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Aims: To test equivalence between two different cephalonium dry-cow therapy (DCT) products with regard to the incidence of bacteriological cure of existing infections of quarters, the incidence of new intramammary infections (IMI) from 2-5 days postpartum and the incidence of clinical mastitis from drying-off to 21 days postpartum., Methods: All cows (n=1,570) on four commercial dairy farms in Southland were eligible for inclusion. Power analysis using a variance inflation factor indicated a required minimum of 2,101 quarters per treatment group. Cows were blocked by herd, parity, previous history of mastitis and most recent somatic cell count (SCC), and then randomly allocated to either of two cephalonium treatments at the cow level, viz the treatment group (new formulation cephalonium) or the positive control group (existing reference formulation cephalonium). All quarters within a cow were assigned the same treatment. Samples collected from all quarters with a SCC ≥500,000 cells/mL at drying-off, and those with a positive culture postpartum were cultured, as well as samples from all quarters collected between 2-5 days postpartum. All cases of mastitis were recorded and sampled for culture. The risk of quarter-level incidence of bacteriological cure, IMI and clinical mastitis was modelled using a GLM and generalised estimating equation (GEE), including the effects of treatment group, age,farm, udder-health status at drying-off and length of dry period., Results: For 829 infected quarters, the estimated incidence of bacteriological cure for all pathogens was 78.0 (95% CI=64.3-91.6)% for the treatment group and 75.7 (95% CI=61.6-89.8)% for the positive control group (p=0.71). Incidence of cure varied with the farm (p=0.001), type of pathogen pre-treatment (p=0.009) and log(10)SCC at drying-off (p<0.001). For 4,530 quarters, the estimated incidence of new IMI, with any pathogen, for treatment and positive control groups was 16.1 (95% CI=13.1-19.7)% and 16.0 (95% CI=13.0-19.5)% respectively (p=0.91). Incidence of IMI varied with farm (p<0.001), pathogen and SCC status before treatment (p<0.001), and length of the dry period (in days) (p<0.001). For 5,136 quarters, the estimated incidence of clinical mastitis was 1.0 (95% CI=0.5-1.7)% in the treatment group and 1.1 (95% CI=0.6-2.0)% in the positive control group (p=0.64)., Conclusions: Use of the two different cephalonium products at drying-off in four dairy herds in Southland resulted in equivalence with respect to incidence of bacteriological cure of existing infections, incidence of new IMI postpartum, and incidence of clinical mastitis in early lactation.