Advanced Profiling of Blood Metabolites in Dairy Cattle: From Milk Mid-Infrared Predictability to Genomic Architecture

In dairy cattle, metabolic priorities are predominantly directed to the mammary gland, with energy and nutrients largely devoted to lactogenesis and, subsequently, galactopoiesis. Consequently, the incidence of metabolic disorders such as negative energy balance followed by ketosis is particularly high during the transition period in highly specialized dairy breeds. The objectives of this thesis were to: (i) investigate the sources of variability of major blood traits in healthy cows in the first 38 days post-partum; (ii) assess the predictive ability of milk mid-infrared (MIR) spectroscopy for estimating hematic parameters; (iii) estimate the heritability and genetic correlations of MIR-predicted blood traits in the Italian Holstein population, the cosmopolitan most important specialized dairy breed; (iv) estimate genetic parameters of the same phenotypes in the Italian Simmental, a dual-purpose cattle breed; and (v) identify the genomic regions associated with major blood biomarkers of ketosis in Holstein. Chapter I provides an overview of the phenotypic variability of major blood traits in healthy cows during early lactation and assesses the predictive ability of milk MIR spectra for these metabolites. In Chapter II, the predictive models were applied to a large database with spectra recorded in the Italian Holstein population to estimate the heritability and genetic correlations of blood energy profile traits determined via milk MIR (β-hydroxybutyrate, non-esterified fatty acids, urea, cholesterol, and glucose), and to evaluate their genetic associations with milk yield and composition. Chapter III applies the same approach to the Italian Simmental population, a dual-purpose breed with different metabolic priorities compared to highly specialized breeds. Finally, Chapter IV focuses on a genome-wide association study aimed at the identification of (non)coding genomic regions associated with blood biomarkers of subclinical ketosis and providing insights into their polygenic architecture. Overall, the findings of the present Thesis highlight the potential of predicted blood phenotypes as novel selection criteria to promote genetic progress toward healthier and more resilient dairy cows. Moreover, the genome-wide association study offer new insights into the biological pathways underlying negative energy balance and subclinical ketosis in Holstein cows, suggesting potential targets for future functional studies and breeding strategies.