Milk withholding period based on dilution policy

MPI has an operational policy for consideration of milk dilution when setting milk withholding periods.

1. Introduction

This policy provides the basis for considering dilution of milk within a herd when setting milk withholding periods (WHPs). It covers

  • use situations where dilution can be considered when setting the milk WHP
  • product types where dilution can be considered when setting the milk WHP
  • the proportion of animals within a herd that can be treated when a WHP based on dilution is used.

2. Purpose of policy

This policy provides guidance on the circumstances when milk dilution within the herd could be considered when setting WHPs for veterinary medicines used on lactating dairy cows producing milk for human consumption. The policy provides guidance so that

  • all parties are clear what principles must be applied if milk dilution within the herd can be considered when setting a milk WHP, and
  • due care is taken to ensure risks to trade, market access and domestic food residue standards are considered.

Refer to the Agricultural Compounds and Veterinary Medicines Act 1997 (ACVM Act)

See the ACVM registration standard and guideline for determination of a residue withholding period (RWP) for veterinary medicines:

3. Background

3.1 Residue levels

Maximum residue levels (MRLs) are established on whole herd treatment. Furthermore, to manage risks to trade with the European Union the MRL must be met at the farm gate. While other trading partners do not require residue levels to be managed at the farm gate, this is the only practical way to ensure that residues will not exceed limits, and it gives a built-in safety factor for any dilution that will occur in further processing.

3.2 Setting of withholding periods

To ensure veterinary medicine residues in milk comply with milk MRLs, WHPs are set. Thus the WHP stated on the product label provides advice to the end user as a means to comply with the MRL established. Because MRLs are set on a whole herd basis, WHPs are also established on a whole herd basis.

There is considerable dilution of the milk from an individual cow as the milk is combined with the herd’s milk. This gives allowance to consider dilution within the herd’s milk when setting WHPs for veterinary medicines where the use pattern and good veterinary practice dictates that the product would be used only in a small percentage of the herd.

3.3 Risk Management

Under the ACVM Act, MPI has to take into account risks and benefits along with ensuring there is least cost to the public. This requires MPI to establish the appropriate level of risk management, and this clearly is not the case in some instances when establishing milk WHPs based on an entire herd treatment. Considering the dilution of milk from an individual cow, the setting of a WHP based on the whole herd being treated is a very conservative risk management tool. An outcome of this conservative risk management approach is the potential to unnecessarily increase the total cost (based on cost of product and lost income from discarded milk) of some treatment options to the veterinarian and farmer when managing the welfare of animals.

The risk assessment process also takes into consideration the role of key risk managers and level of compliance with the risk management tool. Veterinarians play an important role as risk managers and MPI’s expectation is that veterinarians will provide clear advice to end users to enable them to comply with legal milk residue requirements. If end users do not comply with this advice, then (as has always been the case) they may be subject to compliance action.

4. Policy

4.1 Criteria for consideration of dilution for setting milk withholding period

a) Dilution of milk containing residues within the herd must be considered only where use limited to a small proportion of the herd is consistent with good veterinary/agricultural practice in New Zealand. It will not apply to veterinary medicines where there is potential for whole herd treatment in usual circumstances.

Product types likely to have a use pattern consistent with limited use within a herd would include those veterinary medicines with restricted use patterns, such as those used to treat occasional sick animals – for example, non-steroidal anti-inflammatory drugs, antihistamines, and sedatives.

It is expected that no greater than 10% of the herd would ever be likely to be treated with such products and this would be the maximum percentage that would be considered for limited treatment within a herd for this policy. The value of 10% has been considered as it allows for ease of calculation and understanding by the product user.

The policy will not apply to any antibiotic, because of concern raised by stakeholders that residues and use will not be managed appropriately. In future it may be appropriate to consider application of the policy to some antibiotic products with restricted use patterns.

The dilution policy would not be applied to active ingredients that are likely to be used on a large number of animals at any one time (for example, anthelmintics).

b) All the products that would be considered as suitable for the dilution policy require veterinary oversight. Veterinarians provide additional assurance that communication and risk management is appropriate for residue management. This therefore precludes over-the-counter medicines. Both the standard WHP and the WHP with dilution considered will be available for the veterinarian to use. It is up to the veterinarian to advise the most appropriate WHP depending on the individual situation.

4.2 Withholding period information

Where a WHP for a veterinary medicine is to be set based on dilution, the appropriate information must be available on the product label. This is important to provide sufficient consumer information to users, especially those using products on individual animals such as house cows or considering giving the milk to bobby calves without causing residues. It also provides advice on the WHP to apply if a greater proportion of the herd must be treated in exceptional circumstances, particularly in early lactation when only a few cows are being milked.

It is important the herd used for the calculation is the currently lactating herd producing milk or colostrum for human consumption. There will be two WHPs stated on the label – one where dilution has been factored in and one calculated in the standard way, which does not include the dilution factor. To use the dilution WHP, only the percentage of cows treated within the time frame of the standard WHP can be used for calculations. This avoids a percentage being treated each day with an additive effect on the WHP.

An example of how the WHP can be worded is as follows:

When less than 10% of the currently lactating herd are treated within 5 days, the following milk WHP applies: Milk intended for sale for human consumption must be discarded during treatment and for x hours (x milkings) following the last treatment.

When greater than 10% of the currently lactating herd are treated within 5 days, the following milk WHP applies: milk intended for human consumption must be discarded during treatment and for 5 days (10 milkings) following the last treatment.

In this example the dilution has been calculated on a maximum of 10% of the herd being treated within the standard WHP for this drug, which is 5 days.

The WHP must be easy to understand. To avoid confusion the proportion of cows that can be treated must be calculated easily (for example, 5% or 10%) and should not be variable to achieve different WHPs. No greater than 10% of the herd should be allowed treatment to achieve the WHP calculated based on dilution, as this could lead to difficulty with record keeping and calculations. It also allows greater potential for error, leading to non-compliant residues in the milk. It is also not considered that drugs with a limited use pattern would ever be required to treat greater than 10% of the herd at any time in normal circumstances.

As the products will be available under veterinary overview only, it will be the veterinarian’s role to advise which WHP should be applied. This will depend on individual circumstances, such as confidence that the percentage of the herd the product is used in can be calculated and is not likely to be exceeded within the standard WHP of the drug, confidence in record keeping and communication on the farm, and the stage of lactation (whether the herd size is likely to fluctuate or not).

4.3 Calculating withholding periods

The WHP should be calculated in accordance with the guidance in the ACVM Registration Standard and Guideline for Determination of a Residue Withholding Period for Veterinary Medicines. Suitable data will need to be provided at all relevant time points.

When calculating the maximum percentage of the herd that can be treated, 10% is to be the maximum value that is considered appropriate for drugs with a limited use pattern. This figure is not based on any specific information other than the subjective view that it may be inappropriate or difficult to manage at a different level. However, this figure can be reviewed at any time as to whether it is appropriate or not.

A 50% safety margin will be incorporated into the calculation. This means the calculated residues when the maximum percentage of the herd is treated will be 50% of the approved MRL at the farm gate. This is a greater margin of safety than with standard WHPs. This recognises products are frequently used in circumstances other than those in residue trials and existing dilution can play a role in residue management. A further margin of safety is provided by the dilution that will occur at the factory, but this should not be accounted for in the calculations.

It is not expected the maximum figure would ever be approached with the product types under consideration, allowing a further margin of safety.


Last reviewed:
Has this been useful? Give us your feedback