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Glyphosate Fact Sheet
Glyphosate Fact Sheet
This fact sheet describes the basic properties of
glyphosate and the issues surrounding glyphosate resistance and weed control.
Glyphosate is a broad spectrum herbicide used to kill crop weeds. Monsanto’s
trade name for this is Roundup. Roundup Ready crops are engineered to withstand
exposure to glyphosate. This allows applications of the herbicide after crop
emergence, killing weeds but not Roundup resistant crop plants such as RRS
(Roundup Ready Soybeans).
Description
Chemically, glyphosate is an organophosphate like many
other pesticides but it does not affect the nervous system as other
organophosphates do. It is a broad spectrum, non-selective herbicide which kills
all plants, including grasses, broad leaf and woody plants. It is absorbed
mainly through the leaves and is transported around the whole plant, killing all
parts of it. It acts by inhibiting a biochemical pathway, the shikimic acid
pathway. At low levels of application it acts as a growth regulator.
There are three forms of glyphosate used as weed killers;
glyphosate-isopropylammonium and glyphosate-sesquiodium patented by Monsanto and
glyphosate-trimesium, patented by ICI (now Zeneca). Other common brand names are
Rodeo, Accord and Vision.
Glyphosate is technically extremely difficult to measure in
environmental samples. Only a few laboratories have the sophisticated equipment
and techniques necessary. This means that data is often lacking on residue
levels in food and the environment and existing data may not be reliable.
Use In Weed Control
Glyphosate product sales are worth $1,200 million a year.
In the US, glyphosate was used on about 12-25 million acres annually in the
1980s. In the UK it was used on almost 800,000 acres in 1994. Because it is
broad spectrum in action it is used to control a great variety of annual,
biennial, and perennial grasses, sedges, broad leafed weeds and woody shrubs. It
is used in fruit orchards, vineyards, conifer plantations and many plantation
crops (e.g. coffee, tea, bananas); in pre-crop, post-weed emergence in a wide
range of crops (including soybean, cereals, vegetables and cotton); on non-crop
areas (e.g. road shoulders and rights of way); in cereal stubble; forestry;
gardening and horticulture. Other uses of salts of glyphosate are in growth
regulation in peanuts and in sugarcane to regulate growth and speed fruit
ripening.
Human Toxicity
Because the shikimic acid pathway does not exist in
animals, the acute toxicity of glyphosate is very low. Glyphosate can interfere
with some enzyme functions in animals but symptoms of poisoning are only seen at
very high doses. However, products containing glyphosate also contain other
compounds which can be toxic. In particular most contain surfactants known as
polyoxyethyleneamines (POEA). Some of these are much more toxic than glyphosate.
These account for problems associated with worker exposure. They are serious
irritants of the respiratory tract, eyes and skin and are contaminated with
dioxane (not dioxin) which is a suspected carcinogen. Some are toxic to fish.
In California, glyphosate is the third most
commonly-reported cause of pesticide related illness among agricultural workers.
Glyphosate is the most frequent cause of complaints to the UK’s Health and
Safety Executive’s Pesticides Incident Appraisal Panel. New formulations, with
less irritating surfactants, have been developed by Monsanto (e.g. Roundup
Biactive), but cheaper, older preparations are still available.
Environmental Toxicity
Glyphosate is one of the most toxic herbicides, with many
species of wild plants being damaged or killed by applications of less than 10
micrograms per plant. Glyphosate can be more damaging to wild flora than many
other herbicides, as aerial spraying with glyphosate can give average drifts of
1200 to 2500 feet and ground spraying with glyphosate may cause damage to
sensitive plants up to 300 feet from the field sprayed. Glyphosate use is
thought to affect hedgerow trees, causing die-back, and may reduce trees' winter
hardiness and resistance to fungal disease
The direct toxicity of glyphosate to mammals and birds is
low. However, its effect on flora can have a damaging effect on mammals and
birds through habitat destruction. The US EPA concluded that many endangered
species of plants, as well as the Houston toad, may be at risk from glyphosate
use.
Fish and invertebrates are more sensitive to formulations
of glyphosate. As with humans, the surfactants are responsible for much of the
harm . Toxicity is increased with higher water temperatures, and pH. In
Australia, guidelines state that most formulations of glyphosate should not be
used in or near water because of their toxic effects on tadpoles and adult
frogs. The newer, non-irritant formulations such as Roundup Biactive are not
included in this advice.
Of nine herbicides tested for their toxicity to soil
microorganisms, glyphosate was found to be the second most toxic to a range of
bacteria, fungi, actinomycetes and yeasts. However, when glyphosate comes into
contact with the soil it rapidly binds to soil particles and is inactivated.
Unbound glyphosate is degraded by bacteria. Low activity because of binding to
soil particles suggests that glyphosate's effects on soil flora will be limited.
However, some recent work shows that glyphosate can be readily released from
certain types of soil particles, and therefore may leach into water or be taken
up by plants.
Impact Of Genetically Engineered Herbicide Resistant
Crops
The introduction of crops engineered to be resistant to
glyphosate could have two particularly damaging effects. Firstly, it will
increase the use of the herbicide, and secondly, it may encourage the emergence
of herbicide resistant weeds.
