Comparative efficacy of three dry-cow antibiotic formulations in spring-calving New Zealand dairy cows

Aim: To evaluate the efficacy of a dry-cow antibiotic preparation containin g cloxacillin plus ampicillin in a formulation that gives a 10-week duratio n of action, in comparison to products containing cephalonium (10-week acti on) or cloxacillin alone (7-week action). Methods: A total of 493 cows were selected from 6 spring-calving dairy herd s in the Manawatu region of New Zealand, according to the criteria of the S AMM plan, to receive intramammary antibiotic therapy at the end of lactatio n (drying off). Cows were randomly allocated to receive 1 of the 3 dry-cow antibiotic products under investigation. Cows were examined twice during th e dry period and twice daily during the first 10 days of their subsequent l actation for the presence of mastitis. Milk samples were collected from ind ividual quarters at the time of drying off and at 7 and 28-35 days after ca lving, for determination of milk somatic cell counts (SCC). Bacteriology wa s carried out on milk samples taken from cows that developed mastitis durin g the first 10 days after calving. Results: No cows developed mastitis during the dry period. Sixteen cows dev eloped clinical mastitis within 10 days of calving; there was no difference in incidence between treatments. Streptococcus uberis was the most commonl y isolated organism. Mean SCC on Day 7 were lower (p = 0.019) in cephaloniu m-treated quarters (189.9 +/- 28.4 x 10(3) cells/ml) than in cloxacillin-tr eated quarters (388.7 +/- 71.2 x 103 cells/ml); values in quarters receivin g cloxacillin plus ampicillin were intermediate (252.0 +/- 47.0 x 10(3) cel ls/ml). SCC were similar between treatment groups on Day 28-35. Conclusions: The use of a combination of cloxacillin plus ampicillin was ef fective for the prevention of mastitis during the dry- and peri-calving-per iods in pastured dairy cattle.