Integrated weed management

Biological control

Biological control of weeds uses a plant's natural enemies such as insects, mites and diseases to reduce and control its population. It is an economical, effective and environmentally sound method, but is a long-term technique with extensive development and establishment phases. Biocontrol does not eradicate a weed, but can reduce it an acceptable level, or suppress it to a level where it can be controlled with other methods.

Inundative biological control uses mycoherbicides - plant pathogens such as rusts and fungi - applied as a treatment. They are likened to a species-specific natural herbicide, are not self-sustaining and have a short active period.

Classical biological control is the release and establishment of control agents such as insects, rusts and mites into the target weed infestation, creating a natural balance between the weed and its control agent – similar to that found in the weed's native range.

If an agent establishes a population, control becomes self-perpetuating and self-regulating as the agent becomes part of the region's ecology. Monitoring an agent's population dynamics is an important part of a biocontrol strategy.

Biological control should be considered when a control agent has achieved good control in other areas. However, successful programs may take more than 10 years to be effective, and results may vary from area to area. Biological control is practical and effective for:

inaccessible areas such as timbered, rocky and steep locations
areas of low-priority for control
situations where biocontrol is the only option, for example sensitive aquatic areas
situations where chemical control may be too expensive or not effective.

Flaming

Flaming is not a common or well-developed control method in Australia, however in Sweden it has been used for many decades for:

weed control on organic farms
pre-emergent weed control in carrots and other slow-germinating row crops
selective post-emergent control in heat-tolerant crops
general weed control on hard surfaces in urban areas.

Liquefied petroleum gas or propane is used in flame weeders. The process does not require a weed to be burnt, but ruptures the plant's cell membranes by raising its water content to temperatures to above 100°C.

Small seedlings are generally more susceptible to flaming. Species with upright habits and thin leaves are more sensitive than species with a low stature and protected growth points.

Steaming

Steaming is a relatively new weed control method, still in the developmental stage. Applying hot water to a weed results in the loss of the plant's waxy coating, a reduction in moisture, dehydration and death.

The system operates by plumbing water under pressure through a heated chamber, and applying it to the weed. The combination of heat and water pressure breaks down the cellular structure, causing discolouration and death within hours or over a few days.

Field trials carried out in New Zealand have shown that steaming 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.

City councils in Australia have trialled the equipment with mixed results. Trial work and assessments in various situations are still being conducted.

Goats

Controlling weeds with goats is a medium-to-long-term proposition and can be highly effective in certain situations. Goats can be integrated with sheep, cattle and cropping enterprises to provide weed control and pasture improvement. Generally, goats should be only one aspect of an integrated weed control program and stocking rates, timing, weed palatability and farm management strategies need to be considered. It is usually important to have a competitive pasture to colonise bare areas.

Goats control weeds by selectively grazing their foliage, bark, stems and flowers. Goats eat a variety of weed species that sheep and cattle avoid, such as blackberry, sweet briar, scotch broom, thistles, Paterson's curse and horehound. The nutritional value of these species can be quite high. Occasionally goats will eat fireweed, groundsel bush, St John's wort, serrated tussock and spear grass. Goats are grazers of weeds in inaccessible areas where conventional control methods are not possible.

Herbicides

Herbicides are widely used to control weeds in agricultural, commercial and domestic situations. Herbicides are chemicals that kill plants by affecting their enzyme systems, interfering with their growth processes, replacing their hormones or blocking their chemical reactions. Herbicides are effective and practical in a wide variety of situations, and often provide the most economical means of control.

Some herbicides act on contact with the plant; others need to be translocated through the plant's system.

Contact herbicides

Contact herbicides kill the parts of the plants they are applied to - usually limited to leaves and stems of the plant. They are more effective on annual weeds or on seedlings of perennial weeds. Contact herbicides can be either selective (i.e. they only kill broadleaf plants) or non-selective (i.e. they kill all plants). Plants need to be actively growing when contact herbicides are applied, and g good coverage is required to achieve effective results. Contact herbicides include paraquat and diquat.

Translocated herbicides

Translocated herbicides must be moved around a plant's system. They disrupt growth processes and interfere with biochemical reactions. This usually occurs where cells are actively dividing in growth tissue, such as at the bases of stems in grasses, and in growing tips or buds in broadleaf weeds. Translocated herbicides include glyphosate and metsulfuron-methyl.

Regulation of herbicides

The Pesticides Act 1999 (NSW) provides for registration of herbicides, labels and containers. Only registered herbicides can be used to control weeds according to the directions on the product label. Labels are designed to prevent misuse of a product, and users have a legal obligation to read and follow the instructions on it.

Handling and applying herbicides

Herbicides can have potentially harmful effects on human health, livestock, and the environment. Trained users can avoid adverse effects by following the instructions on the product label.

Equipment for herbicide applications includes boom sprayers, hand guns, knapsacks, wick-wipers, granular applicators, aerial sprayers and gas guns. Application methods include foliar spraying, basal bark and cut stump applications, stem injection, and wick-wiping.

