Herbicide-resistant crops will not provide a magic solution to weed control; they need to be used as part of an integrated weed management program. These were the strong messages to emerge from a workshop in Canberra earlier this year, where researchers and environmentalists gathered to discuss the role of such crops and pastures in Australian agriculture.
Update on breeding
Herbicide-resistant crops bred by traditional plant breeding methods are already being grown in Australia. For example, a spring Canola, Siren, resistant to the herbicide triazine was released in 1994. It allows Canola to be grown in areas where major weed problems occur. However, Siren produces about 25 per cent lower seed yields than conventional Canola varieties.
Genetic engineering offers the potential to add herbicide tolerance without sacrificing the other agronomic and disease resistance attributes already incorporated in the cultivar.
The Genetic Manipulation Advisory Committee (GMAC) has considered five proposals for transgenic plants containing herbicide resistance. These include glyphosate-tolerant cotton, chlorsulfuron-tolerant roses and glufosinate-tolerant lupins and subterranean clover. Except in cotton, the genes were not inserted primarily for their herbicide-resistant traits. They serve as markers that another desirable gene has been transferred.
Genetic engineering experiments often include herbicide resistance or antibiotic resistance genes to test whether plants have been transformed. The transgenic plants are grown on a medium containing the herbicide or antibiotic; growth in that medium indicates that the new plants contain the transferred resistance (and hence the desired gene).
In the case of transgenic sub clover, TJ Higgins of CSIRO's Division of Plant Industry added a sunflower gene to sub clover to make it more nutritious for livestock. In the transgenic plant he included a marker gene, which also makes the plant resistant to the broadspectrum herbicide glufosinate.
Glufosinate-resistant lupins could provide a valuable tool for farmers in Western Australia where ryegrass, resistant to selective grass-killing herbicides, is a major problem that prohibits the option of early planting of wheat after lupins.
Plant breeder at the Cooperative Research Centre for Legumes in Mediterranean Agriculture (CLIMA), John Hamblin, has started a backcrossing program to incorporate the resistance gene into advanced lines of lupins in breeding programs in Western Australia and New South Wales. He will start preliminary glasshouse trials on the transgenic lines of the most recently released variety, Merrit, this year. Field trials at three sites on Department of Agriculture field stations will begin in 1996.
Resistant crops: the pros and cons
Proponents of growing herbicideresistant crops argue that with careful use and management these plants could provide new options for weed control and promote the more efficient use of low-dose, low-toxicity herbicides. If farmers had crops that were resistant to a broad-spectrum post-emergence herbicide, for example, they would not need to use pre-emergent herbicides.
Opponents of the technology are concerned that making herbicidetolerant plants available will encourage farmers to use more herbicides. Both groups agree that more research into integrated weed management for both crops and pastures is needed so that farmers do have other options available to control weeds.
Some scientists believe that the selective pressure on weeds caused by introducing herbicide-resistant crops, and thus increasing reliance on a single chemical, will inevitably lead to weeds becoming resistant to some of the most valuable herbicides.
Glyphosate, the active component of Roundup™, is a non-selective contact herbicide commonly used for postemergent weed control. It has been used extensively for more than 20 years. There is no known resistance to it and Monsanto researchers argue that the difficulty of conferring glyphosate tolerance in-the laboratory indicates that none is likely to develop.
However, scientists such as John Huppatz, Assistant Chief of the CSIRO Division of Plant Industry, believe the risk, however small, of losing such a valuable herbicide because of resistance problems must be weighed carefully against the potential benefits of glyphosate-resistant crops.
Dr Huppatz told workshop participants that more than 80 per cent of the genetically engineered crops in field trials so far carry resistance to either glyphosate or glufosinate. He expressed concern that the lack of broad-spectrum herbicides that can be used to exploit this technology could prove a significant limitation to its widespread adoption.
interaction with no-till
Steve Duke of the United States Department of Agriculture told the workshop that crops made resistant to good, non-selective, post-emergent herbicides could help efforts to reduce or eliminate tillage. Farmers need only apply herbicides to treat weed problems, rather than using soil-incorporated or soil-applied pre-emergent treatments.
Using only foliar-applied herbicides in no-tillage agriculture could also have the benefit of reducing herbicide movement to ground- and surface waters. This is because of the generally lower doses needed, the higher microbial activity in no-till soil and reduced water runoff from no-till paddocks.
However, in some cases the potential for plants to become volunteer weeds could compromise minimum tillage. Dick Medd of NSW Agriculture raised the hypothetical case of glyphosatetolerant wheat becoming a weed that would be expensive to control with herbicides and might have to be controlled by cultivation. Dr Medd also pointed out that a number of important weeds have some natural tolerance to broad-spectrum herbicides such as glyphosate.
Any widespread use of glyphosateresistant crops could change the weed spectrum so that tolerant weeds gained the ascendancy and any advantages of the herbicide-resistant crop would be lost.
The potential for herbicide-resistant genes to move into wild plant species is seen to be a real risk where the transgenic crops have a high degree of outcrossing, particularly those with a large number of weedy relatives.
For chemical companies such as Monsanto the stakes are high. Over the past 10 years, the company has invested $1 billion in biotechnology, including the development of herbicide tolerance in crops such as Canola, soybeans, cotton and sugar beet.
Bob Phelps, coordinator of the Australian Gen-Ethics Foundation, is concerned that the herbicide tolerance option maintains farmers' dependence on chemicals. It offers chemical companies continued control of markets when patents on herbicides run out. By developing and patenting glyphosatetolerant plants, for example, Monsanto would be able to maintain a monopoly with seed/herbicide packages and lock other companies into selling Roundup™ after the product patent expires.
Dr Duke said farmers growing minor crops for which few herbicides are available would find herbicide-resistant crops most useful. But for economic reasons chemical companies will probably be interested only in developing the technology to cover a few herbicides in a few major crops. Selective cultivation is one alternative.
This is an edited version of an article appearing in the CSIRO publication Rural Research, Winter 1995.
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