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
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.
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
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.
Monsanto recommends the following general guidelines for minimising the risk of weed resistance:
1. ACT NOW TO STOP WEED SEED SET
Research and plan your WeedSmart strategy
Understand the biology of your weeds
Be strategic and committed
2. CAPTURE WEED SEEDS AT HARVEST
Consider your options – chaff cart, narrow windrow burning, baling, Harrington Seed Destructor
Compare the financial cost per hectare
3. ROTATE CROPS AND HERBICIDE MODES OF ACTION
4. TEST FOR RESISTANCE TO ESTABLISH A CLEAR PICTURE OF PADDOCK-BY-PADDOCK FARM STATUS
5. NEVER CUT THE RATE
6. DON’T AUTOMATICALLY REACH FOR GLYPHOSATE
Diversity, diversity, diversity
Consider post-emergent herbicides where suitable
Consider strategic tillage
7. CAREFULLY MANAGE SPRAY EVENTS
8. PLANT CLEAN SEED INTO CLEAN PADDOCKS WITH CLEAN BORDERS
Plant weed-free crop seed
The density, diversity and fecundity of weeds is generally greatest along paddock borders and areas such as roadsides, channel banks and fencelines
9. USE THE DOUBLE KNOCK TECHNIQUE
10. EMPLOY CROP COMPETITIVENESS TO COMBAT WEEDS
Increase your crop’s competitiveness to win the war against weeds
Row spacing, seeding rate and crop orientation can all be tactics to help crops fight