Individual study: Influence of pasture forage species on invasive nodding thistle Carduus nutans seedling emergence, growth and development near Ohaupo, Waikato, New Zealand
Wardle D. A., Nicholson K. S., Ahmed M. & Rahman A. (1995) Influence of pasture forage species on seedling emergence, growth and development of Carduus nutans. Journal of Applied Ecology, 32, 225-233
In New Zealand nodding thistle Carduus nutans, native to Eurasia, is a widespread and invasive problem weed. As an alternative to herbicide control, this field study investigated the influence of different pasture forage species on C.nutans seedling emergence, growth, flowering and mortality.
Study site: The Field experiment was undertaken at a dairy pasture heavily infested with C.nutans near Ohaupo (37º53'S, 175º18'E) in Waikato region, North Island, New Zealand. The dominant forage species were perennial ryegrass Lolium perenne and white clover Trifolium repens growing on a slit loam soil of pH 5.3 and 1% nitrogen and 0.13% phosphorus content.
The pasture vegetation was killed by herbicide spraying (glyphosate at 0.72 kg/ha and tribenuron-methyl at 22.5 g/ha) on 19 March 1990. Ground limestone was applied (15,000 kg/ha) to raise soil pH on 29 March.
Treatments: Eleven treatments were established in a randomised block design with four replicate (3.8 x 3.8 m) plots. Ten comprised monocutures (six grasses and four legumes) sown on 2 April 1990 at the following rates:
i) Phalaris aquatica - 8.5 kg/ha
ii) prairie grass Bromus catharticus - 43 kg/ha
iii) cock's-foot Dactylis glomerata - 6.6 kg/ha
iv) perennial rye grass Lolium perenne - 15 kg/ha
v) Yorkshire fog Holcus lanatus - 3.2 kg/ha
vi) tall fescue Festuca arundinacea - 16.8 kg/ha
vii) red clover Trifolium pratense - 5 kg/ha
viii) subterranean clover T.subterraneum - 9 kg/ha
ix) white clover T.repens - 3.4 kg/ha
x) lucerne Medicago sativa - 13.6 kg/ha
The 11 treatment was kept free of vegetation by hand weeding (excluding of C.nutans) as a control. A 2 x 1 m subplot was established in each plot in which total thistle seedling emergence and pasture species composition were assessed over a 26-month period. In the remainder of each plot, 10 thistles were monitored for subsequent growth rate, development and survivorship.
C.nutans seedling emergence was greater in some of the legume-sown treatments than in the grass-sown treatments for the first 8 months (especially M.sativa and T.subterraneum). Seedling emergence was negatively related to sown grass cover and the cover of annual meadow-grass Poa annua (which arose from the seed bank), with few differences between grass species. However, seedling emergence in bare ground plots was often substantially less than for the other treatments, probably as at least some vegetation cover was necessary to provide an adequate microclimate for establishment.
Pre-flowering mortality of thistles was lowest in the bare-ground treatment (2.5%), intermediate in the legume-sown treatments and highest in the grass-sown treatments and was strongly related to plot grass (including P.annua) content. All thistles which survived to flowering in the bare-ground and legume plots behaved as annuals, while 3 to 16% of thistles in the various grass-sown plots continued into their second year as rosettes, many flowering as biennials. No thistles survived a third year.
Flowering thistles in the bare ground plots grew larger than in the sown plots and produced substantially more capitula (bare ground: 245 capitula; compared to red clover, the next highest: 146 capitula; and Yorkshire fog, the lowest: 26 capitula). Generally, the thistles in the legume-sown plots also grew larger (37.5 cm stem diameter pre-flowering average) than those in the grass-sown plots (29.3 cm stem diameter pre-flowering average).
Conclusions: Grass-sown plots were likely to be invaded by a non-sown grass P.annua in the first few months following sowing, while the legume-sown plots tended to be invaded by broadleaved weeds. The sown grass - P.annua associations were substantially more effective than the sown legume - broadleaved weed associations in reducing nodding thistle invasion over the two year two month duration of the study.
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