Factors affecting the performance and crop phytotoxicity of a new rice herbicide,cinmethylin. I. Effects of water depth and soil type on the distribution and uptake of cinmethylin by transplanted and direct-seeded rice

Following successful glasshouse evaluation, first-year field trials showed that cinmethylin could give excellent control of barnyard grass (Echinochloa crus-galli) in rice paddies. However, for the successful development of a herbicide in rice, a thorough understanding of its distribution over time between water, soil and plants as affected by water depth and soil type is necessary. An application of an emulsifiable concentrate formulation to laboratory paddies containing transplanted rice showed that more of the cinmethylin became associated with and moved into the soil of shallow paddies than in the more usual deep paddies. After the first day this resulted in slightly higher concentrations of cinmethylin being accumulated by the apical meristem, the site of action of cinmethylin, in shallow-paddy plants and explains the higher phytotoxicities observed in this situation. Rapid removal of cinmethylin by the soil from the paddy water could reduce that available for absorption by the apical meristem of barnyard grass, just below the water surface, and might result in poorer control of this weed. Deeper paddies are therefore recommended for the use of cinmethylin in transplanted rice. An experiment involving direct-seeded rice confirmed that a decrease in paddy water depth and higher organic-matter soils increased the association of cinmethylin with the paddy soil. Thus paddy-water concentrations and availabilities of cinmethylin to various parts of plants were influenced by water depth and soil type. In contrast to transplanted rice, the apical meristem of direct-seeded rice is submerged in the paddy water and the stem concentrations of cinmethylin were then generally correlated with the paddy-water concentrations. They were highest from the shallow, sandy paddies and lowest from the deep, silt clay loam paddies. These relative concentrations correlate with observed relative phytotoxicities of direct-seeded rice, activities against barnyard grass and the narrower selectivity margins between the weeds and the direct-seeded crop. The cinmethylin concentrations in the apical meristems of the transplanted rice were always lower than those from the direct-seeded rice and give an illustration of a classical case of depth protection' of a crop from an applied herbicide.