A number of related 1,3,4-thiadiazole derivatives have been synthesized and their herbicidal activities have been evaluated by pre-emergence, postemergence, and irrigated water treatment methods. Some l-substituted-3-(5-substituted-l,3,4-thiadiazol-2-yl)ureas showed excellent herbicidal activity. This activity depends upon the molecular size of the substituent in the 5-position of 1,3,4-thiadiazole ring. In particular, activity is maximized when the substituent is the tertiary-butyl group, while replacement of the O atom in the urea moiety with a S atom resulted in decreased herbicidal activity. Further correlations of chemical structure to herbicidal activity are discussed.
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