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Big Dry Poses Prickly Problem

Thistles are rearing their ugly heads again in drought-affected central New Zealand.

WoolPro extension specialists in Wairarapa, Nelson/Marlborough and Canterbury are expecting thistles to be particularly fierce in those regions this year due to the hot, dry summer.

“Existing thistles have now gone to seed, and the next generation are waiting in the wings,” says Wairarapa extension specialist Richard Gavigan.

“Unfortunately for farmers, the dry weather has knocked pastures back, which will give thistles a big head-start when the autumn rains come.”

Good pasture cover is the best defense against scotch, winged and nodding thistles, which thrive in these summer dry areas.

Thistles are a problem because they reduce pasture production and stock carrying capacity, and increase scabby mouth and parapox infections in stock. One North Island trial showed thistle covers of 30 per cent reduced ewe liveweight gain over a year by 29 per cent, or 9 kg.

Thistle heads are a particular problem in wool. Undetected, they go right through the scouring process, and literally explode at carding, spreading throughout the wool and ending up in the spun yarn.

WoolPro is advising farmers to plan a course of action now to keep thistles under control.

AgResearch Poukawa’s Mike Slay says maintaining a dense pasture sward is the key to keeping thistles at bay, but it’s easier said than done.

Hot, dry summers, and pasture damage from insects, such as porina caterpillars and New Zealand and Tasmanian grass grubs, have left ground bare, encouraging thistle seeds to germinate.

Farmers should take care of existing pasture by trying not to graze it too hard during dry periods, especially on dry ridges where thistles are most likely to strike.

For areas where thistles have already taken hold, broadcast spraying with MCPA, 2,4-D or MCPB is advised. Used correctly, these sprays are designed to be ‘soft’ on pasture, but sufficiently hard on thistles.

“Remember, the objective is to encourage a good recovery of pasture. The dose should be set so it’s high enough to fix the problem, but no higher than that.”

In the case of nodding thistle, Mr Slay says the plant root should be examined to determine its age. Older thistles will have longer roots. He suggests farmers seek advice on how to deal with mixed-age thistles in the same block.

Farmers should now start planning to spray established thistles – delaying spraying until most thistles have grown two or more leaves off the crown and clovers are past the seedling stage. This is usually around April.

On dry ridges and hill country, farmers should ‘break the cycle’ of thistles by sowing drought-tolerant grasses such as cocksfoot or grazing brome after spraying, he says.

Farmers should also be on the lookout for thistles that don’t die after spraying. Thistles that become resistant to spray create a real control problem as the herbicides needed to kill them are not so pasture-friendly.

To prevent thistle contamination in wool, WoolPro urges farmers to avoid grazing woolly sheep in paddocks where thistles are a problem.

WoolPro has recently released a WoolPak, which gives more advice on controlling thistles throughout the year. Copies are available from WoolPro by calling 0800 4 WOOLPRO (0800 496 657).


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