What Is An MRL & Where Does It Fit In With Withdrawal Periods?

Rose explains more about maximum residue limits that are allowed in either milk or meat – and factors that can cause a positive test in animals out of their withdrawal period.

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MRL stands for Maximum Residue Limit which is the legally allowed maximum concentration of a veterinary medicine allowed in either milk or meat.

All medicines have to be tested for safety before they can be licensed for use, and the withdrawal period for any given medicine is based on the MRL. These rules sometimes seem to be onerous, but we must remember that these exist ultimately to protect consumers.

Very occasionally, milk from animals that are out of their withdrawal periods may still test positive for antibiotics on an on-farm residue test such as the Delvo SP. There might be several possible reasons for this:

1. If antibiotics are used ‘off license’ (not exactly as stated on the data sheet), e.g. a 6 day course instead of 4 days (see diagrams 1 and 2), or tubing twice a day instead of once a day (see diagram 3). In this case, antibiotic levels in milk can build up to above the MRL so it is necessary to use a standard minimum 7 day milk and 30 day meat withdrawal period for these ‘off license’ protocols as the normal withdrawal will not be appropriate. Please note that only a vet can recommend off license treatment protocols so please speak to one of us if you are unsure.

2. If two antibiotics of the same class but in different preparations (e.g. mastitis tubes plus injectable) are given together and this is not recommended on the data sheet, MRLs may again be exceeded for longer so a standard minimum 7 day milk/30 day meat withdrawal should be applied.

Diagram 1. 4 day antibiotic course
Diagram 2. 6 day antibiotic course. The time taken for the antibiotic concentration in milk to fall below the MRL is an extra 4 days, despite only 2 extra days of treatment being given.
Diagram 3. Effect on the WHP as a result of twice-daily, rather than once-daily, tubing.

3. For some commonly used antibiotics (including Betamox and Noroclav), the MRL is actually higher than the level detected by the Delvo SP test so an individual cow milk sample may occasionally fail for 24-48 hours after the end of the withdrawal period. If this occurs, it should be reported back to the drug company via your vet as it is technically an ‘adverse reaction’. Our advice is to believe the Delvo test first as the dairies use the same technology as their first line test.

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