Negatively controlled trial investigating the effect of dry cow therapy in first-lactation dairy cows on the prevalence of intramammary infections in the subsequent lactation

Research has shown that selective dry cow therapy (SDCT) can be implemented in well-managed dairy farms without negatively affecting postpartum udder health and performance. This raises the question of whether dry cow therapy (DCT) is necessary in well-managed farms. Our primary objective was to assess the effect of withholding DCT from cows at the end of their first lactation on postpartum quarter-level IMI prevalence. Our secondary objectives were to evaluate the effect of DCT omission on postpartum quarter-level IMI prevalence in subgroups of quarters, stratified by quarter-level SCC and California Mastitis Test (CMT) results at dry-off, and to use those findings to predict the effect of SCC- and CMT-guided SDCT programs on antibiotic use and udder health. This randomized controlled trial enrolled 446 cows in a single herd, which were assigned at dry-off to receive either intramammary antibiotics and an internal teat sealant (ATB+ITS) or only an internal teat sealant (ITS-only). Quarter-level milk samples were collected at dry-off to determine SCC and CMT results. During the first 2 wk after calving, quarter-level milk samples were collected and submitted for milk culture. Logistic regression was used to investigate the effect of DCT omission on postpartum IMI prevalence overall and in subgroups stratified by SCC (50, 100, 150, 200, 400 × 1,000 cells/mL) or CMT results (no reaction, trace, positive, or more) at dry-off. Estimated marginal means were used to predict the effect of SCC- and CMT-guided SDCT on antibiotic use and postcalving IMI prevalence. Withholding DCT from all quarters at dry-off in first-lactation cows increased the early postpartum prevalence of gram-positive pathogens during the second lactation (relative risk [RR]: 1.29; 95% CI: 0.99, 1.65), particularly NAS and mammaliicocci (RR [95% CI]: 1.95 [1.05, 3.57]), whereas the prevalence of Staphylococcus aureus or Streptococcus spp. and Streptococcus-like organisms remained low in both groups. Quarter-level SCC at dry-off was demonstrated to be useful for guiding DCT decisions. Quarters with high SCC at dry-off in the ITS-only group had a higher risk of postpartum gram-positive IMI compared with ATB+ITS quarters, with RR ranging from 1.36 to 2.29. In contrast, among quarters with SCC equal or below the investigated cutoffs, the risk of postpartum IMI was similar between ITS-only and ATB+ITS groups, with RR ranging from 0.96 to 1.18, even at cutoffs as high as 400,000 cells/mL. Estimated marginal means from statistical models in this study found that limiting antibiotic treatments to quarters with SCC >400,000 cells/mL could reduce antibiotic treatment at dry-off by 85.1% without significantly affecting gram-positive IMI postpartum prevalence (RR [95% CI]: 1.07 (0.78, 1.47), when compared with blanket DCT. However, CMT-based SDCT appeared less effective, as both CMT-positive and CMT-negative quarters showed a numerical increase in postpartum gram-positive IMI prevalence when DCT was withheld (RR between 1.24 to 1.28 and 1.28 to 1.86, respectively). In conclusion, omitting DCT at the end of the first lactation may increase gram-positive IMI and is therefore not recommended. However, quarter-level SCC-guided SDCT appears to be a viable strategy to reduce antibiotic use without substantially affecting postpartum IMI prevalence.