The choice of equipment and application method depends on the size of the infestation, type and susceptibility of weed, topography, access, and potential environmental and health hazards. For herbicide treatments to be safe and effective, weather, soil conditions and the timing of the treatment must be considered. Weather conditions should be assessed and monitored during treatments to reduce the risk of drift and off-target damage. If heavy rain follows application, effectiveness can be reduced and contamination of waterways can occur through run-off.

Cultivation

Destroying weeds through cultivation is a proven method of control. It is particularly effective on young weeds. Implements are used to dig up and destroy weeds, ranging from large tractors, discs and ploughs to hand tools such as mattocks and chip hoes. Shoots can be separated from their roots or buried deeply to prevent regrowth, and roots can be dragged to the surface to dry out. Some types of weeds can be controlled with repeated passes; however eradication of perennial weeds can be difficult and depends on their root systems.

Cultivation is more effective if weeds are cultivated before they flower and under reasonably dry conditions. Manual cultivation is a viable means of weed control in small-scale situations or as a follow-up control measure.

Slashing

Slashing can be done mechanically with a tractor and slasher or by using a hand-held brush-cutter. It is cheaper than cultivation and preserves ground cover, reducing soil erosion and allowing access in wet weather. Continual slashing may provide control if a desirable pasture species is present and encouraged to replace the weed, but slashing will not eradicate a weed, and can't be used for weed control in crops.

Slashing can:

prevent tall weeds from flowering and seeding
remove unpalatable or inedible weeds left after stock have selectively grazed a paddock
temporarily control weeds until they re-shoot
control vegetation and weeds along roadsides

However, slashing can also have negative effects, such as encouraging the growth of less desirable weed species, or spreading weeds that grow vegetatively.

Mulching

Mulching involves the use of physical barriers such as black plastic or woven weed matting to exclude sunlight and prevent weed establishment. Mulching is used for weed control in row crop production such as strawberries, where machinery lays black plastic between rows. Woven weed matting is useful along roadsides, steep banks and cuttings where areas need to be revegetated and where bank stabilisation is necessary.

Natural mulches include sawdust, timber chips, straw, manures and grass clippings. These have other beneficial effects including adding organic matter and nutrients to the soil. However there can be a risk of introducing weed seeds in the mulch material. Most perennial weeds can penetrate mulches such as sawdust and wood chips.

Fire

The success of fire as a weed control method depends on the amount of fuel, the speed and intensity of the fire, and the time of year that burning takes place. Fire is a major control method for woody weeds in western regions of NSW, and can be a useful for controlling lantana and blackberry in certain situations. Fire is best used as part of an integrated weed management program.

Unlike wildfire, a controlled burn - where only the desired area is burned using firebreaks and back-burning techniques – is the best approach for woody weed control. Direct costs are lower than alternative methods such as herbicide treatments or mechanical clearing. A controlled burn:

minimises damage to the environment
avoids damage to property and livestock
helps restore land to an open condition suitable for pasture
creates access for further weed control.

Reafforestation

Reafforestation is a long-term method of weed control, where a dense tree canopy is formed to restrict sunlight penetration to weeds on the forest floor. Reafforestation can be in the form of revegetation with native species or through establishment of plantation forests. A weed control program can involve agro-forestry principles, which include growing trees in conjunction with other agricultural enterprises such as cropping or domestic animals. Reafforestation is suitable over large areas where other forms of weed control are uneconomic or impractical.

Mature trees compete for moisture, nutrients and sunlight and restrict potential weed establishment and growth. It can take 5 to 10 years before trees form a dense canopy and during this establishment phase it is critical to undertake other forms of weed control. A competitive, desirable, shade-tolerant grass or legume can also assist with weed control.

Land management

Good land management is critical to reducing the incidence and impact of weeds. The initial increased costs associated with better land management are compensated with reduced weed control. Management strategies that help to reduce weed problems include:

maintenance of pastures or desirable ground covers
reduced disturbances and tillage
management of nutrient run-off
grazing management
early weed identification
good weed hygiene.

Grazing and pasture management

Competitive, desirable pastures can provide effective weed control. Stocking rates must be managed so as not to cause overgrazing, as weeds will establish in overgrazed areas.

A vigorous pasture competes more effectively with weeds and has added benefits of increased production. Weeds can be controlled in a pasture situation by improving the existing pasture or replacing it with a more suitable or competitive species. Pastures can be improved by adding fertilisers and lime according to soil test results.

Crop management

Crop rotations can minimise weed problems, help control diseases and insects, and improve soil fertility and structure – producing increased yields. Crop rotations can break the seeding and germinating cycle of the weeds.

Weed hygiene

Weed hygiene includes sowing only weed-free seed, cleaning machinery and vehicles, checking clothing and equipment for weed seeds or weed fragments, and removing sources of weed reinfestation around a control site.

New livestock being introduced to a property should be quarantined for several days so any potential weed seeds can pass through their systems into a known area, and be treated later.

Weed identification

When weeds are identified in the early stages of their infestation, eradication is more likely. Control is economical when carried out early, rather than waiting until the weed infestation has spread and become established.