Integrated Weed Management
Rod Ensbey, Regional Weed Control Coordinator, NSW Agriculture,
Grafton
Integrated weed control is the coordinated use of a range
of suitable chemical and non-chemical control methods. The
aim is to incorporate a variety of control methods which are
cost-effective and practical and which reduce reliance on
herbicides. Successful implementation of integrated weed management
programs requires long-term planning; knowledge of the weed's
biology and life cycle; and the appropriate weed control methods.
Many weed infestations can be controlled or eradicated using
integrated methods. For example, the treatment of large infestations
of lantana with herbicides is not economically feasible. Lantana,
though, can be controlled by using a combination of fire,
pasture improvement, herbicides, mechanical methods and grazing
management.
A suggested control program could be: remove stock for several
months; burn at the appropriate time; sow an improved pasture
in early summer; continue to exclude stock until pasture establishes;
and follow-up with herbicide spot-spraying on regrowth. This
regime may need repeating for 2-3 years. Depending on the
terrain and access, burning could be substituted with bulldozing
or slashing to reduce the bulk of the mature plants and then
followed up with spot-spraying. A similar integrated program
could also be used for blackberry control.
Bitou bush is another invasive environmental weed which infests
up to 75 % of the New South Wales' coastline. A long-term
integrated control program is being implemented to combat
this weed threat. This involves a combination of manual, biological
and chemical means to control this problem.
In many situations, weed control becomes more cost-effective
and practical when methods are integrated ― combining
herbicide and non-herbicide controls. However, a knowledge
of the weed; the appropriate control methods; the timing of
each control; and planning; is essential for success.
The various control methods used in integrated weed management
are discussed below.
Biological Control
Weed biological control involves the use of the plant's natural
enemies such as insects, mites and diseases to control weed
populations. It is an economical, effective and environmentally-sound
method of weed control.
However, biocontrol is a long-term technique with extensive
development and establishment phases. Biocontrol also will
not eradicate a weed but, if successful, reduces it to an
acceptable level where it can be controlled by other means.
There are two main types of weed biological control, Inundative
and Classical.
Inundative Control. Inundation is the use of mycoherbicides
to control single weed species that escape mechanical or chemical
control.
Mycoherbicides are plant pathogens such as rusts and fungi
applied to control a specific weed. They can be likened to
a species-specific natural herbicide, but are not self-sustaining
and have a short active period.
In Australia, researchers are investigating the potential
of mycoherbicides for controlling Noogoora burrs and Bitou
bush.
Classical Control. Classical biological control is
the release of control agents such as insects, rusts and mites
into a region to permanently suppress selected target weeds.
The aim is to establish a natural balance between the weed
and its control agent - similar to the balance found in the
native range of the weed.
Classical biological control alone does not eradicate a
weed species. If the agent successfully establishes however,
control becomes self-perpetuating and self-regulating as the
control agents become a permanent part of the region's ecology.
Biological control can also be an attractive option when
the release control agent has established in other areas and
has shown good results. Successful programs however may take
more than 10 years to be effective and results may vary from
area to area.
Biological control may be more practical and effective
in areas such as:
- inaccessible areas such as timbered, rocky and steep locations;
- low-priority areas for control;
- situations where biocontrol is the only option, for instance
salvinia in sensitive aquatic areas;
- where chemical control may be too expensive or not effective.
Many weeds in Australia have been targeted for biological
control. Some programs have been extremely successful; other
programs have varied from partial to completely unsuccessful.
The most spectacular success involved the control of the
common pest, prickly pear during the late 1920s. The Cactoblastis
moth was introduced in 1926, but it wasn't until six years
later that the moth made its full impact.
The result was that millions of hectares of land was freed
from prickly pear and again became viable for agriculture.
The Cactoblastis moth hasn't totally eradicated prickly
pear, with isolated areas still remaining, normally in particularly
cold and wet locations.
Other successful agents include the Cyrtobagous salviniae
weevil introduced for the control of the aquatic weed, salvinia.
This agent is particularly successful in Queensland and
northern New South Wales, becoming less reliable as the climate
becomes cooler.
There have been numerous examples of insects which have
been released and either have failed to establish, or have
established but had little impact on the weed. Lantana has
had 25 agents trialed and released for its control. Some have
been partially successful, with the majority having little
impact.
In a worldwide review of biological control of weeds, it
was calculated that while 63 % of agents released became established,
only 24 % of releases have been considered effective in controlling
their weed host.
Current programs underway in New South Wales are in the
early stages of implementation. Success, if it is to occur,
is a long way down the track. Most of today's biological control
programs will benefit the next generation of land managers.
Flame Cultivation
Flame cultivation, or flame weeding as it is also known,
has been recognised for a number of years but has never developed
into a legitimate weed control method in Australia. Currently
there are a number of trials underway assessing this form
of weed control in crop and non-crop situations.
