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Weed management — protecting the pasture investment

Weeds are estimated to cost Australian livestock industries $2.1 billion per annum in control costs and lost production.

The following principles provide a basis for managing weeds.

  1. Awareness - be aware of existing and potential weed problems.
  2. Detection - be on the lookout for new weed infestations before they become too large and difficult to contain.
  3. Planning - prioritise efforts and plan a strategy for successful control.
  4. Prevention - is better than a cure, so preventing new weed infestations and containing spread of existing weeds will make life easier.
  5. Intervention - and do it early. Controlling weeds now rather than later will prevent them spreading out of control.
  6. Control and monitor - as always, monitoring is a critical component in weed management. Managers need to gauge how well they are controlling weeds and re-plan their efforts for the future.

Use the 3D weed management approach to minimise the impact of priority weeds on pasture and livestock production. The '3Ds' of weed management are:

  • Deliberation -  consider the current weed problem and the desired level of weed control.
  • Diversity - use a combination of tools to control weeds.
  • Diligence - continue to manage the weed problem to keep it at the desired level of control.

Apply the '3Ds' of weed management to increase pasture competitiveness while reducing the proportion of weed species in the pasture.

Ideally, weed management in pastures is undertaken through a weed replacement approach - encouraging desirable pasture species to replace and exclude weeds.

Fire is a strategic tool to manage weeds in northern production systems.

What constitutes a weed?

Pasture weeds are not always easily determined, as most weedy species have some feed value at some point in their life cycle.  

This complicates weed management, as the weedy status of a plant depends on the time of year, its location, the livestock enterprise and management objectives.

Weed impact varies according to season, climate, soil type and, most importantly, the management of the pasture. However, weeds generally have one or more of the following characteristics:

  • poisonous to livestock
  • produce plant parts that affect animal health, prevent grazing or reduce the value of animal products (eg grass seeds that damage the skin or meat)
  • lower digestibility
  • occupy space and resources that could otherwise be used to grow more productive and desirable species
  • rapidly spread to neighbouring areas
  • costly to control

Undesirable or ’weedy’ species in pastures generally fall into one of the following four categories.

Annual grass weeds

Annual grasses, such as silver grass, barley grass and brome provide quality feed from the autumn break until early spring. After that:

  • seed heads contaminate wool and damage hides, eyes and mouths
  • digestibility drops markedly in late spring and over summer
  • seed production is very high (up to 500,000/m2).

Annual grasses are a valuable component of pastures where soil type and climate limit the success of perennial grasses, but can dominate a pasture at the expense of more desirable species.

Replacing annual grasses with perennials will reduce seed problems and reduce weed invasion.

Shallow-rooted annual grasses usually allow more moisture and nutrients to pass through the soil profile, and this can contribute to increases in dryland salinity and soil acidity.

Broadleaf weeds

Broadleaf weeds such as Paterson’s curse, St John’s wort and thistles can rapidly dominate pastures, especially where bare patches are increasing and pastures are set-stocked.

Broadleaf weeds have the following characteristics:

  • often have poor digestibility compared to desirable pasture species
  • seeds of some broadleaf weed species contaminate wool, damage hides and cause animal health problems
  • some species can poison livestock slowly over time, eg St John’s wort and Paterson’s curse
  • many species form dense patches over large areas, excluding stock from grazing
  • often produce a lot of long-lasting seed, eg St John’s wort, Onopordum or Illyrian thistle seeds can remain viable for 12 years or more
  • often not grazed by livestock, except at the early rosette stage, when broadleaf weeds are more digestible

Livestock will graze young broadleaf weeds when stock densities are high enough to overcome selectivity, and the pasture has been rested to encourage a more upright growth habit in the weeds.

Perennial grass weeds

Managing perennial grass weeds in perennial grass pastures is difficult and challenging. 

Common perennial grass weeds like serrated tussock, African lovegrass, Chilean needle grass, giant Parramatta grass and the weedy sporobolus grasses have low digestibility.  

Livestock avoid grazing the lower quality species when other feed sources are available, allowing the perennial grass weeds to dominate.

Other perennial grasses, such as bent grass or couch, use the space and resources that could otherwise sustain more productive species.

Prevent perennial grass weed invasion by identifying them soon as they appear as isolated plants, and applying the 3D weed management principles.

Some perennial grass weeds can be used for livestock production by keeping them leafy with targeted grazing management.

Producers need to decide whether to control or use perennial grass weeds on their particular circumstances.

Perennial woody weeds

ntroduced woody weeds such as blackberry, gorse, sweet briar, mesquite and parkinsonia are long-lived and can dominate pastures if left uncontrolled.

Several selective herbicides are effective on these weeds with minimal damage to pasture cover but can be expensive and time consuming to apply over large infestations.

Establishing and/or maintaining a competitive pasture will reduce woody weed establishment.

Woody weeds have the following characteristics:

  • many produce a fruit, which is readily spread long distances
  • many invade and dominate pastures where pasture competition has been weakened eg at water points
  • most are not grazed by sheep and cattle (but often eaten by goats)
  • many restrict livestock movement due to dense sharp thorns
  • seeds last many years in the soil
  • many provide habitat and protection for pests such as rabbits and foxes.

Know your weed

Recognise pasture species and become familiar with the life cycle of key weed species, particularly seed production and recruitment periods. 

               

A clear understanding of a weed’s life cycle can help to identify the weed’s strengths and weaknesses, assisting the development of an appropriate management strategy.

Identify:

  • how weeds are transported and spread
  • where each species prefers to grow and the type of grazing management that encourages its growth
  • the relative lifespan (hardseededness) of seeds of different species.

Manage in response to season

Favourable seasons offer greater flexibility for weed control. Pastures generally respond better to management treatments under favourable conditions as they are actively growing.

In poor (dry) seasons, pastures may be stressed, dormant or inactive and management tactics to control weeds and encourage desirable species will therefore have limited impact.

Adjust management according to the prevailing seasonal conditions.