For the purpose of this guideline, a failed crop is considered to be any crop that, because of adverse conditions, could be grazed by livestock during its vegetative growth stage rather than grown to maturity. These are crops that normally would be grown to maturity. This description only relates to crops that are palatable to livestock and where consumption by livestock is an accepted means of disposing of the failed crop. Crops for which this is not an accepted practice or never happens will not require residue data related to the failed crop situation, but appropriate statements restraining livestock from feeding on the crop material may be required on the label. Further details are provided in the Animal transfer studies guideline.
1. Objectives of failed crop residue studies
The objectives of failed crop residue studies are to:
- determine the level of pesticide residue that will occur during the vegetative growth stage of a crop
- provide sufficient residue data to allow appropriate grazing restraint statements to be set for failed crop situations
- provide data to determine whether animal transfer studies will be required to be submitted so that maximum residue limits (MRLs) can be set in primary food commodities of animal origin.
2. Conducting residue trials for failed crop situations
Residue data to cover the failed crop situation would normally be generated as part of crop residue trials. Information on how to conduct crop residue trials is provided in the Food and Agriculture Organization’s guidelines on residue trials, OECD Guidance Documents and Test Guidelines and other Australian Pesticides and Veterinary Medicines Authority residue guidelines. Crops that could become failed crops will require additional sampling points during the vegetative growth stage. If a withholding period is necessary, the sampling regime should be spread across the time range within which the withholding period is expected to occur. It is essential to sample at the recommended withholding period; otherwise, the withholding period will be set at the next longest sampling time.
As a guide, additional sampling points for failed crops are:
- on the day of application immediately after the pesticide has dried (or at the earliest time after application that sufficient plant material exists for sampling)
- at the time of any proposed withholding period
- at least one point in between these 2 sampling times (unless the initial sample and withholding period sample coincide)
- at least one point after the sample taken at the proposed withholding period.
Sufficient data points should be taken to allow construction of a residue decay curve. If an extended withholding period is proposed, additional samples should be taken at the time of the proposed extended withholding period. Residues should be determined on a dry-weight basis and moisture content reported.
3. Animal transfer studies
If significant residues are present in the crop or crop parts that could be consumed by animals following the proposed withholding period, animal transfer studies may be necessary. Further details about assessing the level of significance of the residue and any additional data that may be required are provided in the Animal transfer studies guideline. That guideline provides advice on the use of label restraint statements for livestock feeding if no failed crop residue data is generated.
4. Proposals for maximum residue limits and withholding periods
Maximum residue limits (MRLs) for the appropriate animal feed commodities relevant to the failed crop situation should be proposed for inclusion in Table 4 of the MRL standard. Appropriate withholding periods should also be proposed.
MRLs for inclusion in Table 1 of the MRL standard should also be proposed for primary food commodities of animal origin that relate to the relevant grazing animals. Further details are provided in the Animal transfer studies guideline.