Farm Tech

In The Paddock
Seasonal sheep flock management tips – December 2002

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Thistles can be controlled – but there's no silver bullet

Key points

• For control purposes there are two types of thistle—Californians and the rest
• Annual and biennial thistles are best controlled by pasture management practices that encourage the maintenance of a tight sward
• Topping, grubbing and herbicides also have a place in thistle control
• Californian thistle can be controlled by regular rotational grazing, associated with the use of either topping or herbicides to deplete root reserves and weaken the plant.

From a control point of view, there are two groups of thistle—Californians (calies) and the others.

Calies are the most economically important weed of New Zealand pastures. They can rapidly colonise pastures from their network of thick fleshy rhizomes. Their seeds are also spread by the wind.

Other thistles are mainly annuals or biennials. Of these, nodding thistles are among the hardest to control. They can also be serious weeds of improved pastures, especially in summer dry areas, and in open ground in new pastures.

Methods that control nodders will work well with other non-Californian thistles.

Annual and biennial thistles

As annual and biennial thistles only reproduce through seed, control measures should focus on reducing seed production and germination.

Where flower heads are immature and at a similar stage of maturity, topping or mowing may prevent regrowth and reduce seed production by as much as 80 per cent.

If thistles are cut too early—before terminal buds have formed—they are likely to re-grow. But if cut too late, when flowers are close to maturity, viable seeds can still be produced in the severed heads.

Hand grubbing is effective for small infestations, or for follow-up treatment after broadacre treatment. To prevent re-sprouting, plants must be grubbed 5–10 cm below the surface and detached from the sod.

Deep ploughing will bury thistle seeds too deeply for them to emerge if they germinate. The downside is that deeply buried seeds survive longer in the soil—about 10 years.

Shallow cultivation risks creating an ideal seedbed for new thistle seedlings. But these are easily killed by herbicides or by repeated cultivation. Seeds remaining near the surface have far shorter life—about two years.

The best way to achieve long-term control is to establish and maintain dense pastures, especially during late summer and autumn when the thistles are shedding their seed.

Seedling thistles are intolerant of intense competition, especially for light. A pasture cover of 6 cm or more can reduce nodding thistle by more than 90 per cent.

Rotational grazing with high grazing pressure for short periods, followed by long spells, will encourage rapid dense pasture re-growth. Set-stocking and overgrazing open up pastures in late summer/autumn, favouring thistle infestation.

If pasture is short, feed silage or hay at this time, rather than risk over-grazing. Over-sowing of open pastures with vigorous grass species will help reduce thistle infestation.

The seedhead weevil, Rhinocyllus conicus, reduces seed production by about 60 per cent a year, but will not reduce thistle populations when open pasture allows seedling establishment. It only takes one seedling in 2,000 to survive and reproduce to maintain thistle populations and it takes at least seven years before soil seed stocks are exhausted.

Goats eat thistle flowers as well as re-growth, reducing the ground cover of some thistle species by more than 90 per cent. They reduce the height and vigour of tall thistles and depress flowering.

Several herbicides are useful for thistle control in pasture, but the phenoxy (hormone) herbicides MCPA and MCPB are generally preferred.

Phenoxy sprays work best when thistles are small. In trials at Ruakura, nearly 90 per cent of small (under 10 cm diameter) nodding thistles were killed using MCPA, compared with 75 per cent of large thistles. If thistles are taller than the sward, a wick or boom wiper can be used, to reduce damage to clovers.

MCPA is superior to MCPB, but is hard on clovers. It is best used in winter, when clovers are not actively growing.

In most parts of the country, blanket spraying in autumn with MCPB is recommended to kill seedlings. In warmer districts where germination occurs in winter, many farmers prefer early spring treatments (August–early September).

In newly-sown pastures, MCPB should be applied only when clovers have two true leaves and pasture cover exceeds 50 per cent.

Sprays containing glyphosate (e.g. Roundup®), Picloram (e.g. Tordon®) and dicamba are very effective for spot treatment of large rosettes. However, they are non-selective to clovers and should not be used for broadacre thistle spraying.

Rain within 1–2 days of herbicide application is one of the main reasons for poor control. If 20–25 cm of rain falls a day after spraying, control is likely to be reduced by 50 per cent.

Herbicide treatments are not 100 per cent effective against seedlings in autumn or rosettes in spring, and are no substitute for maintaining a dense sward. They are probably most appropriate to use in seasons when drought has allowed pastures to open up.

Repeated use of phenoxy herbicides can lead to the development of herbicide resistant weeds, as has occurred with nodding thistle. To reduce the risk of this occurring, observe label directions and ensure your equipment is properly calibrated.

Californian thistles (calies)

Calies are deep-rooted perennials, which reproduce vegetatively and have a different growth habit to other thistles. They are best controlled by depleting their root reserves.

Cultivation chops up and spreads rhizomes. However, repeated cultivation that focuses on exposing and desiccating the roots can be effective, if you have the time.

Repeated topping can be used to deplete root reserves. Best results are achieved by topping at the ‘ball bearing’ stage and then wiping at the ‘blue haze’ stage with clopyralid (e.g. Versatill®).

Because calies have a more erect growth habit for longer periods than annual thistles, wick or boom wiping with herbicides is very effective. They can also be sprayed using MCPB at the ‘ball bearing’ stage. Timing is less critical than with other thistles because germination from seeds is not such a major issue.

Some farmers combine regular topping and rotational grazing to achieve very good control. In one trial, calie cover was reduced by 99 per cent within two years when topping to 3–4 cm was followed by hard grazing and/or spraying with MCPA.

The repeated removal of vegetative growth by regular short-term rotational grazing over summer and autumn weakens the plant and its ability to produce new growth. It is very important to graze at the ‘soft prick’ stage, when growth is still lettuce-like.

To use this method of control, high stock pressure needs to be applied, so it is best to concentrate on controlling calies on one paddock at a time.

More information

Keeping thistles out of the crop, WoolPro 2001.

Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local Meat & Wool Innovation extension specialist: