**9. Antibiotic residues in milk following prepartum lactating cow treatment**

A disadvantage of prepartum lactating cow antibiotic therapy for controlling mastitis in heifers is the potential for antibiotic residues, especially if heifers calve sooner than expected. In one study (Oliver et al., 1992), it was shown that 17% of colostrum samples from heifer mammary glands infused with cloxacillin were positive for antibiotic residues by the *Bacillus stearothermophilus* disc assay, the majority of which were from heifers that calved within 5 days of treatment. Only 4.5% of samples obtained at the first milking after parturition were positive for antibiotic residues if intramammary infusion of cloxacillin occurred > 7 days before parturition. All samples obtained 3 days after parturition, the time when milk would likely be marketed for human consumption, were negative for antibiotic residues. In contrast, 85% of colostrum samples and 28.2% of samples obtained 3 days after parturition were positive for cephapirin residues, and marked variability between time of antibiotic treatment and parturition with persistence of antibiotic residues was observed. Thus, antibiotic treatment of heifer mammary glands earlier in gestation may be advantageous from a residue standpoint, but the timing of antibiotic treatment and subsequent persistence in mammary secretions could impact efficacy.

Another study was conducted to determine if antibiotic treatment of heifer mammary glands earlier in the prepartum period reduced occurrence of residues in milk (Oliver et al., 1997a). A total of 82 Jersey heifers was used. Approximately half served as negative controls (n = 42) and half received an intramammary infusion of 200 mg cephapirin sodium (n = 40) 14 days before calving. Forty percent of samples from cephapirin-treated quarters were positive at the first milking after calving, but only 3.1% of samples obtained from antibiotictreated quarters at the sixth milking (3 days) after calving were positive; 3 of the 4 positive samples were from a heifer that calved early and within 3 days of treatment.

Thus, the interval between prepartum antibiotic treatment and calving was related to persistence of residues during early lactation, and infusion of antibiotics 14 days prepartum (Oliver et al., 1992) compared with 7 days prepartum (Oliver et al., 1997a) reduced occurrence of residues in milk during early lactation. Similarly, Middleton et al. (2005) found that after heifers were treated 10-14 days prior to expected calving date with a pirlimycin lactating cow product, prevalence of IMI in early lactation was decreased without causing pirlimycin residues in milk at 3 days postpartum, even when a heifer was treated 1 day before calving (heifer calved early).
