Improved PADDY model including photoisomerization and metabolic pathways for predicting pesticide behavior in paddy fields: Application to the herbicide pyriminobac-methyl

Abstract

Rice production is one of the major non-point sources of pesticide pollution in Japan because more than half of all arable land is paddy fields, from which pesticides can easily be transported to rivers. Public concerns regarding the adverse effects of pesticides in runoff on aquatic ecosystems and drinking water are increasing. In 2003, the Ministry of the Environment of Japan imposed a new pesticide registration scheme concerning ecological risk in an aquatic environment based on concepts adopted in the EU and the USA. In this scheme, the acute effect concentration in the assessment species (i.e., fish, Daphnia, or algae) is compared with the predicted environmental concentration calculated using environmental models based on the standard scenario of the pesticide use. 1)Recently, mathematical models (e.g., PADDY 2,3) and PCPF-1 4-6) ) have been developed to simulate the transport and fate of paddy rice pesticides in Japan; however, these models do not consider metabolites derived from the parent compound by hydrolysis, photolysis, or microbial degradation. The RICEWQ model, developed in the EU and the USA, is the only one that considers metabolites under paddy conditions. 7)The metabolites of some pesticides, such as organophosphorus insecticides, may be more toxic to aquatic organisms than the parent compound. Therefore, for ecological risk assessment it is necessary to evaluate the exposure of aquatic organisms not only to the parent compound but also to its metabolites. PADDY 2,3) is used to predict pesticide behavior in paddies. The objective of this study was to develop and evaluate an improved PADDY-based model that includes photoisomerization and metabolic pathways for the herbicide pyriminobacmethyl [methyl 2-(4,6-dimethoxy-2-pyrimidinyloxy)-6-(E,Z)-(1-methoxyiminoethyl)benzoate], which is used for selective control of early water grass. To validate the model, values calculated by the improved model were compared with measured values from paddy lysimeter and field experiments. Theory, Materials, and Methods ChemicalsPyriminobac-methyl herbicide consists of a mixture of its (E)-isomer (I) and (Z)-isomer (II) as active ingredients, with an (E)/(Z) ratio of ca. 5/1 in a technical grade of the active ingredient. The photo-transformation of this compound is the Improved PADDY model including photoisomerization and metabolic pathways for predicting pesticide behavior in paddy fields:Application to the herbicide pyriminobac-methyl Kumiai Chemical Ind. Co., Ltd., Tamari, Kakegawa, Shizuoka 436-0011, Japan (Received May 16, 2009; Accepted September 17, 2009) An improved simulation model for predicting pesticide behavior in paddy fields based on PADDY was developed to include photoisomerization and metabolic pathways and applied to the herbicide pyriminobac-methyl. Isomerization of pyriminobac-methyl in paddy water by sunlight was modeled as a reversible first-order reaction between the (E)-and (Z)-isomers in response to UV-B irradiation. The formation and degradation of its main metabolites in flooded so...

Herbicide discharge from rice paddy fields by surface runoff and percolation flow: A case study in paddy fields in the Lake Biwa basin, Japan

“…34) The K oc was reported as 220-1300 35) and 372-741 31) for pyriminobac-methyl, 160 36) and 754 34) for imazosulfuron, and 161-362 37) for pyraclonil, which is comparable to the values presented in the current study.…”

“…31) The (E)-and (Z)-isomers can be analyzed separately using a GC-MS; however, pyriminobac-methyl concentrations in this study are described as the sum of the (E)-and (Z)-isomer.…”

“…Previous reports [31][32][33][34] showed that the elimination rate constant of herbicide concentrations in paddy water was assumed to follow biphasic first-order kinetics; an initial rapid-elimination phase followed by a second, slow-elimination phase. In the current study, the turning points between the two phases for herbicides were set to 10 days after application.…”

Behavior of isoprothiolane and fipronil in paddy water, soil, and rice plants after nursery-box or submerged applications

“…29) Daily percolation and discharge of paddy water were calculated from a water-balance equation 20) using the measured water-balance components. Figure 2 shows the field management and the sampling schedule for the paddy water, soil, and rice plants.…”

“…We also discuss how the key factors that determine mass distribution of the pesticides in the root-zone and inter-plant areas could be used to improve the previously developed PADDY model [18][19][20] for simulating the behavior of nursery-box-applied pesticides in paddy fields. 21) Materials and Methods…”