A Singh, P R F Adkins, M Wieland
Individual quarter dry-off (QDO) is used to manage cows with recurrent clinical mastitis or chronic subclinical mastitis, yet little is known about how the pathogen identified in the affected quarter before QDO influences subsequent udder health, milk production, and culling risk. The objective of this retrospective matched cohort study was to investigate the association between QDO and these outcomes during the first 100 d after the procedure. Data were retrieved from a 4,900-cow dairy herd milked 3 times daily. We compared outcomes of cohorts that consisted of 2,967 cows detected with either recurrent clinical or chronic subclinical mastitis and subjected to QDO with outcomes of 2,967 randomly selected healthy herd mates. Cows in the QDO cohort were classified into pathogen groups based on milk culture results obtained from the quarter undergoing QDO in the same lactation. Within pathogen groups comprising at least 50 QDO cows, cows were analyzed separately according to the indication for QDO (recurrent clinical mastitis or chronic subclinical mastitis). Before QDO, cows subjected to the procedure had lower projected 305-d mature equivalent milk yield and experienced more previous clinical mastitis events than matched healthy control cows. The following groups were analyzed together with their respective healthy control cows: No culture, Negative, Streptococcus uberis, Streptococcus spp., and Escherichia coli pathogen groups were analyzed for both selection criteria, whereas the Klebsiella spp., Pasteurella spp., and Streptococcus dysgalactiae pathogen groups were analyzed only among cows subjected to QDO because of recurrent clinical mastitis. Other pathogen groups were not evaluated because sample sizes were insufficient to permit meaningful statistical analyses. Generalized linear mixed models were used to evaluate milk yield and SCC, whereas Cox proportional hazards and logistic regression models were used to evaluate clinical mastitis and culling. Compared with their matched healthy control cows, QDO cows had lower milk yield after QDO, although the duration of these differences varied among pathogen groups. Among cows subjected to QDO because of chronic subclinical mastitis, SCC declined after QDO but remained higher than that of matched healthy control cows during early follow-up for selected pathogen groups. Increased hazards of subsequent clinical mastitis were observed in the No culture, Negative, Streptococcus spp., and Streptococcus dysgalactiae pathogen groups among cows subjected to QDO because of recurrent clinical mastitis, and in the No culture, Negative, and Streptococcus spp. pathogen groups among cows subjected to QDO because of chronic subclinical mastitis. An increased hazard of culling was observed only in the No culture pathogen group. Our findings indicate that responses following QDO differed according to both the pathogen identified before QDO and whether the procedure was performed because of recurrent clinical mastitis or chronic subclinical mastitis.