Effects of reduced intramammary antimicrobial use during the dry period on udder health in Dutch dairy herds

Dry cow therapy (DCT) in the Netherlands changedfrom mainly blanket to selective antimicrobial DCT.This transition was supported by a national guideline,with the individual somatic cell count (SCC) at thelast milk recording before dry-off as the main selectioncriterion for antimicrobial DCT. The aim of this retrospectiveobservational study is to evaluate the SCCdynamics during the dry period at the herd and individualdry period level following the national transitionfrom mainly blanket to selective antimicrobial DCT.At the herd level, we used 2 data sets to evaluate theSCC dynamics during the dry period: (1) a nationaldata set containing 3,493 herds with data availablefrom 2011 through 2015 and (2) a veterinary practicedata set containing 280 herds with data available from2013 through 2015. The herd level analysis was carriedout using key performance indicators provided via milkrecording (CRV, Arnhem, the Netherlands): the percentageof cows that developed a new intramammaryinfection (IMI) during the dry period and the percentageof cows cured of an IMI during the dry period.The effect of DCT at individual dry period level wasanalyzed with a mixed-effects logistic regression modelbased on 4,404 dry periods from 2,638 cows in 20 herdswithin the veterinary practice data set. For these 20herds, individual SCC data from milk recordings andindividual cow DCT were available from 2013 through2015. No significant changes were observed to the SCCdynamics during the dry period at the herd level. Thepercentage of cows that developed a new IMI duringthe dry period ranged between 16 and 18%, and thepercentage of cows cured from an IMI during the dryperiod ranged between 74 and 76%. At the individual dry period level, a low SCC at the first milk recordingfollowing a dry period was associated with the use ofintramammary antimicrobial DCT with or without theconcurrent use of an intramammary teat sealer [oddsratio (OR) = 2.16 and OR = 2.07, respectively], the useof DCT with an intramammary teat sealer only (OR =1.35), and a low SCC at the last milk recording beforedry-off (OR = 1.78). This study demonstrates that theselection of cows for DCT without antimicrobials basedon SCC thresholds at the last milk recording is possiblewithout significant changes to udder health andreduced the use of antimicrobials