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44. What is a herbicide?
Herbicides are substances used to kill plants, especially weeds. There are many different herbicides. They can be broad spectrum or selective; short-acting or residual; applied before a crop is planted or when a crop is starting to grow. Some work through the system of the plants; others through its leaves; some kill the plant on contact, others move around within the plant.
45. Why are herbicides necessary?
Farmers use herbicides to control weeds that compete with their crops for soil, moisture and sunlight reducing yield and productivity. Weeds cost the Australian grains industry around $1.3 billion per annum in control costs, lost production and contamination.
Without herbicides weeds would need to be removed manually. Disrupting the soil to remove weeds reduces its organic matter content, increases moisture loss and exposes the land to wind and water erosion. In addition, manual removal is highly labour intensive.
46. What are herbicide tolerant crops?
Conventionally-bred herbicide tolerant canola varieties are already used throughout Australia. Herbicide tolerant crops can tolerate higher than normal doses of particular herbicides. In the case of the GM varieties in the pipeline, the herbicide resistance characteristic comes from bacteria.
There are a wide variety of herbicide tolerant crops under development throughout the world. They include canola, cotton, potatoes, corn, sugar beet, rice and lupins. The most dominant GM crop in the world is herbicide tolerant soybean. It comprises 51 percent of the total area of GM crops.
47. Do we have herbicide tolerant crops growing in Australia?
Conventionally-bred herbicide tolerant canola is grown in Australia, and two GM varieties have been approved by the OGTR and could be grown commercially in New South Wales and Victoria in 2008.
48. Why use herbicide tolerant crops?
Herbicide tolerant crops offer growers more options for weed control. They also have several other potential benefits, including:
- increased control of difficult weeds;
- reduced risk of damage to crops if herbicide is applied at the wrong dose;
- reduced use of herbicides because treatment can be delayed until weeds appear rather than being applied in anticipation of weeds developing;
- less mechanical cultivation required to remove weeds; and,
- increased farming flexibility, including paddock rotation, because shorter acting herbicides can be used instead of older, more persistent products.
49. Will herbicide tolerant crops lead to greater use of herbicides?
It is possible that herbicide tolerant crops may lead to greater herbicide use, however evidence to-date suggests otherwise. Also, the herbicides used on such crops to-date, are those which have the least environmental impact.
According to the results of a survey conducted of 650 canola growers in Canada by the Canola Council of Canada in 2000, “Overall, the impact of the adoption of transgenic canola production has contributed significantly to the reduction of chemical herbicide usage.”
Traditionally, growers have had to apply herbicides before planting in order to prevent any herbicide damage to their crop. The herbicide tolerant crops allow growers the flexibility to wait and see if weeds are going to threaten their crop before they use the chemicals.
50. Could herbicide resistance spread to wild plants and weeds?
This is one of the areas that Australia’s gene technology framework would consider in relation to environmental safety before approving a herbicide tolerant GM crop for field trials or commercial release.
Certain crops are capable of reproducing with wild plants and weeds, however there are many conditions that would need to be favourable for this to happen. Herbicide resistance genes could only transfer to related weeds species if the crop and the weed flower at the same time and the pollen can be carried by wind or bees between the plants. The two species must also be sexually compatible and capable of producing fertile hybrid offspring.
There are strict management guidelines in place for herbicide tolerant GM crops to minimise the chances of resistance developing during field trials. These guidelines are developed by the Regulator and include provisions such as buffer zones.
In the event that the herbicide tolerance characteristic was passed into a weedy relative, the weed would still be susceptible to a host of other herbicides which could control it.
It must be noted that two herbicide tolerant canolas, bred through conventional means, are already grown in Australia so these weed management issues are not new to farmers.
51. What is a ‘superweed’?
Some people are concerned ‘superweeds’ will develop if crops modified for herbicide tolerance cross-breed with their weedy relatives and pass on the herbicide resistant characteristic. The resulting plants, so-called ‘superweeds’, could be resistant to herbicide controls.
Both Australian and international research has concluded that canola can cross-breed with relatives such as Chinese mustard (Brassica juncea), wild turnip (Brassica rapa), and wild radish (Raphanus raphanistrum). However, the numbers of offspring of any such crosses are very low, with reduced fertility and fitness.
Wild radish is the most severe broadleaf crop weed in Australia. As a result, further research was undertaken into the likelihood of this weed crossbreeding with conventional herbicide tolerant canola. A three-year Australian study at the Cooperative Research Centre for Weed Management Systems found that the outcrossing rate from canola to wild radish is very low, one in 26 million.
In the event that outcrossing did occur, the environmental impact is considered to be minimal because the new gene only gives the weed resistance to the one herbicide that is applied to the herbicide tolerant crop. It would still be susceptible to other herbicides, and it would not have any competitive advantage in the wild.
52. Will herbicide resistant plants damage the environment?
Most herbicide resistant GM plants are resistant to glyphosate (commonly known as Roundup) or glufosinate (sold as Basta). These broad spectrum short-acting herbicides are deactivated in the soil and break down into harmless by-products with few residues.
If herbicide tolerant crops continue to be developed with resistance to safer herbicides, the environment may benefit from better soil management, fewer residues and reduced cultivation.
The commercial release of a GM crop requiring the use of a harmful herbicide is unlikely to be permitted.
Two conventionally-bred herbicide tolerant canolas are already used widely in Australia. One of these canolas is tolerant to the herbicide atrazine, which is soon to be phased in Europe because of the negative impact it is having on the environment. According to a Melbourne University report published in 2007, if the two GM canolas approved for commercial use in Australia (but not currently grown) replaced just 50 per cent of the triazine tolerant canola, 640 tonnes less triazine would be used each year. The triazine group of herbicides includes atrazine, being banned in the EU as mentioned already.
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