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Prevention of antibiotic residues

Prevention of antibiotic residues in milk is necessary to ensure health protection of consumers and to avoid failures during milk processing. In practice residues of antibiotics in milk are most often determined by microbial inhibitor tests. Treatment of mastitis with antibiotics is one of the most important causes associated with positive results of inhibitor tests on bulk tank milk. In conventional milking management factors like accidental milking of treated cows, not attending the withholding time and failures during milking are the main reasons for antibiotic residues in bulk milk together with insufficient cleaning of milking equipment. In automatic milking like in conventional milking management failures are reported as the main reasons for residues in milk. Little is known about the impact of milking intervals on the excretion of antibiotic residues in milk. Previous investigations were mainly focused on the effect of stripping on concentrations of antibiotics in milk after treatment of mastitis.

The aim of the study reported here was to determine the excretion of antibiotics in milk in dependence on milking frequency. Four drugs containing six different antibiotics were studied in healthy cows milked with three different milking frequencies. The concentration of residues in milk at the end of the withholding period was compared to Maximum Residue Limits (MRL) set by Council Regulation 2377/90/EEC.

Excretion trials:
Excretion trials were performed under experimental conditions simulating milking frequencies as observed in AM systems. The healthy cows were treated with one injector for each quarter (worst case) in intervals according to manufacturers instructions. Cows were milked 2 times per day (interval 14/10 hours (reference), 3times per day (interval 8h) or 1.5 times per day (interval 16h). One drug was selected for further analysis in cows with clinical mastitis. Only the diseased udder quarter was treated. The cows with mastitis were milked twice a day or 1.5times per day.

Results and Conclusions:
The excretion time of antibiotic residues in milk of healthy cows varied with different milking frequencies after intramammary treatment. For three of the tested drugs significantly shorter excretion times were observed in cows milked 3 times per day compared to cows milked 1.5 times per day. Longest excretion times were observed for all antibiotics except for one drug in cows milked less frequently with 1.5milkings per day.

Treatment trials in cows with clinical mastitis showed large variation in excretion times of individual cows and much shorter excretion times than in worst case studies in healthy cows. Although no significant differences were determined between groups milked 2 times or 1.5 times per day it should be considered that only one drug was tested in a low number of cows. A more pronounced influence of milking frequency may be observed for other drugs. Although results from studies in healthy cows cannot necessarily be transferred to cows with clinical mastitis, controlled milking of cows at least 2 times per day is recommended after treatment with antibiotics.

If cows in AM systems are milked in shorter intervals after treatment higher concentrations of residues in milk have to be expected. This could increase the risk for carry over of antibiotics into milk of the next cow milked at the same place, especially if failures in cleaning procedures occur.

According to the results presented, prolonged excretion of residues in milk of individual cows in connection with milking frequencies deviating from regular milking times twice per day, seem to be of minor importance for positive results of inhibitor tests in bulk tank milk, when recommendations for treatment are followed.

Verwandte Informationen

Reported by

Federal Research Centre for Nutrition and Food (former Federal Dairy Research Centre)
Hermann-Weigmann-Strasse 1
24103 Kiel
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