Effects of antimicrobial treatment at the end of lactation on milk yield somatic cell count and incidence of clinical mastitis during the subsequent lactation in a dairy herd with a low prevalence of contagious mastitis

OBJECTIVE: To determine whether treating cows with antimicrobials at the end of lactation would lower the incidence of clinical mastitis, improve milk production, and decrease somatic cell count (SCC) in the subsequent lactation. DESIGN: Randomized blind field trial. ANIMALS: 233 Holstein cows from a single herd. All cows were in lactation 2 or greater. PROCEDURE: Cows were randomly assigned to treatment groups. Treated cows were given procaine penicillin G and novobiocin by intramammary infusion. Control cows were not treated. Farm personnel recorded cases of clinical mastitis. Milk yield and SCC were recorded during the subsequent lactation. RESULTS: Treatment did not significantly reduce the incidence of clinical mastitis when data for all cows were grouped or when data were stratified by lactation groups (lactation 2 vs lactation > or = 3) or by last SCC ( 500,000 cells/ml). Somatic cell counts (first, mean of first 5, maximum of first 5) for treated and control cows were similar, and proportions of treated and control cows with SCC > 500,000 cells/ml at least once were not significantly different. Treated cows produced 179 kg (394 lb) more milk during the first 17 weeks of lactation than did control cows. CLINICAL IMPLICATIONS: Treating cows with antimicrobials at the end of lactation increased 17-week milk production during the subsequent lactation and, at current milk prices, was financially preferable to not treating them