EFFECTS OF ANTIBIOTIC-THERAPY AT DRYING OFF ON MASTITIS IN THE DRY PERIOD AND EARLY LACTATION

Four strategies for selecting cows for intramammary therapy with benzathine cloxacillin at drying off were compared in 12 Victorian dairy herds. The bacteriological status of all quarters of all (1044) cows was determined just before drying off, within 2 d of calving, and again 3 to 5 months after calving. All cases of clinical mastitis (from calving to mid-lactation) were recorded. Cows not infected at drying off were allocated randomly to 2 subgroups of approximately 350 cows each: not infected, not treated (NI-NT), or not infected, all quarters treated (NI-AT). New infection rates in the dry period (3.8%) for NI-NT) vs 2.1% for NI-AT) and in early lactation (4.1% for NI-NT vs 3.9% for NI-AT) were low and these differences were not significant. Incidence of clinical mastitis in early lactation was almost 50% higher for the treated group of uninfected cows compared with the untreated group (0.05 < p < 0.1). Cows infected in one or more quarters at drying off were split randomly into 2 subgroups of approximately 170 cows each: infected, all quarters treated (I-AT), or infected quarters treated only (I-QT). The new infection rate during the dry period was nearly 4 times higher for I-QT (15.3%) due to significantly more new infections by Staphylococcus aureus and Streptococcus uberis. In early lactation there was a significantly higher rate for S. aureus in group I-AT (34 vs 13 new infections), but no overall difference for total pathogens between groups. S. uberis had a significantly higher rate in the I-QT group, both at calving (26 vs 5) and at mid-lactation (15 vs 6), compared with the I-AT group. Incidence of clinical mastitis in early lactation was not significantly different between treatment groups although there were 25% more clinicals for the I-QT group (25 vs 20).