Monsanto claim that the introduction of herbicide resistant
crops will reduce the overall amount of herbicide used. They argue that
glyphosate will replace other, more environmentally damaging herbicides, because
only glyphosate need be used rather than several different compounds. They also
argue that weed killer will be used less frequently on resistant crops.
Importantly they also consider glyphosate to be 'environmentally friendly' and a
'safe' herbicide, basing this claim on its reduced soil particle binding and low
toxicity to humans.
Other herbicides used on soybeans and other crops are
unquestionably harmful to the environment and human health. The question is
whether glyphosate is really any less harmful and whether herbicide resistant
plants will reduce the amount of potentially damaging chemical to the
environment. Evaluating overall amount of use on a weight or volume basis does
not allow for the differences in toxicity between chemicals. Weight or volume of
total herbicide may decrease simply because glyphosate is more effective at
killing plants than many other chemicals. Glyphosate is already the eleventh
most widely used pesticide in the US on a volume basis. Its damaging impacts on
the environment have already been described.
Whether there will be a reduction in the number of times
herbicide is used is also questionable. In their documents prepared for the US
authorities, Monsanto say that under current regimes, between one and five
applications of different herbicides or herbicide mixtures are needed to control
weeds in soybean crops, while with Roundup Ready soybeans only "one or
possibly two" applications of Roundup will be needed. Yet in their
information for farmers in Argentina, Monsanto recommends Roundup be used with
Roundup Ready soybeans before sowing, when the young plant has three to four
leaves and then whenever the farmers find weeds. This is "at least twice
and probably more frequently".
Herbicide Resistance In Weeds
One of the major concerns of weed scientists is that the
emergence of herbicide resistant weeds may be encouraged by the use of herbicide
resistant plants. Herbicide resistance arises in an analogous fashion to the
emergence of antibiotic resistance in bacteria. Mutations occur in plants and
when one arises which makes it resistant to the herbicide, it will have an
advantage and grow and flourish when other plants are killed Resistance to
glyphosate is easy to induce in plants in the laboratory. Monsanto claims
resistance to glyphosate is unlikely to emerge in the field because it does not
persist in soil. However, weed resistance to paraquat, an herbicide which has a
shorter soil persistence than glyphosate, is already a serious problem. One weed
specialist concluded, by comparison to paraquat, that "Presumably
glyphosate resistance can also be obtained with multi-annual treatments" (Gressel,
in Cassley et al, 1991). Roundup Ready soybeans are intended to be used with
"multi-annual treatments" and so the emergence of resistance will be
encouraged. Even before the increased use of glyphosate expected with the
introduction of resistant crops, there has already been a report of glyphosate
resistance in a weed which occurred in ryegrass in Australia.
Glyphosate resistant weeds could also arise if there is
gene flow between the soybean and a related wild plant or if the soybean
survives to emerge as a weed ("a volunteer") in the subsequent crop.
Gene flow is possible in the Far East where soybean originated and wild related
plants exist. Herbicide resistant volunteers may be a problem where mild
climates occur and overwintering of soybean is possible.
Herbicide resistant crops are an expensive problem for
farmers. Having weeds resistant to another herbicide, triazine, have been
estimated to cost farmers up to $10 an acre in extra weed control expenditure.
There would be an extra penalty for farmers growing glyphosate resistant crops
if glyphosate resistant weeds evolved, because not only would they have to
change their weed control practices but they would have paid a premium for the
herbicide resistant seed in the first place.
Thus herbicide resistant soybeans promise increased
herbicide use and associated damage to the environment, together with an
increased risk of weed resistance, which would be a costly problem for farmers.
Sources:
Active Ingredient Fact Sheet: Glyphosate. Pesticide News 33
pp28-29, September 1996.
Breeze, V, Thomas, G & Butler, R (1992) Use of a model
and toxicity data to predict risks to some wild plant species from drift of four
herbicides. Annals of Allied Biology 121: 669-677
Carlisle S.M. & Trevors, J.T. (1988) Glyphosate in the
environment. Water Soil and Air Pollution. 39: 409-420
Casley J C., Cussans G W & Atkin R K (eds) (1991)
Herbicide resistance in weeds and crops. Oxford: Butterworth-Heinmann
Marrs, R H, Williams, C T, Frost, A J & Plant, R A
(1989) Assessment of the effects of herbicide spray drift on a range of plants
of conservation interest. Environmental Pollution 59: 71- 86
New Scientist, 6 July 1996, p6
Petition for determination of nonregulated status of
soybeans with a Roundup Ready gene. Agricultural Group of Monsanto to APHIS,
USDA, 1993.
US-EPA RED Facts: Glyphosate, September 1993
Yates W E., Akesson N B & Bayer D E (1978) Drift of
glyphosate sprays applied with aerial and ground equipment. Weed Science 26 (6):
597-604
GREENPEACE, April 1997
1436 U St. NW, Washington DC 20009
This report © Greenpeace 1997
Original Article: http://www.greenpeaceusa.org/media/factsheets/glyphosatetext.htm
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