In Sweden, flame weeding has been used for many decades,
particularly in organic farming situations for pre-emergence
control in carrots and other slow-germinating row crops. Flaming
has also been used in Sweden for selective post-emergent control
in heat tolerant-crops and for general weed control on hard
surfaces in urban areas.
Liquefied petroleum gas or propane is the fuel most commonly
used in flame weeders. The efficacy of flame weeding is attributed
to a direct effect of the flame on the plant's cell membrane
and an indirect effect during subsequent desiccation.
The weed flaming process does not require the weed to be
burnt - it can also raise moisture temperatures to above 100°,
where the moisture turns to steam and ruptures the plants'
cells.
Small dicotyledons are generally more susceptible to flaming
than large ones. Species with upright habit and thin leaves
are also more sensitive than species with a low stature and
protected growth points. Theoretically then, upright-growing
species such as Parramatta grass and setaria should be more
susceptible than lower, prostrate-growing species such as
couch and kikuyu.
Until flame weeding is full-evaluated; trial work completed;
and suitable equipment has been developed; this control method
is not a viable option.
However, within the next few years, flame weeding may become
a viable weed control option, particularly for organic farming
and other environmentally sensitive areas.
Goats
The ability of goats to control weeds in Australia has been
well documented. Goats have been used for sustainable pasture
management and weed control for a range of weed situations.
They can be integrated with sheep, cattle and cropping enterprises
to provide weed control and pasture improvement. In most situations,
goats should be seen as only one aspect of an integrated weed
control program, which can also include burning, mechanical
removal, spraying and pasture improvement.
Goats control weeds by preferentially grazing them, thereby
placing the weeds at a disadvantage by preventing the weeds
from flowering and by ring-barking and structurally-weakening
some shrub species. Goats eat a variety of undesirable plants
and shrubs that sheep and cattle avoid, and quite often the
nutritional value of these species is quite high. They are
efficient browsers and graziers of weeds in steep, rocky areas,
around trees and other inaccessible areas where conventional
control methods are not applicable.
The use of goats for weed control is a medium to long-term
proposition and, therefore, expectations should be realistic.
In some situations, goats can give effective control of a
weed. In other cases, they may only limit the spread or have
very little effect on the weed at all.
For goats to be effective: stocking rates; timing; weed
palatability and farm management strategies, need to be considered.
In most cases, it is also important to have a competitive-based
pasture to overcome the weed and colonise bare areas.
There are many weed species that are eaten by goats with
the degree of control depending on the palatability of the
weed.
Highly-palatable weeds include: blackberry, sweet
briar, scotch broom
Palatable weeds include: scotch thistles, variegated
and nodding thistles, Paterson's curse, lantana, horehound.
Other species that are moderately palatable and eaten occasionally
include fireweed, groundsel bush, St John's wort, serrated
tussock, and spear grass.
Hot Water Application
Hot water application or steaming is a relative new weed
control method. Applying hot water to a weed results in the
loss of the plant's waxy coating, a reduction in moisture
and dehydration.
The system operates by plumbing water under pressure through
a heated chamber onto the weeds. The combination of heat,
pressure, and water volume breaks down cellular structure
causing discolouration and death within hours or a few days.
One treatment can kill most annuals and some young perennials.
The top growth of older perennials are scorched off, but the
impact on the roots is minimal unless treatment is repeated
frequently.
This form of weed control is still in the developmental
stage. A number of large city councils have trialed the equipment,
reporting mixed results on its effectiveness.
Hot water application is still a relatively new weed control
method and thus trial work and assessments of the practicability
and effectiveness in various situations is still being evaluated.
Field trials carried out in New Zealand have shown that
hot water application has similar results to glyphosate, except
in controlling perennial weeds.
Preliminary observations indicate that hot water treatment
kills annual weeds in 24 hours. The foliage from some perennials
also dies within 24 hours, but regrowth recurs from the roots
within a week or two.
Herbicide Control
Herbicides are widely used for control of weeds in both agricultural
and non-agricultural situations. The early herbicides such
as arsenic trioxide and iron sulphate were mostly by-products
of the chemical industry. Specifically manufactured materials
are now used.
These newer materials generally act on specific enzyme systems
in plants. In comparison with the past, herbicides are now
generally used at significantly lower rates with declining
amounts of active ingredients.
Herbicides kill weeds by interfering with the growth processes
of the plant by replacing hormones in the plant or by blocking
chemical reactions in other ways. Some herbicides do this
where they make contact with the plant, others need to be
translocated in the plant system to the site of action. Herbicides
are therefore grouped as either translocated or contact.
Contact herbicides kill the parts of the plants they touch,
which is usually limited to leaves and stems of the plant.
