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Keeping thistles out of the clip

Winning the battle

Good pasture management is the key to controlling thistles and keeping them out of wool:

  • Avoid over-grazing, pugging and soil compaction
  • Maintain a good pasture sward
  • Control thistles by mechanical or chemical topping
  • Don’t buy or make hay with thistle contamination
  • Check and clean all vehicles and machinery coming on to your property
  • Grub scattered plants, removing at least 5 cm of the taproot
  • Keep woolly sheep out of thistle-infested paddocks

Thistles can be serious pasture weeds and a major contaminant of wool. They cost farmers money, reduce pasture production and can be a major headache for wool processors.

History and biology

Thistles were introduced to New Zealand in the 1800s, and have spread throughout the country.

The most common and serious species are: Californian thistle (Cirsium arvense), Scotch thistle (Cirsium vulgare), plumeless thistle (Carduus acanthoides), nodding thistle (Carduus nutans), variegated thistle (Silybum marianum), winged thistle (Carduus tenuiflorus) and slender winged thistle (Carduus pycnocephalus).

Californian thistles have extensive, creeping root systems, which is how they over–winter and reproduce. All other thistles are annuals, biennials or short-lived perennials that reproduce only by seed.

The ones that grow like annuals germinate in autumn or winter and flower the following summer. Scotch thistle is an exception. In pasture it germinates in autumn or winter but doesn’t flower until the second summer.

Depending on when they germinate and how quickly they grow before temperatures drop, nodding and plumeless thistles can behave as annuals, biennials or triennials.

While Californian thistles spread through their root system, nodding and Scotch thistle seeds are spread by wind, feeding out hay or silage and movement of agricultural equipment and vehicles.

Why they are a problem

On farm

On farm, thistles can reduce pasture production and stock carrying capacity, and increase scabby mouth and parapox infections in stock. They compete with other plants and can prevent useful pasture species growing.

Sheep production, in particular, can be affected if thistles are growing densely enough to suppress clover and grass leaf production. Dense thistle cover can significantly reduce ewe and lamb liveweight gain (for example, in one North Island trial, 30 per cent thistle cover reduced ewe liveweight gain over a year by 29 per cent, i.e. 9kg.)

Nodding thistle is an aggressive pasture weed and can be very serious in areas prone to summer droughts. Thick stands can make it difficult for farmers to move stock around, and can stop stock getting to good pasture species.

In wool

Any vegetable matter in wool can cause unsightly contamination in yarn (e.g. it doesn’t take dye the same way wool does) and can be expensive and time consuming to remove.

Thistle heads are a particular problem. Undetected thistle heads can literally explode during carding, contaminating the yarn with fine thistle head fibres. If this happens, manufacturers may have to use tweezers to laboriously remove the thistle fibres from the yarn or the finished product.

Not surprisingly, processors take a dim view of this, and in bad cases seek financial redress from suppliers. To prevent the problem, some New Zealand mills and exporters are prepared to pay more (up to 40 cents extra per kilo during the 2000 season) for thistle-free wool with a vegetable matter content of less than 0.1 per cent.

New Zealand risks losing the support of important customers if we don’t supply them with fibre that is free of major contaminants like thistles.


Controlling thistles

Non-chemical control

Thistle seed germination is high in open pastures under lax grazing regimes, and low in dense, vigorously growing swards. Maintaining strong pastures, especially in autumn, can reduce thistle invasion.

One way to achieve this is to avoid over-grazing in summer so that pastures recover more quickly, preventing the seeds from germinating in autumn.

However, if thistles are proliferating because of lax grazing, mow three times between late November and March. Alternatively, topping in November, once most of the grass has begun flowering, can also help control thistles, and can pay for itself through improved pasture digestibility and increased lamb growth.

A programme to completely remove the thistle top growth through a combination of low-level mowing and intensive mob stocking as soon as the ewes become available after weaning can be very effective.

Mow with the mower set low. Follow this with two days’ mob stocking by at least 350 ewes/hectare. Repeat this mob grazing at intervals of less than three weeks to keep re-growth to a minimum. This should be repeated five or six times in the first season, and followed up the following year. This intensive defoliation depletes the thistles’ underground food reserves until the plants are exhausted.

Topping two or three times a year, followed by grazing, can give control over a much larger area than is possible with only one topping and more grazings.

