Influence of commercial formulation on the sorption and leaching behaviour of propyzamide in soil

- Weaker sorption of formulated pesticide for centrifugation method but not for batch.
- Greater leaching of propyzamide from commercial formulation than for technical.
- Enhanced leaching from formulation consistent across soils and time points.
- Very strong increase in sorption and decrease in leaching with time.
- Assessments of pesticide fate that ignore co-formulants may be over-simplified.

Experiments compared sorption and leaching behaviour for the herbicide propyzamide when applied to two soils either as technical material or in the commercial formulation Kerb® Flo. Sorption was investigated in batch systems as well as using a centrifugation technique to investigate changes in pesticide concentration in soil pore water over incubation periods of up to 28 days. Studies with small soil columns compared leaching of technical and formulated pesticide for irrigation events (6 pore volumes) 1, 7, 14, 21 and 28 days after treatment. There were no differences in sorption of technical and formulated propyzamide when measured by batch systems. Sorption of technical material was significantly greater than that of formulated pesticide in sandy loam (p < 0.05), but not in sandy silt loam when measured by centrifugation of soil incubated at field capacity. Partition coefficients measured by batch and centrifugation methods were similar after 1 day and those measured by centrifugation increased by factors of 5.3 to 7.5 over the next 4 weeks. The mass of propyzamide leached from soil columns ranged between 1.1 ± 0.33% and 14.4 ± 3.2% of the applied amount. For all time intervals and in both soils, the mass of propyzamide leached was significantly greater (two-sided t-tests, p < 0.001) for the formulated product than for the technical material. Leached losses decreased consistently with time in the sandy loam soil (losses after 28 days were 14-17% of those after 1 day), but with less consistency in the sandy silt loam. There was a highly significant effect of formulation on the leaching of propyzamide through soil (two-way ANOVA, p < 0.001) as well as highly significant effects of time and soil type (p < 0.001). Results are consistent with modelling studies where leaching from commercial products in the field could only be simulated by reducing sorption coefficients relative to those measured with technical material in the laboratory.

164