Home' Otago Southland Farmer : March 8th 2013 Contents 22
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Fertiliser & spraying
FROM Page 21
The ideal time to get N on in hill
country is early spring, so you can
increase pasture cover for lactating ewes.
your steep hills in late spring or even early
summer, and dealing with the easy hills
and flat land in autumn, when more cash
What place has nitrogen fertiliser got on
Of all the nutrients, it is nitrogen (N) that
drives pasture production. If there is
insufficient atmospheric N being fixed by
legumes, the pasture will be dominated by
poor grass species such as browntop.
These grasses are only in a green
vegetative state for a short period in the
spring, suppressing feed supply as well as
The ideal time to get N on in hill country is
early spring, so you can increase pasture
cover for lactating ewes. As for all inputs,
N will only give a profitable return if the
extra pasture is converted into high-value
meat and wool products. The economics
of N application depend on the returns
achieved from product and the cost of
both the product and application services.
-- Information sourced from Ballance
Thistles can be controlled -- but there's no silver bullet
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, says information from Tectra, as 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-
Annual and biennial thistles:
As annual and biennial thistles only reproduce through seed,
control measures should focus on reducing seed production
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. However, 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 a
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
Several herbicides are useful for thistle control in pasture, but
the phenoxy (hormone) herbicides MCPA and MCPB are
Phenoxy sprays work best when thistles are small.
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
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.
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