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The effectiveness of crop competition for better weed control and reducing herbicide rates was determined for Avena ludoviciana and Phalaris paradoxa. Four experiments, previously broadcast with seeds of the two weeds in separate plots, were sown with three wheat densities, and emerged weeds were treated with four herbicide doses (0-100% of recommended rate). The measured crop and weed traits were first analysed across experiments for treatment effects. Grain yield and weed seed production data were then analysed using cubic smoothing splines to model the response surfaces. Although herbicide rate for both weeds and crop density for P. paradoxa had significant linear effects on yield, there was a significant non-linearity of the response surface. Similarly, herbicide rate and crop density had significant linear effects on weed seed production, and there was significant non-linearity of the response surface that differed for the weed species. Maximum crop yield and reduction in seed production of P. paradoxa was achieved with approximately 80 wheat plants m[sup -2] and weeds treated with 100% herbicide rate. For A. ludoviciana, this was 130 wheat plants m[sup -2] applied with 75% herbicide rate. Alternatively, these benefits were achieved by increasing crop density to 150 plants m[sup -2] applied with 50% herbicide rate. At high crop density, application of the 100% herbicide rate tended to reduce yield, particularly with the A. ludoviciana herbicide, and this impacted adversely on the suppression of weed seed production. Thus, more competitive wheat crops have the potential for improving weed control and reducing herbicide rates. [ABSTRACT FROM AUTHOR]

The resistance of weeds to triazinc 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 Rapistrum rugosum L., three of Sisymhriam orientate L., five of Sonchus oleraceus L., one of Fallopia convolvulus (L.) A. Love and one of Sisymhrium thellungii 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 irio L., two of Brassica tournefortil Gouan, three of Emex australis Steinh. and 11 of Phalaris paradoxa L. Two collections of Urochloa 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 trionum L., nine of Salvia reflexa L., two of Parthenium hysterophorus L., two of Amaranthus viridis L. and 14 of Echinochioa 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. [ABSTRACT FROM AUTHOR]