Effect of selective dry cow therapy on dry period intramammary infection dynamics and their association with management factors in Japan

Selective dry cow therapy (SDCT) allocates antibiotics in the dry period to cows or quarters with a high risk of intramammary infection (IMI), potentially improving antibiotic stewardship. However, SDCT has not been used in Japan, possibly due to concerns of negative impacts on udder health. This research aims to evaluate how the SDCT use affected dry period IMI dynamics in Japan. Additionally, the effects of management factors were also considered. At dry-off, 44 cows received antibiotics plus external teat sealant or external sealant in isolation based on their IMI risk, which was assessed using milk culture, a modified California mastitis test (CMT), and mastitis records. The SDCT approach allowed antibiotic use to be reduced by 33.7%. However, quarters with a low risk of infection who received no antibiotics had a numerically higher prevalence of IMI before calving than those who received antibiotics (28.6% vs. 19.2%). In addition, an increased risk of IMI in quarters without antibiotics was also attributed to poor herd hygiene and a shorter duration of external teat sealant adherence. The result suggests that these factors influence the outcome of an SDCT program. Therefore, a uniform recommendation for dry cow herd management may not be optimal.