Evaluation Of Selective Dry Cow Therapy For Controlling Mastitis And Improving Antibiotic Stewardship In U.S. Dairy Herds

Abstract

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.