Comparative effects of intramammary dry-cow antimicrobials and internal teat sealant on postpartum somatic cell count dynamics.

Optimizing dry-cow therapy is essential for maintaining a favorable early-lactation somatic cell count (SCC). However, comparative field data on different classes of intramammary antimicrobials (IMAs) remain limited. This randomized 3 × 2 factorial field trial compared the effects of three IMA formulations, administered with or without an internal teat sealant (ITS), on postpartum SCC dynamics and milk characteristics in a single well-managed herd. Cows (n = 263), stratified by parity and pre-dry-off SCC (<200,000 or ≥200,000 cells/mL), were randomly assigned to treatment groups. Composite milk samples were collected at dry-off and at 10-30 days in milk (DIM). Mammary inflammatory status was classified according to SCC transition categories (Low-Low, Low-High, High-Low, High-High). Transition patterns were analyzed using exact chi-square tests, whereas continuous outcomes were analyzed using linear mixed-effects models. Overall, 84.0% of cows had low postpartum SCC (<200,000 cells/mL). Neither IMA formulation nor ITS significantly affected postpartum SCC or transition patterns (P > 0.05). Most cows (71.1%) remained in the Low-Low category. Pre-dry-off SCC was significantly correlated with postpartum SCC (Spearman's ρ = 0.28, P < 0.0001). Small but significant differences in milk yield and protein percentage were observed among treatment groups (P < 0.05). In this well-managed herd, individual cow factors, particularly pre-dry-off SCC, were more strongly associated with early-lactation SCC status than either IMA formulation or ITS use. Because these findings are based on a single herd without bacteriological culture data, they should be interpreted cautiously and confirmed in adequately powered multi-herd studies.