Evolution and spread of herbicide resistant barley grass (Hordeum glaucum Steud. and H. leporinum Link.) in South Australia

The barley grasses (Hordeum glaucum Steud. and Hordeum leporinum Link.) are important weeds of crops and pastures in South Australia. Populations of both species have evolved resistance to paraquat, primarily following intensive use of paraquat for winter weed control in lucerne (Medicago sativa L.) crops. In the past few years, agricultural consultants have been reporting an increase in problems with controlling Hordeum spp. populations in lucerne crops in South Australia. This research was conducted to determine the relative importance of seed movement compared with independent evolution for paraquat resistance in Hordeum spp. Hordeum glaucum and H. leporinum seed was collected from lucerne fi elds and fi elds immediately adjacent to the lucerne fi elds at several sites in South Australia. The seed was tested to determine which fi elds contained resistant populations. As H. glaucum and H. leporinum are self pollinated, RAPD technology was used to examine diversity within and between populations. Individual resistant populations contained much less genetic diversity than did adjacent susceptible populations indicating the existence of a selection bottleneck caused by herbicide use. Most resistant populations were different genotypes; however, two resistant populations separated by 7 km appeared to be the same genotype. These results suggest that both independent evolution and seed movement are important in the distribution of paraquat-resistant Hordeum spp. in South Australia.