Elimination of ceftiofur hydrochloride residue in postpartum cows' milk after intramammary infusing at dry-off

The purpose of this study was to investigate the residue elimination of ceftiofur hydrochloride in milk of postpartum cows after intramammary infusing at dry-off. An ultra-high performance liquid chromatography-tandem mass spectrometry （UPLC-MS/ MS） method was developed and validated to detect ceftiofur hydrochloride residue in milk. Through preprocessing, ceftiofur hydrochloride was derivatized into a more stable compound dedfuroyl ceftiofur acetamide （DCA） for further analysis. The linear range of DCA was 0.1 to 50 μg kg-1. Average recoveries of DCA were 82.52-105.86％. The intra-day and interday coefficients of variation （CV） were 2.95-9.82 and 6.41-7.43％, respectively. The limit of detection （LOD） and limit of quantitation （LOQ） scores were 0.05 and 0.1 μg kg-1, respectively. These parameters showed this method was reliable and valid. Twelve cows were administrated 10 mL ceftiofur hydrochloride by intramammary infusion （corresponding to 500 mg ceftiofur） to each udder after the last milking before the dry-off period. Milk was collected from each udder of cow at 12, 24, 36, 48, 60 and 72 h after calving and was mixed for each time point and each cow, then subjected to UPLC-MS/MS analysis. The results showed, the DCA concentrations in all milk samples were less than LOQ and the maximum residue limit （MRL） 100 μg kg-1, which suggested the withdrawal time of ceftiofur hydrochloride intramammary infusion used for preventing and curing mastitis in dry cows was 0 day. The study provided guidance for the clinical applications of ceftiofur hydrochloride intramammary infusion （dry cow）.

Persistence of gentamicin residues in milk after the intramammary treatment of lactating cows for mastitis

This study was designed to investigate persistence of gentamicin residues in milk after the intramammary treatment of lactating cows for mastitis. Milk samples were collected at a 1-d interval after the last administration from 34 individual cows that had received intramammary infusions of gentamicin. The doses and treatment times evaluated in this study represented those that have been applied by veterinarians in practice. The tetrazolium chloride assay was used to determine whether there were significant residues of the antibiotic in the samples. Persistence of detectable drug residues in milk from 33 cows (28 cows, ≤6 infusions at ≤0.7 g gentamicin; and 5 cows, 2 infusions at 0.8 g gentamicin) did not exceed 5 d; but 1 cow (5 infusions at 0.8 g gentamicin) had detectable residues in its milk for 9 d. Our results suggest that a 5-d milk withdrawal period might be insufficient to secure the clearance of the contamination of gentamicin, because treatment times and dosages contribute to the antibiotic clearance. A larger scale of samples are needed for further investigations.