A randomized, controlled trial examining quarter-level somatic cell count and culture-based selective dry cow therapy against blanket dry cow therapy on early lactation production outcomes

The objective of this study was to determine quarters requiring antimicrobial treatment using either a benchtop somatic cell counter (S-SDCT) or culture with gram-positive selective media (C-SDCT) and compare outcomes in these cows to those receiving blanket dry cow therapy (BDCT) in a randomized, controlled trial. Two novel methods of identifying cows with intramammary infections followed by selective antimicrobial treatment were evaluated at a commercial dairy farm to determine their usefulness in decreasing antibiotic usage during the dry period without significant detrimental effects on milk quality and production. Cows (n = 840) were randomly allocated to one of 3 groups (BDCT, C-SDCT, S-SDCT) the day before dry-off and quarter-level milk samples (QLMS) were collected. The QLMS from cows in the S-SDCT group were evaluated using the cell counter and quarters were treated if somatic cell count (SCC) was ≥200,000 cells/mL, while QLMS from cows in the C-SDCT group were cultured and quarters were treated if the culture showed growth. All cows in the BDCT received antimicrobial therapy and all cows received an internal teat sealant regardless of treatment group. Outcomes measured were first and second DHIA test somatic cell count, milk production through 60 d in milk, cows leaving the farm, clinical mastitis, and bacteriologic new infections in a subset of quarters. Cows in both SDCT groups had fewer antimicrobial treatments than cows in the BDCT group as was expected, and cows in the C-SDCT group had fewer treatments than those in the S-SDCT group. Cows in both SDCT groups had higher linear score at the first DHIA test (BDCT: 1.8, S-SDCT: 2.2, C-SDCT: 2.2), however there were no other differences between groups regarding any other outcomes measured. While antimicrobial use was significantly reduced, farms should use caution in adopting the benchtop analyzer and the selective media described in this study as ways to identify infected cows for dry cow therapy as they may result in increased linear score early in lactation.