Susceptibility of bovine mammary gland to infections during the dry period.

Quarter foremilk samples (1,574) were collected for bacteriological analysis from 40 cows during late lactation, early involution, prior to parturition, parturition, and during early lactation. Six of 160 quarters were infected with major pathogens during late lactation. Twelve new infections occurred during early involution. Twenty-two quarters were infected at parturition. Of these infections, 12 occurred after early involution, and 10 infections that originated during early involution persisted throughout the dry period. During early lactation, 21 major pathogen infections were observed. Nine occurred after calving, and 12 persisted from parturition. Most major pathogen infections (93.8%) were caused by coliforms and streptococcal species other than Streptococcus agalactiae. Numbers of streptococcal infections were highest at parturition and early lactation. Numbers of quarters infected with coliforms were similar during early involution, parturition, and early lactation. Coagulase negative staphylococcal infections increased during the dry period, whereas Corynebacterium bovis infected quarters decreased. Udder infections were few in cows completing first or second dry periods. The incidence of udder infection was highest in cows completing third or later dry periods.