Persistence,penetration, and breakdown of chlorthiamid and dichlobenil herbicides in field soils of different types

The persistence, penetration, and breakdown of the herbicides chlorthiamid (‘Prefix’, 2,6-dichlorothio-benzamide) and dichlobenil (‘Casoron’, 2,6-dichlorobenzonitrile) have been studied in the field in clay, medium loam, sandy loam, and peat using applications of unincorporated granules and wettable powders at 9 kg/ha (active material). Conversion of chlorthiamid to dichlobenil was rapid in the soil. After 4 weeks, less than 3% remained of chlorthiamid that was applied to clay, medium loam, and sandy loam and less than 17% remained in peat. The initial half-life of dichlobenil residues (following dichlobenil application) and ‘total nitrile’ residues (chlorthiamid + dichlobenil residues following chlorthiamid application) under wet conditions was near 2 weeks in clay, medium loam, and sandy loam, and ranged from 3-20 weeks in peat for different applications. There was very little penetration of chlorthiamid or dichlobenil below the 4 in depth of the soil within 32 weeks of the single application of herbicides except in peat. 2,6-Dichlorobenzamide was formed in all four soil types after application of either chlorthiamid or dichlobenil. At 8-10 weeks after application the benzamide residues in the soil were generally greater than those of the dichlobenil, except in peat. In clay, which was studied in most detail, the benzamide penetrated to greater depths than did the dichlobenil or chlorthiamid.