A cheaper alternative to topping is to wound the thistles by towing a clod-crusher over them to improve their palatability, and then grazing with 200 to 250 ewes/ha within 48 hours. This is best done in December.

Californian thistles

Repeated mob grazings can reduce the vigour of Californian thistles, especially when combined with mowing. Grazing 200 to 250 ewes/ha for four days each fortnight during spring, and around 200 ewes/ha for four days each month during summer can give extremely good control of thistle shoots in the following year.

Nodding thistles

Once nodding thistles are established, different grazing regimes appear to make little difference to thistle growth. However, goats will readily eat, and kill, the plants once the thistles have bolted.

Hand grubbing scattered nodding thistle plants can be effective, but the plants must be cut to 5 to 10 cm below the soil surface and the soil removed from the roots so the plants do not re-sprout.

Chemical control

Spraying nodding thistle should start in early spring. For Californian thistles, spraying in early summer when the thistles are in their early flower bud stage, followed by a further spray treatment in the autumn will keep them under control. Missed or large thistles should be spot treated.

To achieve the best control of thistles and maintain pasture quality, graze paddocks so the thistles are exposed, wait two or three days for the paddock to recover, then spray. Wait at least 14 days after spraying before grazing the paddock again.

Broadcast spraying

The sprays commonly used to control thistles include MCPA, 2,4-D and MCPB. These herbicides are more effective on younger plants. None of them are very effective after the plants start to bolt. MCPB is more effective on young thistles, and 2,4-DB is effective only on seedling thistles.

MCPA and 2,4-D suppress clovers for weeks or months after spraying, which can make pastures more susceptible to further thistle invasion. 2,4-D ester is more effective than 2,4-D amine in killing thistles in pastures.

MCPB does not affect clovers in pastures, but it has to be used at the higher rate than MCPA or 2,4-D, and is more expensive.

Several herbicides, such as clopyralid (Versatill(r)), dicamba or picloram (Tordon(r)), can be added to MCPA or 2,4-D to improve thistle kill, but all of them kill clovers and will seriously weaken pastures, making them more susceptible to further thistle invasion.

Nodding thistle has developed resistance to MCPB, MCPA and 2,4-D as a result of repeated spraying of the same populations.

Spot spraying

Many herbicides (such as 2,4-D, MCPA, clopyralid, dichlobenil (Prefix-D(r)), glyphosate and picloram) can be used for spot spraying, but most of them damage other pasture plants, so they should be used with great care. They can be applied by knapsack, hand-gun, hand-lance, drench gun fitted with extension lance or granules.

Rotary carpet wipers

Carpet-rollers and weed-wipers are effective ways to apply non-selective herbicides. They can be effective in controlling bolted thistles, but because thistles do not all bolt at the same time, several applications may be necessary.

Pasture must be grazed down hard before using a roller or wiper, so the thistles stand clear of the pasture which would otherwise be harmed by the chemical. Herbicides that can be used include Escort(r) (metsulfuron) and glyphosate.

Biological control

Several biocontrol agents have been released for the control of nodding and Californian thistles. The nodding seed-head weevil (Rhinocyllus conicus) is widespread and is known to reduce seed production by half in Hawkes Bay, although it is less effective in the South Island. The crown-feeding weevil (Trichosirocalus horridus), which attacks rosettes, and a gall-forming fly (Urophora solstitialis) have also been released for nodding thistle.

Biocontrol agents have been released for Californian thistle, and a mycoherbicide (a fungal disease used as a weedkiller) based on the fungus Sclerotinia sclerotiorum is being developed.

Wool handling

Preventing thistle heads getting into wool should be the farmers’ first priority, through good pasture management and a well planned spraying programme. If that fails, wool handlers are the last line of defence.

With a premium of up to 40c/kg clean, it is worth spending a few extra seconds to ensure that good quality fleece is thistle-free.

For quality-critical lines (good colour fleece, second shear and lambs body wools), wool handlers may find it quicker to pull the good wool from the thistles, instead of trying to find and remove all the individual thistle heads from each fleece. It only takes one thistle head to create a problem for spinners.

Contact details

For more information about thistle control, contact AgResearch, phone (07)834-6600 (www.agresearch.co.nz/agr/publications.htm). then search AgFacts for the subject of your choice.

For more information about thistles and wool preparation, contact Meat and Wool Innovation, 0800 496 657, email info@mwi.co.nz.


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