A whole-plant bioassay system for the detection of residues of ALS/AHAS inhibiting herbicides in soils and determination of safe planting intervals for sensitive crops
In Ontario, the ALS/AHAS herbicides, including the sulfonylureas, imidazolinones and triazolopyrimidines, are used to control a wide range of weeds in many crop species. These herbicides have very high biological activity (i.e. dose ranges of g ai ha-1 vs. kg ai ha-1 for older generation chemistries) which has potential to cause significant negative impact to non-target plants or subsequently planted sensitive crops such as ' Beta vulgaris' (sugar beet), and 'Brassica spp.' (canola and cole crops) among others. The injury to sensitive crops varies with herbicide type as well as climatic and edaphic factors. Sulfonylurea herbicides degrade primarily by chemical hydrolysis and tend to persist in alkaline soils. Imidazolinone herbicides degrade primarily by microbial activity and are more persistent in acidic soils. This research was conducted to develop a whole-plant bioassay system to detect injurious concentrations of these herbicides in different soil types across southern Ontario. A loam soil was fortified with 0.0, 0.01, 0.05, 0.1, 0.5, 1, 2.5, 5 and 10 [mu]kg-1 of each herbicide into which indicator crops of sugar beet, canola, rutabaga and sweet corn were seeded and grown for 30 days under optimized growth room conditions. Plant dry weight data were subjected to a log-logistic regression analysis to determine the I50 concentrations of each herbicide by indicator species combination. I50 values for chlorimuron were 0.11 (se<0.005), 0.09 (se<0.004), and 0.10-0.11 (se<0.009) [mu]g kg-1 for canola, rutabaga and sugar beet, respectively. For flumetsulam the I 50 values were 0.95-1.10 (se<0.075), 0.69-0.85 (se<0.062), and 0.67-1.76 (se<0.139) for canola, rutabaga and sugar beet, respectively while I'50' values for imazethapyr were 0.42-0.65 (se<0.057), 0.54-0.91 (se<0.071), and 0.30-0.46 (se<0.032) [mu]g kg-1 respectively, for canola, rutabaga and sugar beet. The whole-plant bioassay system was utilized to determine the in situ concentration of chlorimuron and imazethapyr in field samples collected at various intervals following field application to a soybean crop in 1998 or 1999 in four-replicate RCBD experiments at five sites across Ontario. Each of the sites differed in soil pH and other edaphic factors. Growth response of sugar beet in herbicide-treated field soil samples was compared to general and soil-specific dose-response curves developed by spiking untreated site-specific soil with 0.0, 0.05, 0.1, 0.5, 1.0, 2.5 [mu]g kg-1 of each herbicide. The results indicate that the degradation rate of chlorimuron and imazethapyr varied by site but were similar within a site, reflecting differences in soil conditions among sites (pH, texture, OM, etc). However, three years following herbicide application there was no difference in herbicide concentration detected by bioassay among the sites. I10 values would be a more appropriate economic indicator of acceptable crop injury for these herbicides and were estimated to be 0.005-0.090 and 0.067-0.405 pg kg-1 for chlorimuron and imazethapyr, respectively, using sugar beet as the indicator species. The whole-plant bioassay system was developed and validated across a range of southern Ontario soils and was able to detect injurious concentrations of chlorimuron and imazethapyr that were at levels less than the detection limits of liquid chromatography and capillary electrophoresis. This bioassay system can be used to determine when sensitive crop species can safely be seeded into sites previously treated with the ALS/AHAS herbicides.