Chairperson, Graduate Committee: Megan Van EmonKnuth, Ryan MichaelW. C. Stewart, J. B. Taylor, C. J. Yeoman, B. Bisha, M. L. Van Emon and T. W. Murphy were co-authors of the article, 'The prevalence and etiology of subclinical mastitis and association with milk somatic cell count in western range flocks' submitted to the journal 'Journal of animal science' which is contained within this thesis.W. C. Stewart, J. B. Taylor, C. J. Yeoman, M. L. Van Emon and T. W. Murphy were co-authors of the article, 'Udder morphometry and health and their relationship with ewe somatic cell count and productivity in extensively manged research flocks' submitted to the journal 'Journal of animal science' which is contained within this thesis.2020-02-062020-02-062019https://scholarworks.montana.edu/handle/1/15619Mastitis results from a bacterial infection of the mammary gland and is a devastating disease to all sheep producers from both an economic and animal welfare perspective. Clinically infected ewes display visually apparent symptoms, however, subclinically infected ewes do not although this form is more common. Since milk is a direct commodity of dairy animals, much of the past research has been conducted here and the production and economic impacts of subclinical mastitis are less clear in non-dairy (e.g., meat- and wool-type) ewes. The objectives of the first study were to identify bacteria species present in milk collected from clinically healthy ewes and evaluate somatic cell count (SCC) thresholds relating to intramammary infection. Milk samples were collected from two research flocks in the Western U.S. (Montana State University = MSU; U.S. Sheep Experiment Station = USSES). Bacteria were identified by both culturing and identification via mass spectrometry and polymerase chain reaction methods. Overall, 60 bacteria species were identified using mass spectrometry and the most common belonged to the Bacillus and Staphylococcus genera. The ideal SCC thresholds to predict intramammary infection ranged between 240 x 10 3 to 1370 x 10 3 cells/mL, depending on the flock and time of collection. In the second study, milk samples were collected and udder and teat morphometric traits were observed to predict ewe productivity via dam 120 day adjusted litter weaning weight (LW120). Udder and teat characteristics were assessed on a linear scale at each sampling and included teat length, udder symmetry, and presence of supernumerary teats, to name a few. The effect of log10-transformed SCC (LSCC) on ewe productivity was dependent on lactation stage and production year, but when significant, indicated a 9.2-14.7 kg reduction in LW120 associated with a 1-unit increase in LSCC. Factors which influenced LSCC included parity, production year, and presence of supernumerary teats in USSES ewes and, for MSU ewes, included teat length, external teat damage, udder symmetry, and presence of supernumerary teats. The results indicate subclinical mastitis is common and additional studies investigating techniques to mitigate its severity and prevalence in meat- and wool-type ewes are warranted.enMastitisInfectionSheepEwesBreastMilkThesisCopyright 2019 by Ryan Michael Knuth