FOR 45 years, infusing a long-acting antimicrobial preparation into each teat of a dairy cow at the end of lactation has been one of the most important tools in the National Institute for Research in Dairying/Central Veterinary Laboratory (NIRD/CVL) five-point management strategy to minimise intramammary infections. The strategy has been widely adopted internationally, aimed at both treatment of, and protection from, intramammary infections. The success can be measured in the reduction of national herd bulk milk cell counts from in excess of one million cells/ml in the mid-1960s to values commonly around 150,000 cell/ml annual average or less today (for example, Booth 1988, AHDB 2017). More importantly, the prevalence of intramammary infection at the end of lactation has fallen from around 60 per cent to as low as 10 to 20 per cent of cows, varying with herd, farming system and even country (Wilson and Kingwill 1975, Bradley and others 2007, Hillerton 2017). As a consequence of the reduced prevalence of intramammary infections at drying off, the routine use of antibiotic dry cow therapy is no longer required …