Management of dry cows to reduce mastitis.

The objective of mastitis control during the dry period is to have as few infected quarters as possible at calving. This depends on enhancing elimination of infections present at drying off and on reducing the new infection rate during the dry period; prevention of new infection appears to offer greater long-term benefits. Incidence of new infection is high in the dry period with periods of high susceptibility shortly after dry off and again prepartum. Pathogens of both contagious and environmental origin cause new dry period infections; however, exposure to contagious pathogens probably decreases with cessation of regular milking, whereas exposure to environmental pathogens continues throughout the dry period. Varying susceptibility over the dry period may be affected by bacterial loads on the teat skin, characteristics of the teat canal, and internal protective mechanisms. At present, antibiotic therapy at the end of lactation is the most effective means of eliminating existing infections and preventing new infections. Although there are reasons to prefer selective therapy, present evidence favors a recommendation for treatment of all cows at the time of drying off. A shortcoming of present therapy regimens for the dry period is that they provide little or no protective effect against new infection prepartum. Other mastitis control methods and management practices have not been shown conclusively to reduce new dry period infections. However, it appears that reduction of exposure to environmental pathogens during dry period should be recommended. More effective means to reduce new infections in the prepartum period are needed.