Breakdown of the herbicide 14 C‐chlorthiamid II. —Glasshouse studies of breakdown in soils and in crops grown in the soils

Plants were grown to harvest in a glasshouse in soils treated once with 14 C‐chlorthiamid (2,6‐dichlorothio‐benzamide, ‘Prefix’) at dosage levels (1‐3 lb/acre active material) near those recommended for commercial field use. Rice plants (stems and leaves) grown in soil treated at 1 lb/acre contained dichlobenil (2,6‐dichlorobenzonitrile, ‘Casoron’, 0.03 ppm) and 2,6‐dichlorobenzamide (0.06 ppm) and the grain itself showed a total residue of 14 C‐components of only 0.06 ppm (expressed as equivalent weight of chlorthiamid) at harvest. Winter wheat plants (stalks and leaves) grown in medium loam treated at 1.2 lb/acre contained residues of 2,6‐dichlorobenzamide and free and conjugated 3‐hydroxy‐2,6‐dichlorobenzamide, together with traces of sugar conjugates of 3‐hydroxy‐and 4‐hydroxy‐2,6‐dichlorobenzonitrile. The fruit of apple grown in a glasshouse in soil treated once at 1.5 lb/acre, showed no detectable residue (<0.004 ppm) in spite of total residues of up to 2.4 ppm in the leaves. The latter probably resulted from sublimation of dichlobenil from the soil. Such a mode of uptake would be unlikely to lead to as much residue in the leaves under field conditions. Traces of dichlobenil (0.005 ppm), 2,6‐dichlorobenzamide (0.02 ppm) with larger amounts of glycosides of 3‐hydroxy‐2,6‐dichlorobenzamide (1.3 ppm) and 3‐hydroxy‐ and 4‐hydroxy‐2,6‐dichlorobenzonitriles (0.6 ppm and 0.5 ppm respectively) were identified in the leaves at harvest. All of the breakdown products of chlorthiamid that have been identified are of low acute oral mammalian toxicity.