Categories of weeds
Weed species generally fall into one of the following four categories:
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.
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.
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.
Introduced 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
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