While the percentage of dairy cull cows with drug residues in meat is less than 1% and appears to be dropping over the past few years, dairy cattle still make up the majority of all cattle with those drug residues.
In 2014, 108,000 dairy cull cows were tested, and 628 of them, or 0.6%, were positive. Just over 20,000 beef cows were tested, but just 81 came back positive (0.4%).
One reason cull dairy cows have higher rates of residues is that dairy cows are often culled after they become sick. Even though the withdrawal period might be observed, drug clearance from cows recovering from illness is likely slower than in healthy cows.
“Drugs clear more slowly with disease, but drug withdrawals are based on healthy cow studies,” says Geof Smith, a veterinarian with the College of Veterinary Medicine at North Carolina State University. “It is likely why we see higher residue risk with cull cows.”
Dairy farmers, he says, need to take this into account when culling cows that have been sick and subsequently treated with antibiotics.
A trial conducted by Smith and colleagues at North Carolina shows how cows producing normally and those with mastitis clear flunixin. Ten cows with mastitis were matched with 10 healthy cows in terms age, days in milk and production. All cows were treated with one dose of flunixin.
The healthy cows were below 2 ppb of flunixin by 36 hours after treatment. The cows with mastitis still had flunixin levels above 2 ppb 60 hours after treatment.
Route of administration also can play a role in how quickly a drug will clear. For example, if Excede is administered correctly at the base of the ear, residues in meat clear in 13 days. However, if Excede is given in the muscle, the meat withdrawal goes to 90 days. If given subcutaneously, the meat withdrawal goes to 140 days.
Records, training essential
When treating sick animals, records are critical. Not only must you record date, time, drug and route of administration, notes should be kept on whether the animal recovers or needs repeated treatment.
Those that respond slowly to treatment may be good candidates for culling once they do get well because they may be less likely to respond to treatment if they get sick again. “Animals with unresolving disease are often at risk of residue violations,” says Smith. “So when possible, try to identify cattle unlikely to respond to treatment.
“For cattle that have been treated extensively, euthanasia may be a better option than culling,” he says.
It is also critical dairy farmers work with their veterinarians to develop antibiotic treatment protocols and withdrawal periods for both milk and meat. If drugs are used extra-label, withdrawal periods are especially critical because drug companies have not established withdrawal periods for those uses.
On larger dairies where employees diagnose and treat sick cattle, the employees must be fully trained and understand how to administer drugs, identify treated cattle, keep records, calculate and record withdrawal times, says Smith. “As farms become larger, there are more employees involved and more opportunities for mistakes,” he says.
This story appears in the June 2017 issue of Dairy Herd Management.