Ensure no survivors: stop weeds from setting seed!
Integrated weed management (IWM) is a strategy to manage existing herbicide resistance and prolong the useful life of herbicide groups. An IWM strategy will also reduce the rate of species shift, manage the cost of future weed control by depleting the number of weed seeds in the soil, and improve crop productivity through effective weed management.
What do you need to know?
Herbicide resistance is normally present at low frequencies in weed populations before a herbicide is first applied. Using any herbicide creates a selection pressure that increases the likelihood that resistant individuals will survive, set seed and therefore pass on their resistance to offspring.
The underlying frequency of resistant individuals within a population will vary greatly with the particular weed species and also the mode of action of the herbicide used. Mix herbicides at full label rates and rotate between modes of action to delay herbicide resistance developing.
Early in the development of a resistant population, resistance commonly occurs in isolated patches. This is the critical time to identify the problem (however many of the possible symptoms of herbicide resistance may also be due to other causes of spray failure). Patch management is an important component of farm biosecurity, it reduces new incursions and prevents distribution of hard to control weeds.
Resistance has been confirmed in 48 weeds in Australia. In cotton growing areas six common grass weeds are known to be resistant to glyphosate - awnless barnyard grass, liverseed grass, sweet summer grass, windmill grass, feathertop Rhodes grass, and annual ryegrass. In addition, two broadleaf species - flaxleaf fleabane and sowthistle - have developed resistance to glyphosate.
Correct weed identification:
To be able to effectively manage resistance and to decide on an appropriate response weeds need to be correctly identified. Herbicide susceptibility can differ between similar weed species hence similar species may respond differently to the herbicide mode of action used.
Use the Weeds of Australian Cotton app to identify weeds during early growth stages.
Good record keeping helps to mitigate problems when they occur and enables growers to determine the effectiveness of their management strategies as well as helping to understand why a problem developed. Records should be kept for all fields, and include cropping history, GPS locations for weed patches, weed control tactics used and their effectiveness after every operation.
Timely intervention of a weed control operation can be the most significant factor in its effectiveness. Herbicides are more effective on rapidly growing small weeds, and may be quite ineffective in controlling large or stressed weeds. An understanding of the critical period for weed control is needed to balance the cost of control with the optimum timing for spray efficacy. (For more, see this chapter of WEEDpak).
Herbicides are classified into groups based on their mode of action. The Australian classification system will be replaced with the international numbering system commencing with a staged rollout from July 2021. Growers will see new numbering on herbicide labels from 2022. This website allows cross referencing of the two systems (an app is also available).
Herbicides groups should be rotated whenever possible to avoid using the same group on consecutive generations of weeds.
Decisions such as cotton planting time, pre-irrigation versus watering-up, methods of fertiliser application, management of rotation crops, stubble retention, pupae busting, dessication timing and in crop irrigation management all have an impact on weed emergence and growth.
Agronomy decisions should be considered as part of the weed management program. For example, modify the timing and method of applying pre-plant N to achieve a ‘spring tickle’ (shallow cultivation in combination with a non-selective, knockdown herbicide) in the same operation.
So, what should you do on your farm?
- Apply herbicides according to label directions and the relevant state Pesticides Act.
- Consider the role of pre-plant residuals to reduce initial weed populations prior to the first glyphosate application. Use late in-crop residual herbicides prior to canopy closure to control late germinating weeds.
- Identify key weeds and assess the weed burden annually. Target strategies to manage problem weeds.
Where possible use 2 non glyphosate weed control tactics in fallow and 2 non glyphosate tactics in crop to target no survivors; these may be other modes of action or timely cultivation. Remember, 2 + 2 & 0 survivors.
- When growing winter crops as part of the cropping rotation use harvest weed seed control tactics such as chaff lining, narrow windrows or seed destructor mills.
- Practice the WEEDsmart summer cropping BIG 6.
- Practice good farm hygiene to prevent the entry of new weeds to the farm, around the cropping area, in fallow and rotation fields, irrigation structures, waste areas, and between fields. Come Clean, Go Clean.
- Scout fields regularly to assess weed pressure and the efficacy of control measures.
- Select herbicides with consideration for rotating their modes of action, residues and re-cropping intervals.
- Identify key weeds at risk of glyphosate resistance through use of a risk assessment tool and adjust management practices to minimise risk.
- Control weeds which survive a herbicide application using an alternative mode of action or management tool before they set seed.
- Control volunteer and ratoon cotton plants in field and non-field areas.
- Keep records of weed management inputs for each field, including alternative management tools (such as cultivation or chipping).
- Use weed control thresholds to determine the timing of in-crop applications.
- Apply all herbicides at the ideal weed and crop growth stages.
- Use precision spray technology to optimise control while reducing chemical inputs.
- Avoid temperature inversions when applying herbicides, especially Group I chemistry such as 2,4-D.
Where should I go for more information?
Eric Koetz – Technical Lead Weed Management
Ph: 0413 256 132
- Herbicide Resistance Management Strategy
- Cotton Pest Management Guide
- Australian Cotton Production Manual
- CottonInfo fact sheet: Come Clean Go Clean
- CottonInfo case study: Managing herbicide resistant barnyard grass
- CottonInfo grower case studies: Controlling your weeds
- CottonInfo/CRDC/GRDC GrowNote: Reducing herbicide damage
- About key weeds: