What are withholding periods?
A Withholding Period (WHP) is the minimum amount of time that has to pass between application of a veterinary medicine or compound (eg internal or external parasite treatment, antibiotic, vaccine, anaesthetic drug or chemical) and harvest (eg of velvet antler or milk) or slaughter.
All veterinary medicines or compound that are registered for use in or on animals must have the Withholding Period printed on the label and people using these medicines or compounds must comply with them or face severe penalties.
Why do we have them?
The Withholding Period is set for each veterinary medicine or compound to ensure that by the time the animal is killed, or the product is harvested, the amount of residual drug or chemical remaining in the produce is so low that it is very unlikely to pose any health risks from consumption of treated produce.
How are they set?
For each drug or compound a maximum residue limit ('MRL') is set by The Ministry for Primary Industry (MPI) (previously The New Zealand Food Safety Authority) based on all available information.
Studies are conducted the determine how long it takes for each compound in each particular formulation to decline to this MRL after treatment by each route of administration for each livestock species in which the drug or compound will be used.
When a veterinary medicine is registered for use in New Zealand, this information is used to set the WHP for that particular formulation of drug or compound using the specified route of administration for the animal species in which it has been tested.
What are 'Default' withholding periods?
If a veterinary medicine or drug is used in an animal species for which it has not been registered then a Default WHP must be used. In the case of deer, the Default WHP is 91 days.
The exception to a Default WHP of 91 days is if there is specific approval for a lesser time. For example parasite treatment “Cydectin injection” is not licensed for use in deer. However, veterinarians can prescibe 'off-label' use of Cydectin Injection which allows for deer to be slaughtered 49 days after treatment. This exception only applies to drugs obtained from a vet with a vet prescription, so if you are contemplating administering a product with a 91 day WHP for deer fewer than 91 days before the intended slaughter date, you must obtain the product under veterinary prescription.