Relative intra-species competitive ability of nodding thistle biotypes with varying resistance to the herbicide 2,4-D

Resistance of pasture weed species to phenoxy herbicides is emerging as a potential problem in New Zealand and is especially apparent in the case of nodding thistle (Carduus nutans). We selected eight biotypes of nodding thistle known to differ in resistance to the herbicide 2,4-D and evaluated their relative competitive abilities during vegetative growth in terms of their effects on growth of the other biotypes, growth response to the other biotypes, and growth and seedling emergence responses to pre-established swards of perennial ryegrass (Lolium perenne). Although biotypes differed in terms of competitive ability for each of these measures, there was no detectable agreement between any pair of measures across the eight biotypes. None of the measures of competitive ability was significantly correlated with herbicide resistance across the biotypes. Our results show no clear causative relationship between competitive ability as measured in this experiment and herbicide resistance. Since resistant populations are apparently of comparable competitive ability to susceptible ones, cessation of herbicide use is unlikely to be followed by a significant loss of resistant individuals from a population.