Genetic variation in herbicide resistance in scentless mayweed (Tripleurospermum inodorum (L.) Schultz Bip.) I. Differences between populations in response to MCPA

Experiments are described in which plants from six different populations of the outbreeding annual weed Tripleitrospermum inodorum were grown with barley in field plots and sprayed with 4-chloro-2-melhyIphenoxyacetic acid (MCPA) at a number of doses. The populations showed significant differences in reduction of weight, the most resistant population having an ED₅₀ of 181 ± 36·9 mg/m² a.e. MCPA, 2·49 limes as great as the ED₅₀ of the least resistant population. In a second experiment in which plants from these two populations were again grown with barley in field plots and sprayed with several doses of MCPA at three different stages of growth, their resistance to MCPA was found to differ consistently at all three stages of growth, the more resistant population having a mean LD₅₀ of 1103 ± 161·5 mg/m² a.e. MCPA, 2·09 times as great as the mean LD₅₀ of the less resistant population. The populations were similar in other respects. The differences in the spraying histories of the two populations were consistent with the supposition that the differences in resistance to MCPA had resulted from natural selection for MCPA resistance in the more intensively sprayed population, but other processes may also have been involved.