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A study of the quantitative analysis of herbicide residues by both chemical and bioassay methods in soils is presented. Field and laboratory residue trials were carried out with a representative member of the following groups of herbicides: ureas, triazines, diphenylethers, phenoxyacetic acids, and dithiophosphates. Representative samples were taken at different time intervals, and degradation curves were established both by chemical methods and by two types of bioassay. Chemical analysis either separated active ingredient and metabolites by chromatographic techniques or compresied total residues. Bioassays were performed using either monocotyledons and dicotyledons or algae. The results obtained by chemical and bioassay analysis for the degradation rates of chlorotoluron, ametryn, 2,4-D and C 19490 showed a correlation coefficient of 0.914, indicating that the two methods gave almost identical results. Especially with the highly adsorbed urea and triazine herbicides, the uptake of biologically active material by test plants was slightly less than the solvent-extractable parent compound plus its metabolities, and so the absolute level of residues obtained by bioassay was lower. In the case of fluorodifen, the correlation between the methods was not established. The bioassay showed higher residues and slower degradation than chemical analysis. Various factors which could explain this anomalous result are discussed.

CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] applied as a seed treatment enables sorghum [Sorghum bicolor(L.) Moench] to tolerate metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide]. Improved tolerance of alexandergrass [Brachiaria plantaginea, (Link) A. Hitchc.]Eleusinespp., proso millet (Panicum miliaceumL.) and rice (Oryza sativaL.) to metolachlor also was demonstrated. Seven sorghum cultivars, representing widely divergent types of germplasm, responded similarly to the safening effect of CGA-43089 when the cultivars were exposed to metolachlor. Treated sorghum seed may be stored more than 1 yr without a loss of safening activity. In these growth chamber experiments, neither extremes of soil moisture nor temperature reduced the effectiveness of the safener. The predominant site of uptake of CGA-43089 is the coleoptile of sorghum seedlings.

Summary In a collaborative study by 12 laboratories the reproducibility of bioassay techniques was analysed by comparison and statistical treatment of the ED50‐values (μg herbicide/g soil). Commonlyused bioassay techniques were investigated; two direct seeding methods, a transplanting method and a shoot extension method. The herbicides studied were two photosynthesis inhibitors, atrazine and metribuzin using Lepidium sativum and Brassica rapa as test plants; two germination inhibitors, tri‐allate and trifluralin using Avena sativa as test plant. The mean ED50‐value of atrazine in the direct seeding method was 0.18 with a variation from 0.12 to 0.29 and 0.12 (0.07–0.68) in the transplanting method. The values of metribuzin were 0.05 (0.02–0.11) and 0.03 (0.01–0.15) respectively. The values for triallate were 1.15 (0.50–2.60) in the direct seeding method and 1.10 (0.54–2.53) in the shoot extension method and for trifluralin 3.11 (0.72–18.31) and 1.33 (0.48–2.94) respectively. The reproducibility was best in the shoot extension method. No outliers (confidence intervals lying entirely outside the confidence interval of the overall mean) in the ED50‐values were observed with atrazine and metribuzin when using the direct seeding method, whereas with the germination inhibitors a high number of results (four with tri‐allate, six with trifluralin) could be considered as outliers. A comparison of the use of fresh weight and dry matter showed good agreement between the ED50‐values they gave, with similar reproducibility.