food (e.g., sawflies) due, in turn, to the absence of the weeds upon
which the insects themselves feed. An effect at the bottom of the food chain led to a
population decline further up. It is worth reflecting that such an effect by herbicides
could not have been forecasted by normal risk assessment (see Chapters 14 and 15).
The herbicides responsible are in general of very low avian toxicity, and ordinary
risk assessment would have declared them perfectly safe to use so far as partridges
and other birds are concerned. Subsequent work has shown that partridge popula-
tions can continue to survive on agricultural land if headlands are left unsprayed,
thereby allowing weeds to survive, weeds that will support the insects on which
young partridges feed.
This study helped to ring the alarm bells about possible other indirect effects of
the wide use of herbicides in agriculture. More recently, further evidence has been
gained of the reduction in populations of insects and other arthropods on farmland
that may relate, at least in part, to the removal of weeds by the use of herbicides. A
study of farmland birds in Britain established the marked decline of several spe-
cies in addition to the grey partridge, which may be the consequence of the indi-
rect effects of herbicides and other pesticides (Crick et al. 1998, and Chapter 12
of Walker et al. 2006). Species affected include tree sparrow (
Passer montanus
),
turtle dove (
Streptopelia purpur
), spotted flycatcher (
Musciapa striata
), and skylark
(
Alauda arvensis
). A study is currently in progress to attempt to establish the cause
of these declines.
Recently, controversy about the possible side effects of herbicides used on agri-
cultural land has intensified with the development of genetically modified (GM)
crops. Some GM crops are relatively insensitive to the action of herbicides, thus
permitting the application to them of unusually high levels of certain herbicides. The
advantage of increasing the dose, from the agricultural point of view, is the control
of certain difficult weeds. From an ecotoxicological point of view, though, increas-
ing dose rates of herbicides above currently approved levels raises the possibility that
this may cause undesirable ecological side effects. It is very important that any such
change in practice is rigorously tested in field trials as part of environmental risk
assessment before approval for marketing is given by regulatory authorities. Such
new technology, based on GM crops, should only be introduced if it is shown to be
environmentally safe.
One problem that has arisen with the use of herbicides in agriculture is spray
or vapor drift. When fine spray droplets are released, especially if applied aerially,
they may be deposited beyond the target area due to air movements to cause damage
there. In the first place, this is a question of application technique. Herbicides, like
other pesticides, should not be applied as sprays under windy conditions. In most
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