Weeds resistant to chlorsulfuron and atrazine from the north-east grain region of Australia

The resistance of weeds to triazine and sulfonylurea herbicides has been recorded in several countries. The extent of the problem in the north-east grain region of Australia is uncertain. In an initial study, resistance to chlorsulfuron and atrazine in 15 weed species was investigated. The study showed that at least six of them have evolved resistance to one or other of these herbicides. Two collections of Rapistvum rugosum L., three of Sisymbrium orientale L., five of Sonchus oleraceus L., one of Fallopia convolvulus (L.) A. Love and one of Sisymbrium rhellungii O. Schultz were resistant to the recommended rate of chlorsulfuron (15 g a.i. ha^-1). Resistance to chlorsulfuron was not discovered in three collections of Sisymbrium L., two of Brassica tournefortii Gouan. three of Emex australis Steinh. and 11 of Phalaris paradoxa L. Two collections of Urochtoa panicoides Beauv. were resistant, and three collections possibly resistant, to the recommended rate of atrazine(1.8 kg a.i. ha^-1). Resistance to atrazine was not found in 12 collections of Hibiscus urionum L., nine of Salvia reflexa L., two of Parthenium hysterophorus L., two of Amaranthus viridis L. and 14 of Echinochloa colona (L) Link. The resistance status of four weeds (R. rugosum. F. convolvulus, S. thellungii and U. panicoides was confirmed using a multiple dose-response screen and is the first report of resistance for these species. Herbicide usage records show that resistance has developed after 3-10 years of selection with chlorsulfuron and 2-15 years of selection with atrazine, with no correlation between the frequency of use and the degree of resistance for any of the species where eight or more collections were made.