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Understanding MRLs

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 need to remember that these exist, ultimately, to protect consumers. Antibiotic residues in milk can lead to a risk of allergic reaction in some people and can affect cheese and yoghurt production, as well as contributing to antibiotic resistance. The reputation of our dairy industry is also at stake if we are unable to defend the protocols that are put in place to safeguard milk.

Common reasons for bulk tank antibiotic failures

Ultimately, most of these boil down to human error:

  • Un-recorded use of a lactating cow tube or failure to correctly identify a cow under treatment
  • Accidental milking of a recently dried off cow
  • Dry cow calving down early whilst still under withdrawal
  • Deliberately not following the withdrawal period
  • Recently bought in cows – don’t assume that these are out of withdrawal – always check with a Delvo test before putting their milk in the tank

Although mistakes happen, having good protocols in place and communicating them to everyone involved with milking the cows, should help to minimise the risk.

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. This might lead to a bulk tank antibiotic failure and there could be several possible reasons for this:

  1. If antibiotics are used ‘off license’ (not exactly as stated on the data sheet; also known as cascade use) e.g. a 6 day course instead of 5 days or tubing twice a day instead of once a day. Antibiotic levels in milk can build up to above the MRL. It is necessary to use a standard minimum 7 day milk and minimum 30 day meat withdrawal as the normal withdrawal will not be appropriate. Please note, only a vet can recommend an off-license treatment protocol so, please speak to us if you’re not sure. Written justification for cascade use of medicines should be kept in your herd health plan file.
  2. If two antibiotics of the same class, but in different preparations (e.g. penicillin mastitis tubes plus injectable penicillin) are given together and this is not written on the data sheet, MRLs could be exceeded for a longer period so, a standard 7 day milk/30 day meat withdrawal should be applied.
  3. For some commonly used antibiotics (including some Amoxicillin preparations), the MRL is actually higher than the level detected by the Delvo SP test. An individual cow milk sample may occasionally fail for 24-48 hours after the end of the withdrawal period, even when the drug is used correctly. If this occurs it should be reported back to the drug company, via the vet, as it is technically an ‘adverse reaction’. Our advice is to believe the Delvo test first because the dairies use the same technology as their first line test.

Antibiotic Residue Tests

There are various types of antibiotic residue tests available; some of these can be used at the farm level and some are used for more detailed investigation following a bulk tank failure. Inhibitory tests such as the Delvo test (current industry standard for routine bulk tank tests) work by growing a bacteria. The bacteria won’t grow if antibiotics are present and this is linked to a colour change. These can be used to test for a wide range of antibiotics, but can take around 3 hours.

At the other end of the spectrum, a much quicker result can be achieved by using an immune-receptor test such as Beta Star. Generally, these can only be used to detect a smaller range of antibiotics; these are often used for checking a tanker before unloading it into a silo. It is worth noting that there is a new immune-receptor test available called InfiniPlex®. This can detect over 40 different medicines (not just antibiotics) within an hour – these are currently used for investigating residue failures.

The Risk of Bulk Tank Failure

If you are worried about the risk of bulk tank failure on your farm, we would recommend signing up to MilkSure and completing a training course with us. One of the most efficient ways of reducing risk is to focus on reducing the amount of antibiotics on farm, for example, by using selective dry cow therapy. It is a Red Tractor requirement for each farm to have an annual review of antibiotic use so use this as an opportunity to come up with a plan with us on how to cut down your antibiotic use.

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Understanding MRLs
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Forth Valley Veterinary Practice Stirling

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Falleninch Farm Dumbarton Road Stirling FK8 3AB
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Forth Valley Veterinary Practice Perth

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Unit 4 Inveralmond Road Inveralmond Industrial Estate Perth PH1 3UF
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