Genetic and Nongenetic Variation of Blood Metabolites Predicted from Milk Infrared Spectra in Dairy Cattle

The negative energy balance experienced by dairy cows in early lactation caused by the lack of trade-off between energy intake (input) and requests for lactogenesis (output) is responsible for the occurrence of metabolic disorders. Blood metabolites are important indicators to monitor nutritional and energy status of the cows, and to detect the presence of metabolic disorders. In particular, the hyperketonemia (HYK) is one of the most frequent and costly metabolic diseases in early-lactation dairy cows, and it is commonly diagnosed through the determination of β-hydroxybutyrate (BHB) concentration in blood. With this background, the overall objectives of the present thesis were: i) to summarise literature results on phenotypic and genetic aspects of BHB concentration in blood and milk of dairy cows; ii) to develop mid-infrared (MIR) spectroscopy prediction models for routine determination of blood metabolites; iii) to describe phenotypic variation of MIR-predicted blood metabolites in Brown Swiss, Holstein-Friesian and Simmental cattle breeds; iv) to assess the genetic variation of blood BHB and nonesterified fatty acids (NEFA) predicted by MIR spectroscopy, and their correlations with milk production and composition traits in early-lactation Italian Holstein dairy cows. Hyperketonemia is an abnormal concentration of circulating ketone bodies in the blood; in particular, concentration of blood BHB ≥ 1.2 mmol/L is commonly recognized as indicator of HYK. In general, HYK impairs health of dairy cows by increasing the risk of the onset of other early-lactation diseases, and it negatively affects reproductive performance. Although the relationship with milk yield is still controversial, HYK has a detrimental effect on milk composition. Costs of HYK are mainly imputable to impaired fertility and milk loss. From a genetic point of view, results from the literature suggested the feasibility of selecting cows with low susceptibility to HYK. Milk is the most promising matrix to monitor HYK, taking advantage of using MIR spectroscopy during routine milk recording. The effectiveness of using routine milk MIR spectra to predict main blood metabolites in early-lactation dairy cows was evaluated. Blood BHB, urea and NEFA were the most predictable traits. Predicted blood BHB showed an improved performance in detecting cows with HYK, compared with commercial calibration equation for milk BHB. Factors associated with the phenotypic variation of MIR-predicted blood metabolites were investigated on a large spectral multi-breed database. Holstein-Friesian cows had the greatest concentration of blood BHB and NEFA, and the lowest blood urea content. Blood BHB and NEFA concentrations generally increased with parity. The greatest BHB concentration was observed in the first 10 days of lactation, except for Simmental cows. From 5 to 35 days in milk, NEFA concentration decreased, whereas urea content increased for all considered breeds. The maximum levels of blood BHB and NEFA concentrations were recorded in spring and early summer. Blood urea generally increased across the year, from spring to winter. Genetic parameters for MIR-predicted blood BHB and NEFA concentrations were estimated. The greatest heritability for both metabolites was assessed in the first 10 days after calving (0.32 for BHB and 0.23 for NEFA), and their genetic correlation varied from 0.50 to 0.60. Moreover, an unfavourable trend of estimated breeding values for both blood BHB and NEFA concentrations across year of birth of the bulls was detected. Genetic correlations of BHB and NEFA with milk yield, somatic cell score, protein, lactose and urea content were similar or at least in the same direction, whereas opposite correlations were observed with fat content and fat-to-protein ratio.