They work more effectively on annual or seedling perennial
weeds and kill relatively quickly. Contact herbicides can
be either selective or non-selective, depending on weed types
and the crops involved. When applying contact herbicides,
the plants need to be actively-growing and stress-free. Good
coverage is required to achieve effective results. Contact
herbicides include paraquat and diquat.
Translocated herbicides move within the plant to a site
of action. They disrupt growth processes and interfere with
biochemical reactions. This usually occurs where cells are
actively dividing in growth tissue, such as at the base of
stems in grasses, and in growing tips or buds in broadleaf
weeds.
Herbicides are also available in a range of forms including
selective, non-selective, residual and pre-emergent products.
The Pesticides Act 1999 provides for registration
of herbicides, labels and containers. Only a registered herbicide
should be used for the control of weeds. Herbicides are only
to be used according to the directions on the label which
gives an outline of the product's use, mixing, application,
restraints and directions. The labels are designed to prevent
misuse of a product.
Users have a legal obligation to read and follow the instructions
on the label.
Pesticides, including herbicides, should be handled and
applied with consideration of their toxic nature and potentially
harmful effects on human health, livestock, and the environment.
By following label instructions and applying herbicides in
the correct manner using best practices and trained staff,
off-target damage and adverse effects can avoided.
There are numerous forms of application techniques and equipment
available to apply herbicides. Equipment available includes
boom sprayers, hand guns, knapsacks, wickwipers, granular
soil applicators, aerial sprayers and gas guns. Application
methods include folia spraying, basal bark, cut stump, stem
injection and wick-wiping methods.
The type of equipment and application method chosen for weed
control depends on the size of infestation, type of weed,
topography, access, potential environmental and health hazards
and the susceptibility or suitability of a certain weed to
a particular application method.
For application equipment to operate effectively, the weather,
soil conditions and time available for spraying must be considered.
Suitable weather conditions are essential if herbicides are
to be applied safely and effectively. Weather conditions should
be assessed and monitored throughout an application to reduce
the risk of drift and subsequent off-target damage.
Heavy rains following herbicide application can reduce the
effectiveness of a treatment and may cause contamination through
run-off.
With the increasing array of products and the continuing
refinement of application equipment, herbicides are a particularly
attractive option due to their effectiveness and practicality
in a wide variety of weed control situations. In many situations,
herbicides alone or integrated with other control methods
can prove to be the most economical means of control, requiring
less labour, fuel and equipment than other methods.
Cultivation
Cultivation is a proven way of controlling weeds. Implements
range from large tractors and ploughs down to hand tools and
the humble chipping hoe. This method results in direct control
of weeds. The treatment of large infestations of lantana with
herbicides is not economically-feasible. Cultivation is therefore
an option, but one which must be used wisely. Smaller weeds
are more rapidly, efficiently and cheaply destroyed by cultivation.
Shoots can also be buried deep to prevent regrowth; the roots
exposed to dry-out; shoots separated from the roots; or a
combination of all three.
Cultivation has two main objectives, to prevent seeding
and to destroy the existing plants. Cultivation can be used
to cut-off weed problems before they get out of control. Eradication
of perennial plants by cultivation though can be difficult
and dependant on the root system or the rhizomes present.
These type of weeds may be controlled by repeated passes where
the roots and rhizomes are dragged to the surface to dry out
and die. This is however, seldom entirely effective.
Control by cultivation should therefore aim at exhausting
the food reserves through repeated disturbance and removal
of the shoots every week to 10 days. For effective control
by cultivation, weeds should be attacked before flowering
and under reasonably dry conditions.
Tillage should be used strategically, choosing the most appropriate
equipment for the varying stages of crop and pasture production.
Manual cultivation using chipping hoes, mattocks and other
suitable hand tools are another viable means of weed control
in small-scale situations, although this method is consuming
and labour-intensive.
Slashing
Slashing can be used to prevent tall growing weeds from flowering
and setting seed. This method can be undertaken by either
a tractor and slashing implement or by using a hand-held brush-cutting
machine.
Slashing can also be used to remove unpalatable or inedible
weeds left after stock have selectively grazed a paddock and
to prevent these weeds taking over. It may also encourage
the growth of more prostrate, less desirable species.
Slashing is not effective in eradicating a weed, just in
temporarily controlling the plant until it reshoots. Continual
slashing may control a weed to a desired level if a more desired
prostrate-growing species is present and is encouraged to
replace the weed. It can be used for the control of vegetation
and weeds along roadsides but is not suitable for the control
of weeds in crops.
Slashing is cheaper than cultivation and preserves the ground
cover, thus reducing soil erosion and improving access in
wet weather.
