Spray-grazing declared plants

Page last updated: Thursday, 5 May 2016 - 1:34pm

Please note: This content may be out of date and is currently under review.

Weeds sprayed with a sub-lethal dose of a phenoxy, hormone-type herbicide appear to become more palatable to stock. Spray-grazing exploits the weed-killing potential of large numbers of stock grazing an infested pasture previously sprayed with a sub-lethal quantity of certain herbicides.

Spray-grazing

‘Spray-grazing’ is a method of controlling declared plants in pastures. It makes use of the fact that a sub-lethal dose of 2,4-D amine, MCPA amine or 2,4-DB herbicide causes many broad-leafed plants to wilt. This appears to make them more palatable to stock, which graze them in preference to the unaffected subterranean clover and grass species in the pasture.

Glyphosate and paraquat also appear to preserve the quality of some grasses. This makes pasture more attractive as dry feed, compared to untreated grasses.

Method

Please check the herbicide label at the time of purchase for the currency of this method.

  1. Six weeks after the opening rains in autumn, spray the infested area with 0.75L/ha of 2,4-D amine (500g/L) for control of annual thistles and Paterson’s curse. Spray with 1.5L/ha of 2,4-D amine if yellow burr weed (Amsinckia spp.), spear thistle or dock are present.
  2. Seven days after spraying, stock the paddock with sheep at a minimum of four to five times the normal stocking rate for the district.
  3. Leave the sheep at this high grazing pressure for about six weeks, or until the pasture shows signs of overgrazing.
  4. Increase the stocking rate again in the spring to prevent any remaining weeds from flowering.

Although saffron thistle is an annual weed, its seeds germinate slowly throughout the autumn and winter months. For this reason, best results with this weed are obtained if 2,4-D amine is sprayed at 1.5L/ha in early September, before any flowering stalks have formed. The heavy stocking rate should be applied seven days after spraying as described above.

Points to remember

  • The treated pasture must have a reasonably good legume base to compete with the weeds and to provide stock feed when the weeds are killed
  • at the recommended rates of application (0.75L/ha), 2,4-D will not severely affect the clover or other pasture species, nor will it kill any of the weeds listed. The spring rate (1.5L/ha) will delay flowering of subterranean clover. Medics will be killed at both rates.
  • heavy grazing is an essential part of this weed control method. Unless grazed, the weeds will recover from spraying in two or three weeks and make normal growth
  • ‘spray-grazing’ does not eliminate weeds from the pasture. However, if carried out correctly, the weed population will be reduced by the competition provided by the other pasture species.
  • the spraying and grazing process may be repeated each year until the bank of dormant weed seeds in the soil is exhausted. However, in many cases only one treatment is needed to maintain a good balanced pasture. Continual use of the phenoxy herbicides will eventually result in grass dominant pastures.

Application of 2,4-D

  • The amount of herbicide to be applied is not as critical as the timing when a lethal dose of chemical is to be sprayed without grazing. This means that, although boom sprays are highly effective, misters, side delivery or ‘boomless’ nozzles may be accurate enough.
  • do not exceed the recommended rate of application, as this may permanently damage the pasture
  • There are gazetted areas in Western Australia where the use of 2,4-D is restricted or regulated by the granting of a permit (see Herbicide application legal aspects).
Spray-grazing reccomendations

Weed

Time to spray

Rate 2,4-D (L/ha)

Comments

Annual thistles & Paterson’s curse

Six weeks after opening rains in autumn

0.75

Heavily graze with sheep, but do not overgraze

Amsinckia

Six weeks after opening rains in autumn

1.5

Heavily graze with sheep, but do not overgraze

Saffron thistle

Early September

1.5

Heavily graze with sheep, but do not overgraze, normal stocking in winter

Dock and spear thistle

Six weeks after opening rains in autumn

1.5

With spear thistle slash tops, spray regrowth when 15-20cm high

Stock as above

Caution: Medics should not be ‘spray-grazed’

Further information

Further information on controlling declared plants can be found through the Declared plant control handbook link.

Other topics that may interest you include:

Contact information

Andrew Reeves
+61 (0)8 9780 6224