By Kevin Knowler, AgResearch Woodlands

Topping two or three times a year, and a follow-up grazing, provides very good Californian thistle control.

Keeping this costly pasture weed under control will improve pasture quality, which in turn can give better lamb growth rates.

AgResearch Woodlands near Invercargill had 60 hectares involved in a Californian thistle control trial, looking at a topping/grazing programme. All the thistles in the 49 paddocks in the programme were either topped or grazed at least six and up to 10 times over two years.

There was a significant reduction in thistle density in the first year, with the second year used to control the remaining thistles. The effectiveness was directly linked to the number of times the paddock was either topped or grazed.

To date almost three-quarters (73 percent) of the area has either zero or very low thistle density, with the remaining paddocks having a low density with some heavier patches.

AgResearch found that topping two to three times annually, with follow up grazing, has allowed control over a much larger area than would have been possible with only one topping and four grazings.

To achieve a successful result the key points to remember are:

  • The mower must be able to cut clean and low to the ground removing, if possible, all of the leaves.
  • Topping gives better clover re-growth than a hard grazing.
  • After the second or third topping the thistles become easier to graze.
  • Topping or grazing three times is the minimum required, with five giving a better result.
  • Grazing is possibly more effective than topping due to the lower height that the sheep can graze the thistles down to.
  • The thistles should be grazed after three weeks re-growth, before they get too hard.
  • The topping/grazing programme should start in November or December.
  • Fence lines may need to be sprayed.
  • This programme may be difficult to implement in a clover-dominant or young grass paddock, where hard grazing may not be desirable.
  • Regular high topping leaving some lower leaves will have some effect on reducing thistle density but it will take much longer to achieve the desired result.

Note: What the treatments are achieving is depleting the plant's resources to the point where it can't recover. So the less time and area of foliage the plant has available to photosynthesise and recover the better.