Rowe, Samuel2020-08-252020-08-252020-03https://hdl.handle.net/11299/215077University of Minnesota Ph.D. dissertation.March 2020. Major: Veterinary Medicine. Advisor: Sandra Godden. 1 computer file (PDF); x, 210 pages.The objective of this research was to identify strategies that reduce antibiotic use at dry-off (dry cow therapy; DCT) without having negative effects on cow health and productivity. Chapter 2 reports findings from a cross-sectional study of 2,889 late lactation cows from 80 herds in the US. Herds were purposively selected to achieve near-equal representation of four bedding materials of interest. Each herd was visited twice. At each visit, aseptic quarter-milk samples were collected (n = 10,448), along with bedding samples (n = 158). Milk and bedding samples were cultured under aerobic conditions. Quarter-level prevalence of IMI was 21.1%, indicating that selective DCT (SDCT) could result in a more efficient use of antibiotics than blanket DCT (BDCT) in some U.S. herds. Counts in bacteria were positively associated with IMI, indicating that antibiotic use at dry-off could potentially be reduced by preventing IMI during lactation through improved bedding management. Chapters 3-5 report findings from a multi-site, randomized, controlled, clinical trial. Cows (n=1275) from 7 herds at 4 sites were randomized to either BDCT, rapid culture-guided SDCT or algorithm-guided SDCT. Health and productivity were monitored during the dry period and the first 120 days of lactation. Both SDCT approaches reduced antibiotic use at dry-off by 55%. Both SDCT approaches performed similarly to BDCT for dry period IMI dynamics (IMI cure, new IMI and post-calving IMI risk; Chapter 3) and post-calving health and production (clinical mastitis and culling/death rates, somatic cell counts and milk yield; Chapter 4). The agreement (Cohen’s Kappa; κ) and negative predictive values (NPV) for detection of IMI, as determined by the reference test, laboratory-based aerobic culture were rapid-culture (κ = 0.28 , NPV = 0.87) and algorithm (κ = 0.09, NPV = 0.80), indicating that some infected quarters escaped antibiotic treatment at dry-off (Chapter 5). Culture- and algorithm-guided SDCT can be used in commercial dairy farms for reduction of antibiotic use.enAlgorithmDry cow therapyMastitisRapid cultureThesis or Dissertation