A mode of selective action of thiadiazolyl urea herbicides

1,1-Dimethyl-3-(5-tert-butyl-1,3,4-thiadiazol-2-yl)urea(I) was newly synthesized by the authors. (I) was found to have the strongest herbicidal activities among the thiadiazolylurea derivatives and selectivity of (I) was found between barley (tolerant)and wheat (susceptible) plants. Neither absorption by roots nor translocation from roots to shoots of 35S-labeled (I) in barley and wheat correlated with selectively between these species of plants. (I) was metabolized in both species of plants by N-demethylation to 1-methyl-3-(5-tert-butyl-1,3,4-thiadiazol-2-yl)urea(II), and further to non-phytotoxic 3-(5-tert-butyl-1,3,4-thiadiazol-2-yl)urea(III). The rates of the N-demethylating reactions from (I) to (II) and from (II) to (III) were much greater in barley shoots than in wheat shoots, especially in the second demethylation. It was concluded that the selective activity of (I) between barley and wheat plants was mainly due to the difference in rates of N-demethylation of (I). In addition, it was found that cotton plants, well known as tolerant to phenylurea herbicides, were remarkably susceptible to (I). It was indicated that different rates of the N-demethylating reaction between the phenyl and thiadiazolyl herbicides correlated with susceptibility to them.