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AgVetLink Special issue for Veterinarians May 2006

Residues and Bobby Calves

Residue management in bobby calves is an area that frequently results in questions because residues may be from direct treatment, the diet (milk and colustrum) or from treatment of the cow prior to birth.

The ACVM Group is developing a comprehensive policy on the use of withholding periods to manage risks of residues. In the meantime, this article focuses on the issue of withholding periods for neonatal calves, which are commonly called ‘bobby calves’ when they are sent for slaughter.

Withholding time information

In general, residues and withholding time information will relate only to the treated animal as there is no requirement to produce data for calves born to treated cows. Although there are examples of situations where registrants have produced data on calves for products that are likely to be used near calving, this is not common because the trials are relatively complex and expensive. To date, one injectable penicillin and one pour-on ivermectin have produced separate withholding periods for calves born to treated dams. They also take into account residues the calf might receive from the dam’s milk.

Default

The ACVM Group considers that, unless there is information to say otherwise, the following rule should be used as a ‘default’:

If a cow calves within the meat withholding time of a veterinary medicine with which she has been treated, the calf is eligible for slaughter only when the cow is eligible for slaughter.

There are obvious exceptions to this rule where alternative advice may be considered by a veterinarian. The most common examples are where the meat withholding period is dependant on a prolonged depot effect or controlled release formulations such as boluses.

The advice from NZFSA is that calves born to cows treated with long-acting dexamethasone ester products can be sent for slaughter as bobby calves. This advice is based on the low but prolonged residue profile in the dam’s meat and milk as a result of a depot effect at the injection site. After birth residues in the calf are metabolised relatively quickly and, because there are no high peaks in milk, it is unlikely the total dexamethasone residues will exceed regulatory limits in the calf at four days of age.

Milk

Labels of registered products will not normally contain advice on withholding periods for calves fed milk containing residues unless the registrant has produced specific data.

In the past, advice was given that a calf exposed to contaminated milk should be fed residue-free milk for seven days. This advice is not based on actual data and must be treated with a degree of caution. It assumes the level of residues in the milk will be relatively low and the calf will be able to eliminate this residue if it is absorbed. Situations where calves are allowed to suckle cows directly after administration of intramammary products or where a large percentage of a herd is treated could make these assumptions irrelevant.

Antimicrobials

In general, cattle meat withholding periods applied to products also apply to calves with the exception of antimicrobials where the current policy is for labels to carry the statement “Not for use in bobby calves”.

This phrase is not very precise and it will be one of the matters to be addressed in the withholding period policy that is being developed. In the meantime, a calf must not be treated with or directly exposed to antibiotics and then sent for slaughter as a bobby.