Objective of the study

Metabolism of the pesticide includes the processes of transport or translocation within the organism as well as transformation to metabolites or degradation products.

Metabolism studies are conducted to determine the qualitative metabolic fate of the active ingredient and elucidate its metabolic pathway. Many pesticides undergo change during and after application to plants or plant products. The composition of the terminal residue must, therefore, be determined before the residue analytical methodology can be developed and residues quantified.

In metabolism studies, the pesticide is applied to crops in amounts and for durations of time that could occur in practice when the compound is used for pest control (i.e. the existing or intended good agricultural practices (GAPs)). In the case of crops, feed and food commodities are harvested after an interval expected under good agricultural practice.

Further information on studies on metabolism of residues in plants can be found in Chapters 3.2 and 5.3 of the JMPR Procedure Manual.

Worked examples on this topic are provided in Chapter 5 of the JMPR Training Manual.

Circumstances in which the study is required

These studies are required whenever a plant or plant product is treated, unless no part of the plants or plant products will be used as food or feed material, or unless a “zero residue situation” applies (such as bait applications).

Test organism or substrate

A metabolism study should be submitted for each type of crop group for which use is
proposed. Crops can be considered to belong to one of five categories for crop metabolism studies:

• root crops (root and tuber vegetables, bulb vegetables)
• leafy crops (Brassica vegetables, leafy vegetables, stem vegetables, hops)
• fruits (citrus fruit, pome fruit, stone fruit small fruits, berries, grapes, banana, tree nuts, fruiting vegetables, persimmon)
• pulses and oilseeds (legume vegetables, pulses, oilseeds, peanuts, legume fodder crops, cacao beans, coffee beans)
• cereals (cereals, grass and forage crops).

One crop from a group will cover the entire group for purposes of metabolism in those crops within the group. In order to extrapolate metabolism of a pesticide to all crop groupings, metabolism studies on a minimum of three representative crops (from the five different crop categories) should be conducted. If the results of these three studies indicate a comparable metabolic route, then additional studies will not be needed on crops in the other two groups.

Test substance

The crop should be treated with radiolabelled active ingredient, preferably containing formulation ingredients typical of an end-use product as applied in the field.

Typical endpoints of the study

• Portion of extractable and non-extractable radioactivity in edible tissues, as % of total radioactive residues (TRR) and mg parent compound equivalent/kg sample material;
• Portion of characterised but not identified radioactivity in edible tissues and potential feed items;
• Identification of metabolites, described by their systematic chemical name – for all extractable residues at >10% TRR or at >0.05 mg/kg;
• Characterization of metabolites – for all extractable residues at <10% TRR or at 0.01- 0.05 mg/kg;
• Presence of metabolites in conjugated form, extractability of conjugated metabolites;
• Schematic diagram of the metabolic pathways in plants with a brief explanation of the distribution and chemical reactions involved;
• Translocation of the radioactivity within plant tissues. Identification of translocated radioactivity (perhaps only some metabolites translocate from the point of application).
• Proportion of residues which can be removed by surface wash.
• Distribution of radioactivity in peel and pulp, or various plant parts ( e.g. forage, stover, grains, husk;

Testing guidelines

The following testing guidelines may be used for determination of the nature of residues in plants:

• OECD Guidelines for the Testing of Chemicals. Test No. 501: Metabolism in crops.
• USEPA-OCSPP Harmonized Test Guidelines. Test No. OPPTS 860.1300. Residue Chemistry Test Guidelines – Nature of the residue: plants, livestock.
• European Commission. Guidelines for Residue Data – Metabolism and distribution in plants. (Technical Guidelines - Appendix A)