Residue Studies - Conduct of Trials

John Unsworth
7th April 2010

Residues remaining in or on plants are often unavoidable even when the pesticide has been applied according to Good Agriculture Practice (GAP). Residue trials are necessary to determine the level of residue and thus any potential harm to humans and animals. Supervised residue trials should be designed so that the highest likely residues are present e.g. by using the highest application rate and shortest pre-harvest interval (PHI) consistent with the GAP to mimic possible worst case scenarios. Trials also need to be conducted in different locations and in more than one season to take account of environmental and climatic variations.

The EU has published a comprehensive guideline1 on “General Recommendations for the Design, Preparation and Realisation of Residue Trials”. Further information can be found in an FAO document2 “Guidelines on Pesticide Residue Trials to Provide Data for the Registration of Pesticides and the Establishment of Maximum Residue Limits”. Further information can be found for Australia3 and the USA4.


The topics covered are:

·         Design of residue trials

·         Application of the plant protection product

·         Records of plant growth and development data

·         Records of weather data

·         Records of soil data

·         Samples


The main aim of residue trials is to quantify the highest likely residue level in treated crops at harvest or on removal from store following the proposed good agricultural practice (GAP). The trials must be designed and implemented to take into account even the worst case residue levels which might be encountered when applying the product according to the critical GAP.


Design of Residue trials


The design of residue trials should take into account the proposed mode of application, the sampling procedure

and the analysis. In order to set Maximum Residue Levels and pre-harvest intervals, results based on several trials carried out in different localities and under different geo-climatic conditions are required. Trials should be carried out in major areas of cultivation or production and should be sited to cover the range of relevant representative conditions (climatic, seasonal, soil, cropping system, farming, etc.) likely to be met for the intended use of the pesticide. These residue trials, performed using proposed commercial formulations, should be carried out in accordance with the proposed Good Agricultural Practice (GAP). When a product is applied to a crop near maturity, studies on residue disappearance with time, decline studies, are usually needed to determine acceptable pre-harvest intervals.


In general, for a comparable set of conditions, trials should be carried out over a minimum of two growing

seasons. The precise number of trials necessary is difficult to determine in advance of a preliminary evaluation of the trial results. Minimum data requirements only apply where comparability can be established between

production areas, e.g. concerning climate, methods and growing seasons of production etc. In the EU assuming all other variables (climate etc) are comparable, a minimum of eight trials representative of the proposed growing area is required for major crops. For minor crops normally four trials representative of the proposed growing area are required.


Application of the Plant Protection Product


Residue trials should be based on the highest approved or proposed rate of application consistent with Good

Agricultural Practice in the region concerned.  A treatment at twice or three times the recommended rate may be included. Data from such a treatment would indicate what might happen if users deliberately or accidentally apply quantities greater than those recommended.


Whenever possible the application equipment used should correspond to that specified by Good Agricultural Practice. Only the formulated product which is the subject of the authorization procedure, or at least, a very similar formulation, should be used. In the course of a single growing season of a crop, many plant protection products may need to be used several times against one or more harmful organisms. Residue trials should therefore reflect the proposed critical GAP (number of applications, interval between applications). To cover the worst case residue situation the application of the same active substance present in different products against other harmful organisms which may attack the same crop has to be considered; where relevant, the design of the trial should also take this into account.


Data Recording


For reliable appraisal of residue behaviour of plant protection products in and on plants, knowledge of the

vegetative stage is often indispensable. In certain instances the timing of application of the plant protection

product will depend on the vegetative stage to be treated. Sowing time, or alternatively, planting time may indicate whether a given variety may be harvested early or late. The differential growth rates involved may affect residue behaviour to a different extent. Access to data defining the start and end of flowering is important for deciding whether, and, if so, to what extent fruit and other harvest products are likely to have been affected by the spray solution. These considerations also bear upon the comparability of crops. At all times, the date of the harvest is indispensable since it may significantly affect the residue situation. For crops with extended harvest periods records on harvest should include beginning and end of harvest. Records should also be kept of the soil types and the daily weather conditions. Proposed forms for reporting this information are given in the FAO document2.




It is not usually practical to collect the entire crop from a trials plot and it is generally necessary to devise a means of taking a representative field sample.  It is, however, extremely difficult to obtain uniform application of a pesticide in the field, thus in taking a sample for residue analysis, it is necessary to approach the task in an intelligent, realistic manner if the results of analysis are to be valid or useful for estimating maximum residue levels. Generally, the selection of the portions that make up the field sample is done randomly, systematically, or selectively from pre-determined "stations", depending upon the circumstances. In order to determine in-plot variation, replicate samples may be taken and analysed separately. The units of the field sample should be identical with the normal harvested product as regards any trimming or cleaning. Separate guidance is available on the recommended portion of the field sample to be prepared for the determination of pesticide residues5,6.


Control (untreated) samples must always be taken. These are as important as samples from test plots. Control

samples should be of similar quality to that of the test samples and may be from plots treated with another pesticide providing these are specified in the trial details and do not interfere with the analytical method. Control samples should be taken before the treated samples, so as to avoid the possibility of contamination from handling.


In order to have a sample suitable for analysis the sample collected from the field must be sub-sampled and should be equal to 500 to 1000g. This laboratory sample is further sub-sampled to provide the analytical sample.

The analytical sample is taken from the laboratory sample, which is to be submitted to analytical procedures. It

is obtained by removing a representative sub-sample from the laboratory sample. For plants or plant products with inedible skin (such as citrus, banana, kiwi, pineapple) a separate analysis of flesh and skin should be performed on some samples in order to provide data on the distribution of residues between flesh and skin.



Recommended weights for samples from various crops are given in the FAO document2.


All samples should be stored deep frozen. Storage stability studies should be carried out to confirm that any residues present at harvest do not decline due to storage.



1.      Commission of the European Communities,  Directorate General for Agriculture, 7029/VI/95 rev.5, VI B II-1 Appendix B, General Recommendations for the Design, Preparation and Realization of Residue Trials



2.      Food and Agriculture Organisation of the United Nations, Rome 1986, Guidelines on Pesticide Residue Trials   to Provide Data for the Registration of Pesticides and the Establishment of Maximum Residue Limits.



3.      Australian Pesticides and Veterinary Medicines Authority (APVMA), Residue Trials to Obtain Permanent MRLS for Crops, Residue Guideline No. 24, December 2000


4.      U.S. Environmental Protection Agency, Residue Chemistry Test Guidelines, OPPTS 860.1500, Crop Field Trials, EPA 712–C–96–183, August 1996


5.      Codex Alimentarius - 27 - Section 2.1 Volume 2A, Part 1 – 2000, Analysis of Pesticide Residues. Portion of Commodities to which CODEX MRLS Apply and which is Analysed.



6.      Council Directive of 27 November 1990 on the fixing of maximum levels for pesticide residues in and on certain products of plant origin, including fruit and vegetables, 90/642/EEC


Last modified April 7th 2010

Date added: 2010-05-08 00:27:55   
Last Updated 2010-05-10 03:03:49   
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