Withholding periods (residues)

The withholding period (WHP) is defined in the Agvet Code as follows:

Withholding period, in relation to the use of a chemical product, means the minimum period that needs to elapse between:

(a) the last use of the product in relation to a crop, pasture or animal; and

(b) the harvesting or cutting of, or the grazing of animals on, the crop or pasture, the shearing or slaughtering of the animal, or the collection of milk or eggs from the animal for human consumption, as the case may be; in order to ensure that the product’s residues fall to or below the maximum limit that the Australian Pesticides and Veterinary Medicines Authority (APVMA) permits.

1. Objective of a withholding period

The objective of a WHP is to provide users with the information they need to ensure that residues in their treated produce will not exceed the maximum residue limit (MRL). This means that the data you submit 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 you design the residue study. The 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 2 streams for trial design.

2. Residue decline information

Residue decline information is generally required when:

  • application is made close to harvest and there is a short WHP (usually less than 14 days for most crops)
  • application is made after harvest
  • the crop could be grazed or fed to animals (including failed crop situation and pastures; refer to the failed crops and pastures guidelines)
  • there are trade implications for the produce.

Residue decline information is generally not required when:

  • there is an extensive time lapse between treatment and harvest of produce (for example, 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.

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 (that is, the earliest stage that animals could graze)
    • at least one point in between (unless the above sampling times coincide)
    • at least one point after the sample taken at the proposed WHP.
  • For crops that are not failed or grazed crops, 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 on 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: one, 3, 5, 7, 14 and 28 days post-application for late season use of pesticides. If multiple applications of the pesticide are anticipated, a sample taken just prior to the final application would be of value
    • Depletion to zero residue (to the limit of quantitation) will only be necessary if there are significant trade issues involving major export commodities (such as 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.

Your submission should include proposals for the required WHP on produce, as well as in situations where a grazing WHP is needed.

Consult the Agricultural Labelling Code for the appropriate withholding period statement.

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