Influence of Pasture Forage Species on Seedling Emergence, Growth and Development of Carduus nutans

1. A study on the interactions between Carduus nutans L. (nodding thistle), and pasture grasses and legumes was conducted in a New Zealand perennial pasture. 3.8 × 3.8 m plots were sown with each of 10 forage species (six grasses and four legumes), or maintained free of pasture (bare ground plots). A 2 × 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 labelled and monitored for subsequent growth rate, development and survivorship. 2. Thistle seedling emergence was greater in some of the legume-sown treatments than in the grass-sown treatments for the first few months of the study. Seedling emergence was negatively related to sown grass cover and the cover of volunteer Poa annua L. in the plots. However, seedling emergence in the bare ground plots was often substantially less than for the other treatments, probably because at least some pasture cover was necessary to provide an adequate microclimate for establishment. 3. The preflowering mortality of thistles was lowest in the bare-ground treatment, intermediate in the legume-sown treatments and highest in the grass-sown treatments. All thistles which survived to flowering in the bare-ground and legume plots behaved as annuals, while between 3 and 16% of thistles in the various grass-sown plots continued into their second year as rosettes, and many of these flowered as biennials. Preflowering thistle mortality was strongly related to the grass (including P. annua) content of the plots. 4. Flowering thistles in the bare ground plots grew larger than in the sown plots and produced substantially more capitula. Generally, the thistles in the legumesown plots also grew larger than those in the grass-sown plots. 5. Grass-sown plots were likely to be invaded by 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 appear to be substantially more effective than the sown legume ― broadleaved weed associations in reducing the problem of nodding thistle invasions