Effective long-term control of pasture weeds can be achieved by an integrated approach incorporating a range of management tools.
These tools must be directed at addressing the objectives of successful weed management:
- Prevent weeds coming onto the property in bought-in feed or machinery.
- Removing the weed or reducing the weed seed set.
- Restricting weed germination.
- Encouraging competition from desirable species.
The 3Ds of weed management
1. Deliberation: Where am I and where do I want to be?
Producers must understand and assess their pasture composition of desirable species and weeds. Pasture assessment is the starting point in determining the extent of the weed problem and whether there are sufficient desirable species to 'encourage' through targeted weed management.
- Categorise and describe plant species, using tools such as MLA's Pasture Health Kit, according to the following botanical features:
- Weed vs desirable species - indicating whether pasture renovation, restoration or replacement is a better option.
- Annual vs biennial vs perennial.
- Grass vs broadleaf weed vs legume - to describe the potential of the paddock for forage production levels and suitability for different livestock classes.
- After determining what is present it is important producers understand the seed production and germination periods of major weeds and pasture plants, including:
- Main period for germination and flowering/ seed production.
- Effective growing period.
- Important differentiating features or unique qualities, such as known weak links or strong points in life cycle, when the the weed most vulnerable, establishment or reproduction periods.
- Declared weed status - is it a noxious or notifiable weed?
- Determine the longevity of the weed seed - how long the weed seed has been in the soil bank and how long it may last, will influence the control program.
- Determining the level and type of weed infestation will indicate:
- Whether control is warranted.
- Priority paddocks for weed control.
- Potential control and management tactics.
Producers should establish a pasture composition target for each paddock or pasture type eg 'in five years have 65% of the pasture species as perennial grasses'. This should also consider weed type and prioritise weeds and paddocks for control. This will help identify whether a complete renovation is necessary or whether it is possible to improve the pasture through the integrated use of strategies that encourage desirable species and remove weeds.
2. Diversity: What tools do I need?
There are a range of cost-effective 'restoration' tools available to achieve the objectives of successful weed management. These are classified as either 'pasture improvers' which encourage desirable and competitive species, or 'weed removers' which remove weeds from pastures and/or reduce weed seed set.
Effective lasting control will only be achieved through developing an integrated, well planned approach using a combination of these. Managing livestock and grazing pressure and ensuring the presence of competitive species, are fundamental to all programs.
- Fertiliser: Implement a well-planned fertiliser program to increases soil fertility and encourage 'fertiliser responsive' perennial species to dominate.
- Grazing management and rest: Manage grazing to maintain and encourage the competitive ability of desirable species and help to reduce or eliminate weeds.
- Grazing palatable weeds: Vary grazing frequency and density to encourage livestock to consume non-toxic vegetative weeds.
- Cropping phase: Consider a short-term cropping phase of severely depleted pasture to assist in weed removal and increase cash flow.
- Pasture establishment: Assess pasture composition to determine whether pasture is sufficiently degraded to warrant total replacement.
- Pest management: Manage invertebrate and insect pests in order to maintain the health of perennial grasses and legumes.
- Herbicides: Use herbicides to selectively control undesirable weeds, taking care to avoid damage to non-target species.
- Spray grazing: Consider applying sub-lethal rates of the herbicides MCPA, 2,4-D amine and 2,4-DB amine to make broadleaf weeds more palatable so heavy grazing pressure can be used to control these weeds in pastures.
- Biological control: Introduce natural control agents, usually from the weed's country of origin.
- Tactical grazing pressure: Use high-density, strategic grazing pressure for a short period to reduce seed production capacity and weaken the plant.
- Slashing: Cut the pasture in spring before flowering to reduce seed production.
- Reduce weed germination-groundcover retention: Maintain greater than 1,500kg/ha of dry matter (including litter) and at least 80% groundcover at the autumn break in order to reduce weed seedling germination and establishment.
All strategies must be timed to the weed's and desirable species' life cycles. Weed removers and pasture improvers should integrate and complement one another and must be employed at the optimal time to achieve desired outcomes.
A focus on grazing management, together with fertiliser and tactical herbicide usage, should be the starting point for a control program. In most cases, all three areas should be used tactically and concurrently to disadvantage the weed while encouraging the desirable species.
3. Diligence - How do I stay there?
Producers must continue to monitor paddocks annually to assess botanical composition and detect changes in the pasture.
Establishing a 'calendar of operations' in which the lifecycle of each plant is described and management tools identified and superimposed, helps predict 'cause and effect'. Producers should then be able to identify reasons for changes in pasture composition and take early steps to address these.
Just completing an activity, such as spraying each year, is not sufficient to control many weed species where there is a viable weed seed population. The goal of an annual control program must be to achieve a complete cessation of seeding in a paddock.
- MLA publications providing information on weed control:
- 3D weed management:
- Paterson's curse case study and best practice manual
- African love grass case study and best practice manual
- Thistles case study and best practice manual
- Silverleaf nightshades case study and best practice manual
- Chilean needle grass case study and best practice manual
- Serrated tussock case study and best practice manual
- EH Graham Centre for Agricultural Innovation's: Silverleaf Nightshade Best Practice Management Guide
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