Interference between pasture plants and thistles—a review

G.W. Bourdôt, AgResearch, PO Box 60, Lincoln, New Zealand.


The proposition that thistles may be controlled in pasture biologically by interference from neighbouring pasture plants is reviewed. Central to this approach is the hypothesis that the species composition and vegetation cover of a pasture influence birth and death rates in thistle populations, and hence also, population size. Experiments conducted mainly in New Zealand, Australia, USA and UK with species of Cirsium, Carduus, Silybum and Onopordum confirm that pasture grasses and legumes exert powerful inhibitory influences over seedling emergence and seedling and rosette survival in thistles. Pasture grasses, particularly perennial species, are generally more effective than legumes. This appears to be related mainly to their higher cover density, implicating competition as a mechanism, although there is some evidence that allelopathy may also operate. Lolium perenne, Holcus lanatus and Phalaris aquatica are particularly effective inhibitors of thistles. Pasture gaps play a key role in thistle seedling emergence and it is concluded that pasture management that promotes a dominant perennial grass component and an absence of small gaps is most likely to prevent thistle invasions. Research is needed on how grazing management and other factors including soil fertility and soil moisture influence the creation and closure of pasture gaps and the balance between grasses, legumes and thistles in pastures.


Plant Protection Quarterly (1996) 11 (Supplement 2) 265-270.