Strategies for implementation of selective dry cow treatment in spring-calving pasture-based dairy production systems

There is a requirement to reduce the use of antimicrobials in animal production systems. Blanket dry cow therapy (BDCT) using an antibiotic with or without internal teat seal (ITS) at dry-off has been widely practiced in Irish dairy herds. Infusion of cows without an intramammary infection (IMI) at dry-off with an ITS alone has been identified as an area for potential reduction in antibiotic use; a practice referred to as selective dry cow treatment (SDCT). The overall aim of the research carried out in this thesis was to both evaluate the impact and establish strategies when implementing SDCT without negatively impacting SCC and IMI in low bulk tank SCC (≤200,000 cells/ml) Irish spring-calving pasture-based dairy herds. This encompassed four scientific papers assessing the efficacy of ITS in herds, precisely differentiating between cows with and without IMI in the lead up to dry-off using SCC data and establishing the association between SDCT and herd management strategies. In the first paper a total of 842 cows across 5 herds with a cow?level SCC of <200,000 cells/ml were assigned ITS or antibiotic plus ITS (AB+ITS) at dry-off. Cows treated with ITS alone had significantly higher SCC and higher risk of new IMI in the following lactation compared to those treated with AB+ITS. However, there were significant differences to the efficacy of ITS according to the herd studied. In the second paper, cow-level SCC data of 2,074 cows across 21 herds was evaluated to predict IMI at dry-off. The last test-day SCC was the best predictor of infection at dry-off. The cut-point for last test-day SCC which maximised sensitivity and specificity was 64,975 cells/ml. Using the same data set as Paper 2, Paper 3 showed that when using quarter?level SCC, the cut-point for last test-day SCC which maximised sensitivity and specificity was 61,000 cells/ml for primiparous and 101,000 cells/ml for multiparous cows. In the fourth paper the association between SDCT and herd management practices was explored using data from 2,016 cows across 21 herds. There was a significant interaction between SDCT and milk yield at last test-day; cows with milk yield >15kg and treated with ITS had a higher SCC in the following lactation compared to cows treated with AB+ITS. Regarding management practices, using a California Mastitis Test to detect high SCC quarters and cleaning cubicles twice daily over the dry period were associated with lower SCC in the following lactation. In all studies, Staphylococcus aureus was the predominant pathogen accounting for 87.0% to 92.1% of quarter-level IMI. Overall these findings provide criteria and management practices to guide the identification and selection of cows suitable for ITS at dry-off.