RELATIVE INTRASPECIES COMPETITIVE ABILITY OF NODDING THISTLE BIOTYPESWITH VARYING RESISTANCE TO THE HERBICIDE 2,4-D

Resistance of pasture weed species to phenoxy herbicides is emerging a s 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, gro wth response to the other biotypes, and growth and seedling emergence responses to pre-established swards of perennial ryegrass (Lolium pere nne). Although biotypes differed in terms of competitive ability for e ach of these measures, there was no detectable agreement between any p air of measures across the eight biotypes. None of the measures of com petitive ability was significantly correlated with herbicide resistanc e across the biotypes. Our results show no clear causative relationshi p between competitive ability as measured in this experiment and herbi cide resistance. Since resistant populations are apparently of compara ble competitive ability to susceptible ones, cessation of herbicide us e is unlikely to be followed by a significant loss of resistant indivi duals from a population.