Print This PageWeed Resistance
One result of modern agriculture and its reliance upon herbicides is the emergence of weed populations that are resistant to herbicides. All natural weed populations, regardless of the application of any herbicide, may contain individual plants (biotypes) that are resistant to herbicides.
Repeated use of any herbicide will expose weed populations to selection pressure that may lead to an increase in the number of surviving, resistant individuals in the population. As a grower continues to use a particular herbicide without any other herbicide modes of action, or without any other cultural practices, the resistant biotype continues to survive and produce seed. Subsequent populations of the resistant biotype will continue to increase until they are the dominant weed in the field.
Scientists have found that there are particular weed characteristics that can facilitate development of weed resistance. These include:
- Large amount of seeds produced per plant
- High levels of germination of those seeds
- Several weed flushes per season
- High frequency of resistant genes
Monsanto and university weed scientists have also identified specific common factors that are often present in areas where glyphosate resistance has developed:
- Limited or no crop rotation
- Limited or no tillage practices
- A high dependency on glyphosate alone or a limited use of other herbicides
- Reduced rates of glyphosate
Confusion about what is or is not herbicide resistance is common.
Herbicide Resistance: Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to a wild type. In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering or selection of variants produced by tissue culture or mutagenesis
Herbicide Tolerance: Herbicide tolerance is the inherent ability of a species to survive and reproduce after herbicide treatment. This implies that there was no selection or genetic manipulation to make the plant tolerant; it is naturally tolerant.
Weed resistance in context
Farmers have been dealing with the issue of herbicide resistant weeds since the 1950s and it is a reality that growers know how to manage. History demonstrates that growers manage through the occurrence of resistance and that the affected herbicide products continue to be valuable and important. Integrated weed management has been an important part of growing crops where herbicide resistant weeds are present. Managing cropping programs as an ongoing system and planning ahead has allowed farmers to use appropriate weed control methods to effectively manage problem weeds and to reduce the risk of further resistance occurring.
The presence of resistant weeds in a cropping system does mean that changes have to be made. These changes have to be effective in managing resistant weeds to allow the growing of successfully commercial crops.
Worldwide, there are many examples of herbicide resistance where the active ingredient is still used on a wide scale. For example, there are more than 60 different triazine resistant weeds, but atrazine is still used on more than half the corn hectares in the United States
The International Survey of Herbicide Resistant Weeds contains a list of weed species with biotypes confirmed to be resistant to a herbicide.
Weed resistance and glyphosate
Glyphosate resistance can occur, however it is rare and slow to develop in comparison to other herbicides.
Overview of the development of resistance to each of the major herbicide groups (amended for Australian herbicide groups from Dr Ian Heap, May 2006). Glyphosate (Group M) resistance is rare and has been slow to develop.
Annual ryegrass (Lolium rigidum) has developed glyphosate resistance in Australia as have localised populations of Awnless Barnyard Grass (Echinola colona) and Liverseed Grass (Urochloa panicoides). These resistant populations have been found in dryland cropping systems that have relied on continuous glyphosate applications as the only method of weed control.
To date, no glyphosate resistant weeds have been detected in any Australian cotton cropping systems.
Monsanto recommends the following general guidelines for minimising the risk of weed resistance:
- Scout fields before and after herbicide applications
- Start with a clean field
- Control weeds early when they are relatively small
- Use the RIGHT TOOL, at the RIGHT RATE at the RIGHT TIME
- Add other herbicides and cultural practices where appropriate
- Control weed escapes and prevent weeds from setting seeds
Clean equipment before moving from paddock / field to minimise the spread of weed seed
Monsanto’s educational weed resistance management website for farmers provides comprehensive and timely information on weed control strategies for Roundup Ready cropping systems.
