Pastures vary greatly in their composition of legume and grass species. They may be either native or improved and generally consist of a mixture of various pasture species, although stands of greater than 90% species purity also exist (for example, lucerne, clovers, medics, phalaris, buffel grass and barley grass).

Pesticides can be used on pastures:

  • during establishment of improved pastures
  • for pest and weed control in established pastures, or
  • for pasture manipulation using methods such as spray/grazing or pasture topping.

If you are proposing any pasture use for inclusion on a pesticide label, you will need to consider the residue implications for grazing animals. Determination of residues in the treated pasture will indicate whether animal transfer studies are required.

1. Objectives of pasture residue studies

Residue studies in pastures are conducted to:

  • determine the level of pesticide residue that will occur in a variety of representative pastures
  • provide sufficient residue information to allow appropriate maximum residue limits (MRLs) to be set for relevant pasture types as primary animal feed commodities
  • determine whether animal transfer studies are required to be submitted so that MRLs can be set in primary food commodities of animal origin.

2. Conducting residue trials on pastures

When conducting a pasture residue study, you should follow the general guidance given in the Food and Agriculture Organization of the United Nation's guidelines for residue trials and OECD guidance documents and test guidelines, giving particular attention to:

  • trial locations
  • treatment rates and timings
  • samplings  
  • pasture type
  • number of trials.

2.1. Trial locations

The choice of locations for residue trials on pastures will depend on the particular use pattern proposed. You should choose locations so that they represent the range of pasture conditions that would normally be encountered when the product is used as directed. You should include, where appropriate, trial locations that cover native and improved pastures, mixed species and pure stand pastures, as well as the major climatic zones in which the product may be used (for example, tropical and temperate).

2.2. Treatment rates and timings

Residue trials on pastures should be conducted at the maximum proposed label rate and at an exaggerated rate, normally 1.5 to 2 times the label rate. The maximum number of applications, with the shortest spray intervals possible according to the proposed use pattern, should be made.

2.3. Sampling

Field samples for residue analysis should consist of 1 kilogram of pasture taken by subsampling each plot. You should cut plant material close to ground level and avoid soil contamination. Sufficient sampling points should be taken to allow a residue decay curve to be constructed. If you are proposing a withholding period, it is essential that you spread the sampling regime across the time range within which the withholding period is expected to occur. It is also essential to sample at the recommended withholding period; otherwise, the withholding period will be set at the next longest sampling time.

The following is a guide to appropriate sampling regimes.

2.3.1. Pasture establishment treatments

Residue sampling should take place:

  • at the earliest time after treatment that sufficient plant material exists for sampling
  • at the time of any proposed withholding period (that is, the earliest stage that animals could graze the pasture)
  • at least one point in between (unless the above sampling times coincide)
  • at least one point after the sample is taken at the proposed withholding period.

Additional sampling points should be taken, as necessary.

2.3.2. Established pasture treatments

Established pastures can be grazed soon after any pesticide application, except in some circumstances. Residue samples should be taken:

  • on the day of application (immediately after the pesticide has dried)
  • on the day after application (day one)
  • at the proposed withholding period
  • at least one point after the sample is taken at the proposed withholding period.

For established pastures, the day-one and withholding period samples will often be the same. Additional sampling points may also be taken to show a decay curve, which is useful if there is a likelihood of residues transferring to grazing animals.

2.3.3. Pasture manipulation treatments

Pasture manipulation treatments such as spray-grazing and spray-topping will generally follow the sampling pattern described for established pasture treatments. However, sampling should be adjusted according to the implications of the use pattern for grazing animals. For example, the spray-grazing technique may entail a longer period of time before animals graze the treated pasture, so the withholding period sample should be adjusted accordingly. This technique also may make certain weeds more palatable to grazing animals, and therefore those weeds should be sampled and analysed separately for residues. Some pasture manipulation techniques may exclude animals from grazing for a much longer period of time. In those cases, you may be able to argue from plant metabolism and other data that there will be no significant residues in the treated pasture at the time when animals return to it.

2.4. Pasture type

You should record details of the type of pasture for each residue trial. For pure stand pastures, you should state the species; for example, lucerne, subterranean clover or white clover. For mixed pastures, the approximate percentage of each of these categories should be stated; for example 70% grass, 30% legume.

2.5. Number of trials

The number of trials will depend on the proposed pasture use of the product. You should conduct a sufficient number of trials to cover the majority of pasture situations and climatic zones in which the product will be used. If the product is for general pasture use, you should conduct residue trials to cover both situations when legumes predominate and when grasses predominate. If use of the product is restricted to certain types of pastures or pasture species, residue data are required only for the pasture species relevant to the proposed use.

3. Animal transfer studies

You may need to conduct animal transfer studies if significant residues are present in the pasture following the proposed withholding period. Residues in pasture, on a wet weight basis, are considered significant if greater than or equal to 0.1 milligram per kilogram. Refer to the Animal transfer studies guideline for more information.

4. Proposals for maximum residue limits and withholding periods

MRLs for animal feed commodities relate only to those feed commodities that may be traded.

Pure stand pastures may be cut and traded as animal feeds. If the proposed use pattern includes these types of pastures, MRLs for the appropriate animal feed commodities (for example, lucerne or medics) should be proposed for inclusion in Table 4 of the MRL standard. For proposal of a group entry (for example, legume animal feeds), the use patterns should be equivalent and sufficient residue data should be provided from representatives of the group.

Unimproved and native pastures are used only for grazing by animals and are not cut as a traded commodity. MRLs do not need to be recommended for these pasture species. However, residue studies should be conducted to determine the implications for grazing animals.

For all pasture uses, MRLs for inclusion in Table 1 of the MRL standard should 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.

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