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AgVetLink: Number 59, December 2006

Milk Withholding Periods

Current policy for the setting of milk withholding periods assumes whole herd treatment and no dilution. It is recognised that this approach can be considered conservative, particularly for those products that are likely to be used in only a small percentage of cows at any one time.

NZFSA is being asked to consider allowing for dilution in the setting of withholding periods with two different scenarios:

• Drugs that are used infrequently and not likely to be used on a large percentage of a lactating herd. This could include antibiotics, particularly those with restrictions or use patterns that would not be likely to result in treatment of more than a few animals.

• Drugs that are routinely used on a whole herd but where a case can be made that treatment of part of a herd is still good practice (e.g. the use of anthelmintics in lactating cows).

No final decisions have been made as to how this will be managed. However, there is agreement within NZFSA to consider milk dilution as an option when setting withholding periods. The issues that need to be considered include:

• The current system may be compensating for the fact that residue data does not provide for all the possible variations (e.g. differences in diseased animals, higher doses, prolonged treatment and additive residues for different application routes).

• Users must be able to understand and practically follow label advice.

• Label advice has to provide for animals treated over a period of time. If a product is being used regularly, this issue may be difficult to manage and lead to error.

• In extreme circumstances, high percentages of lactating herds may be treated (e.g. at the start of calving).

In order to provide for consistency and to reduce the complexity, set herd percentages are likely to be established as thresholds providing a fixed number of options. It is likely that residue assessments will have an inbuilt degree of conservatism to allow for increased potential for error by the user.

The conditions of registration must be taken into account with products managed under conditions such as prescription providing for more control. Two label statements are currently being considered with one providing for dilution and the other giving the traditional withholding period assuming no dilution.

As indicated in the recent workshops (see page 2), we are happy to consider any comments, either negative or positive, on the proposed change. Email comments to Lucy Johnston (lucy.johnston@nzfsa.govt.nz).