Mulching
Mulching involves the use of physical barriers to exclude
sunlight and to prevent weed establishment. Artificial barriers
such as black plastic sheeting or woven, paper products and
woven cloth have been used in various situations, particularly
in row crop production. Machinery is available which will
lay black plastic and allow for transplanting of crops. Woven
black plastic is also useful along roadsides where areas need
to be revegetated on steep banks and cuttings. This option
is only viable for small areas but can assist in weed control,
bank stabilisation and erosion.
Natural mulches include sawdust, timber chips, straw, matures
and grass clippings. Natural mulches have other beneficial
effects by adding organic matter and nutrients to the soil.
They are however, awkward and time-consuming to apply and,
in some cases, may introduce weed seeds.
Most perennial weeds will also penetrate particulate mulches
such as sawdust and wood chips. Mulching for weed control
can be effective, but is limited to suitable row-cropping
production such as strawberries, isolated areas along roadsides,
organic farming and general backyard garden usage.
Fire
Fire has been used for many years as a form of vegetation
and weed control. Its success depends on the amount of fuel;
the speed and intensity of the fire; and time of year that
burning takes place. Fire can play major role in the management
of woody weeds in western regions of New South Wales and can
also be a useful option for the control of lantana and blackberry
in certain situations.
The best fire strategy for woody weeds is a controlled managed
burn. The aim is to burn only the desired area, using firebreaks
and back-burning techniques. A managed burn ― unlike
a wildfire ― is controlled and minimises the damage
to the environment with, ideally, no damage occurring to property
and livestock.
Controlled burning for the management of woody weeds can
help restore land to a more open condition suitable for pasture
growth and subsequent increase in grazing. The direct costs
of managed burning are far lower than alternative techniques
such as chemical treatment and mechanical clearing. An integrated
management program may be required when using fire for weed
control. For example, lantana can be controlled with a combination
of fire, improved pastures, and follow-up spot-spraying. Large,
dense woody weed infestations are most suitable for fire control
as larger areas burn more effectively where other control
methods such as chemical and mechanical are less economical.
Reafforestation
Reafforestation is a long-term method of weed control. The
aim of reafforestation is to form a dense tree canopy that
restricts sunlight penetration to weeds on the forest floor.
Mature trees compete for moisture, nutrients and sunlight,
therefore restricting potential weed establishment and growth.
It can take 5-10 years before trees form a dense canopy and,
during this establishment phase, weed control can be critical
to the success of the plantation. It may therefore be necessary
to use other forms of weed control, such as herbicides and
mechanical means, to assist in this establishment phase. A
competitive, desirable, shade-tolerant grass or legume can
also assist with forest management and weed control.
Large areas of land are more suitable for reafforestation
as other forms of weed control become uneconomic or impractical.
NSW State Forests can assist with tree selection, site preparation,
planting and general forest management. A weed control program
can also involve agro-forestry principles which include tree
growing in conjunction with other agricultural enterprises
such as cropping or domestic animals.
The effectiveness of reafforestation for weed control depends
on the tolerance of various weeds to shading, the added competition,
and forest management.
Reafforestation to control groundsel bush has been tried
in a number of situations but has not been satisfactory ―
unless good forest management methods are adopted. Trials
are also being carried out to assess the effectiveness of
reafforestation to control serrated tussock and giant Parramatta
grass.
Land Management
Sound farm management strategies are also an effective means
of reducing the impact of weeds. Management strategies such
as pasture maintenance, good crop vigour, crop rotations,
reduced tillage, grazing management, early weed identification
and farm hygiene can all reduce weed problems.
A vigorous crop or pasture competes more effectively with
weeds and has the added benefits of increased production.
Weeds can be controlled in a pasture situation by either improving
the existing pasture or removing it and replacing it with
a more suitable or competitive species. Pastures can be improved
by adding fertilisers and lime according to soil test results,
and the use of a selective herbicide may be required to further
suppress weed competition.
Sound crop rotations can also minimise weed problems. Crop
rotations assist in controlling diseases and insects and help
build-up soil fertility and structure to produce increasing
yields. Increased fertility reduces the impact of weeds and
rotating crops can break the seeding and germinating cycle
of the weeds. All weed control methods are of little use if
crop and weed hygiene are not practised.
This includes sowing only weed-free seed, cleaning machinery
and removing sources of reinfestation around the farm. New
stock being introduced onto a property should be quarantined
for several days so any potential weed seeds can pass through
their systems in a known area and be treated later.
Early identification and awareness of potential new weed
problems will reduce the impact of weeds and save the property
manager time and money. If a potential weed is identified
in the early stages of infestation, it will be a lot easier
and economical to eradicate. Grazing management and the introduction
of competitive, desirable pasture species can also be an effective
weed control method.
Stocking rates should be set at appropriate levels so as
to not overgraze. Proper farm management can play a major
role in reducing the incidence and impact of weeds on a property.
The initial increase in costs for better management will be
compensated for by the reduced amount of weed control required
and the increased long-term farm productivity.
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