Residue Guideline No. 10
The withholding period (WHP) is defined in the Ag and Vet Labeling Codes as the period that must elapse between the last application of a chemical and:
- harvesting of plants;
- grazing or cutting for stock food;
- consumption by a human or animal after post-harvest use.
The objective of a WHP is to provide users with the information they require to ensure that residues in their treated produce will not exceed the MRL. This means that the data submitted must demonstrate that the MRL will not be exceeded when the appropriate WHP is observed. The foundation for the WHP, according to good agricultural practice, must therefore be established before designing the residue study. The consequent residue study then confirms the level of acceptable residue at the desired WHP. Once the desired WHP has been determined, the residue study will follow one of the two streams for trial design.
1. Residue decline information is generally required when:
- application is made close to harvest and there is a short WHP, (i.e. usually less than 14 days for most crops);
- application is made after harvest;
- the crop could be grazed or fed to animals (includes failed crop situation and pastures; see Residue Guideline No. 2, Failed crops and pastures); and
- there are trade implications for the produce.
2. Residue decline information is generally not required when:
- there is an extensive time lapse between treatment and harvest of produce (e.g. pre-emergence herbicides where animal feeding does not occur). However, trials should be designed so that the use pattern followed provides data with the minimum time between treatment and harvest;
- there are no trade implications.
3. The generation of residue decline information should be consistent with the following principles:
For grazing of pastures or failed crops, residue samples should be taken:
- at the earliest time after treatment when sufficient plant material exists for sampling;
- at the time of any proposed WHP (i.e. the earliest stage that animals could graze);
- at least one point in between (unless the above sampling times coincide); and
- at least one point after the sample taken at the proposed WHP.
For crops not grazed or failed, the residue sampling regime should in principle be similar to that described above with the following additional aspects:
- The sampling of residues will be dependent upon factors such as the persistence of the pesticide, its metabolism in the plant, whether it translocates, the use pattern and most importantly, whether expected finite residues at harvest would have implications for trade.
- A suggested regime could be: 1, 3, 5, 7, 14 and 28 days post application for late season use of pesticides. If multiple applications of the pesticide are anticipated, then a sample taken just prior to the final application would be of value.
- Depletion to zero residue (to the LOQ) will only be necessary where there are significant trade issues involving major export commodities, e.g. grains. Such depletion studies will not be required for all residue trials and should be conducted for only a representative portion of crops of any given crop group.
The submission should include proposals for the required WHP on produce, as well as in situations where a grazing WHP is needed.
Consult the Ag and Vet Labeling Codes for the appropriate withholding period statement.
Registration Authority for Agricultural and Veterinary Chemicals 1997,
Vet Labeling Code: Code of Practice for Labeling Veterinary Chemical
Products and Ag Labeling Code: Code of Practice for Labeling
Agricultural Chemical Products, NRA, Canberra.