Prepartum intramammary antibiotic therapy: Effect on risk of antibiotic residues in milk from dairy heifers

Milk was collected at the third, sixth and tenth milkings following parturition from 136 heifers treated with a commercial β-Lactam intramammary (IM) antibiotic preparation containing cephapirin sodium at 10 to 21 d prior to anticipated parturition to determine the risk of antibiotic residues occurring in milk resulting from prepartum antibiotic therapy. This was part of a study to determine the effects of prepartum IM therapy on milk production, reproductive performance and cure rate of existing intramammary infections in 9 herds in the U.S. and Canada. The mean interval (d) between IM antibiotic therapy and parturition was 15.0 (standard deviation = 9.61 d). Milk was analyzed for β-Lactam residues using a microbial inhibition antibiotic residue screening test. Antibiotic residues were confirmed with β-Lactamase treatment and re-tested for residues. The percentage of antibiotic residues detected was 28.0, 8.82, and 3.68 in the third, sixth, and tenth milkings following parturition, respectively. Antibiotic residues occurred only in the third milking for 26 primiparous heifers (19.1%). Residues persisted until the sixth milking for 7 heifers (5.15%) and until the tenth milking for 5 heifers (3.68%). The mean interval for heifers identified with an antibiotic residue in milk at the sixth milking or persisting to the tenth milking was 7.58 d (range = 1 to 15 d). An increase in interval and milking number was associated with a decrease in risk of antibiotic residues (P<0.0001). Prepartum IM antibiotic therapy increases the risk of antibiotic residues occurring in milk in the periparturient period. Increasing the interval between prepartum IM antibiotic treatment and parturition and testing for antibiotic residues in milk postpartum will reduce the risk of antibiotic contamination resulting from this treatment.