Morphological and anatomical effects of mef enacet on the growth of barnyardgrass (Echinochloa oryzicola Vasing.) were investigated in petri-dish and pot tests. In the petri-dish, mef enacet did not inhibit weed germination. After germination, however, leaves failed to emerge through the coleoptile, and the primary root was reduced at high concentrations, 10 and 5 ppm. At low concentrations, 0.5 and 0.1 ppm, the growth of shoot, root and coleoptile was slightly inhibited, while the mesocotyl was not affected by the herbicide. In the preemergence treatment in pots, mef enacet did not inhibit the elongation of mesocotyl or coleoptile, however, it severely retarded the growth of 1st and 2nd leaf primordia and length of the primary root. When the weed was treated at 1, 2 and 3 leaf stages, the growth of primordia of subsequent leaves was retarded, and the elongation of coronal roots was inhibited. Anatomical results showed that mefenacet clearly exerted a strong inhibitory action against meristems, but had little effect on existing plant tissue. When applied at the 2-leaf stage, the growth of subsequent leaves was strongly retarded, as was the leaf sheath of the 3rd leaf. The upward and marginal growth of the 3rd and 4th leaves was inhibited and the growth of primordia of the 5th and 6th leaves was also severely inhibited. The growth of epidermal cell was affected and the intercellular spaces of the mesophyll did not form extensive lacunae as shown in the untreated plant. At the 3-leaf stage application, the growth of primordia of the 4th, 5th and 6th leaves was retarded and abnormal swelling of the 1st, 2nd and 3rd tillers was observed.

1‐Phenyl‐5(4H)‐4‐dialkylcarbamoyltetrazolinones represent a new class of herbicides that are active against barnyardgrass (Echinochloa spp.) in paddy fields. To select the most favorable compound as a paddy herbicide, herbicidal activity and selectivity of the carbamoyltetrazolinone compounds to transplanted rice were determined in greenhouse tests. Higher herbicidal activities were associated with the compounds having a 2‐Cl‐ or 2‐Cl‐6‐Me‐substituted or an unsubstituted phenyl ring, and a combination of C2–3 linear alkyl group and a C4 branched‐chain alkyl or a C5–6 cycloalkyl group on the nitrogen atom in the carbamoyl structure. In addition, the compounds carrying a total of seven to nine carbon atoms in the substituents on the carbamoyl nitrogen exhibited a good selectivity between barnyardgrass and transplanted rice plants. From the results of soil thick‐layer chromatography, the good crop compatibility of carbamoyltetrazolinones was correlated to the mobility of the compounds in paddy soil. The investigation resulted in the selection of an excellent compound, 4‐(2‐chlorophenyl)‐N‐cyclohexyl‐N‐ethyl‐4,5‐dihydro‐5‐oxo‐1H‐tetrazole‐1‐carboxamide (common name: fentrazamide), as a rice paddy herbicide for controlling barnyardgrass.

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The effects of fentrazamide [4-(2-chlorophenyl)-N-cyclohexyl-N-ethyl-4,5-dihydro-5-oxo-1H-tetrazole-1-carboxamide] on the growth and morphology of Echinochloa crus-galli (L.) Beav. and Echinochloa oryzicola Vasing. were investigated. Fentrazamide at 250 g a.i. ha Ϫ1 showed high efficacy on weeds up to the 3-leaf stage. The growth of the subsequent leaves was retarded by the herbicide and dark green coloration appeared, and then the basal part of leaf sheathes underwent necrosis. The cell elongation and cell division of Echinochloa spp. were inhibited by fentrazamide. These effects were also observed on plants treated with mefenacet [2-(2-benzothiazoyloxy)-N-methyl-N-phenyl-acetamide].