| Abstract: | MSMA (monosodium methylarsonate) is non-volatile and degrades in the environment to arsenic species which can be determined at the nanogram level. It is thus an excellent tracer for the study of the efficiency of deposition (defined as the fraction of material sprayed that deposits on target foliage) and subsequent fate of a water-soluble pesticide applied to foliage. MSMA was applied to rice (Oryza sativa L. cv. Labelle) foliage at two rates and at various stages of growth. After spraying the foliage was sampled and analysed for surface (removable by water-washing) and absorbed MSMA. Absorption of MSMA was so rapid that within the two hours between application and washing nearly one-half of the recovered MSMA became unavailable for wash-off. The total amounts of MSMA recovered were approximately proportional to plant size and application rate. When the rice foliage reached approximately 80 cm in height, the canopy closure (ground cover) was complete; nevertheless, only about 50% of the sprayed MSMA was found on and in the plants. As the rice canopy approached maturity, or approximately 130 cm high, complete interception of the spray occurred. These results indicate that canopy volume rather than ground cover was more important in determining the efficiency of spray deposition in this case, and that deposition efficiency may approach 100% with non-volatile active ingredients. |
