Quantitative bioassays for determining residues and availability to plants of sulphonylurea herbicides

Summary A bioassay procedure for quantitative determination of sulphonylurea herbicides is described. Turnips (Brassica rapa) were found very suitable as test plants and gave results within 10 days. Six sulphonylurea compounds were investigated for their activity in three widely differing soils. The potential availability to plants was calculated from the dose‐response curves of vermiculite (non‐sorptive substrate) and the corresponding ED50‐values of the soils. The dose‐response relationship (logistic curve) was described by a computer model by a position parameter, the slope of the curve and the minimum and maximum fresh weights of plants. The limit of quantitative detection in the range of ED30 in vermiculite was 0·06 μg 1−1 for sulfometuron and 1·03 μg 1−1 for DPX‐L5300, methy12‐([4‐methoxy‐6‐methyl‐1,3,5‐triazin‐2‐yl (methyl)carbamoyl]‐sulphamoyl) benzoate. Results with turnips showed that sulfometuron was the most active compound in all substrates (ED50 in vermiculite 0·12 μg 1−1) followed by chlorsulfuron, metsulfuron‐methyl, triasulfuron, DPX‐M6316, methyl 3‐([(4‐methoxy‐6‐methyl‐1,3,5‐triazin‐2‐yl)aminocarbamoyl]‐aminosulphaphamoyl)‐2‐thiophenecarboxylate, and DPX‐L5300 which had ED50 or 1·98 μg 1−1, The Horotiu sandy loam soil showed the highest ED50‐values and the lowest plant availability for all compounds compared to the other soils. Probit and logistic evaluation methods for deriving dose‐response relationships are compared and their applicability is discussed. Résumé Essai biologique pour lu détermination quantitative des sulfonyurées et leur disponibilité potentielle pour les plantes Un protocole d'essai biologique pour la détermination quantitatives des herbicides sulfonylurées est décrite. Les navets (Brassica rapa) conveniennent parfaitement comme plantes‐tests et donnent des résultats sous 10 jours. Six sulfonylurées différentes ont vu leur efficacité testée dans trois sols bien différents. La disponibilité potentielle pour les plantes a été calculée à partir des courbes d'effets doses sur vermiculite (substrat non absorbant) et les valeurs correspondantes efficacité 50 des sols. La relation effet‐dose a été décrite par un modèle mathématique incluant un paramètre de position, la pente de la courbe et son minimum et les poids frais initiaux des plantes. La limite de détection quantitative pour l'efficacité 30 dans la vermiculite était de 0,06 μg 1−1 pour le sulfometuran et 1,03 μg 1−1 pour le DPX‐5300, methyl 2‐([4‐methoxy‐6‐methyl‐1,3,