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Some aspects of the biology, ecology and control of slender thistle, Carduus pycnocephalus L. and C. tenuiflorus Curt : (Compositae) in Tasmania


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Bendall, Glen Max 1973 , 'Some aspects of the biology, ecology and control of slender thistle, Carduus pycnocephalus L. and C. tenuiflorus Curt : (Compositae) in Tasmania', Coursework Master thesis, University of Tasmania.

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The germination, dormancy and nutritional
requirements of slender thistle, and the effects of grazing
management on the ecology of slender thistles in improved
pasture were studied.
Heterocarpy was found to be well developed in
slender thistles. Approximately 85% of the seed produced
contained a soluble germination inhibitor which required
leaching from the seed to facilitate germination. The
remainder of the seed did not possess an inhibitor and would
germinate without leaching. Seedlings arising from seeds
Without an inhibitor had greater root growth at low soil
moisture than seedlings arising from seeds containing an
inhibitor, thus conveying to the former seedlings a greater
tolerance of moisture stress. The survival value of
heterocarpy to slender thistles is discussed.
Following leaching, seed germinated over a range of
well defined temperatures (10°C-30°C) and this was related to
the germination pattern of slender thistles in the field,
Effect of burial of seed at varying depths on
germination, dormancy, and longevity was observed. Seed
buried at a depth of 1.3cm gave the highest percentage
emergence. Twenty to 25% of seed buried at 5cm and 10cm
remained dormant and formed the source for germination in
following seasons, Five percent of seed buried at 10cm
remained dormant for more than two years,
Slender thistles responded more to applications of
nitrogen•than to applications of phosphorus or potassium when
grown man infertile soil, Nitrogen greatly stimulated vegetative growth and also directly increased the reproductive capacity of the plants by enhancing branching and hence the
number of flower heads produced. High pH (pH6.5) also
favoured thistle growth.
The effects of grazing management on slender thistle
populations and botanical composition of improved pasture were
investigated in two field trials. Deferred autumn grazing in
winter and spring significantly reduced thistle population
densities. Thistle control was obtained by two different
ecological mechanisms:
1. Deferred autumn grazing caused pasture/thistle
competition for light which resulted in etiolation of the
thistles to the extent that they were readily eaten during
subsequent grazing. The increase in acceptability of the
thistles to the sheep appeared to be mainly due to
morphological changes. An increase in nitrate (N) and total
reducing sugars with etiolation may have also favoured
2. Deferred autumn grazing apparently reduced the
availability of moisture to thistle seeds and germination was
partially inhibited.
Grazing management, especially spring grazing,
favourably altered pasture botanical composition by reducing
the proportion of weed grasses. These changes are discussed
in relation to thistle control
It is suggested that deferred autumn grazing may be
an alternative to herbicides for slender thistle control in
pastures and that control would be expected to be mainly by
the first mechanism. It is also suggested that deferred
autumn grazing and herbicides may be combined to give effective control, as etiolated thistles were more easily killed with MCPA than normal rosette thistles.

Item Type: Thesis - Coursework Master
Authors/Creators:Bendall, Glen Max
Keywords: Thistles
Copyright Holders: The Author
Copyright Information:

Copyright 1973 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Agr.Sc.) - Tasmania, 1974. Bibliography: l. 87-96

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