Genetic and non-genetic factors influencing milk composition and cow metabolism

The aim of this study was to evaluate the influence of genetic and non-genetic factors on milk composition and cow metabolism. Czech Holstein ( $n=230$ ) and Czech Simmental cows ( $n=289$ ) were analysed. The effects of farm, breed, parity, lactation stage, and polymorphisms on the AGPAT6, SCD1, FASN, LEP, and DGAT1 genes were evaluated. The contents of protein, fat, and fatty acids (FAs); the levels of $\beta$ -hydroxybutyric acid (BHB), citric acid, acetone, and urea; and somatic cell scores were analysed. The farm and lactation stages were the most significant factors influencing the parameters, as well as their interactions. The effects of parity and breed were much weaker. The impact of gene polymorphisms was very low, as were the interactions between gene and farm, between lactation stage and farm, and between parity and lactation stage. BHB levels were influenced by farm in the models including all genes and by parity in the models including AGPAT6, SCD1, and DGAT1 genes. In contrast, the effect of lactation stage on BHB levels was not significant in any model. The contents of protein and lactose were influenced by parity and lactation stage and, in some cases, by farm-lactation stage interactions and by gene polymorphisms. Urea, citric acid, acetone, and FA levels were affected by the lactation stage, and urea was also affected by the interaction of the farm and lactation stage. Daily milk production correlated negatively and significantly with components other than urea, acetone, BHB, and polyunsaturated FAs. The urea content and milk yield were significantly positively correlated. The correlations of BHB with fat $/$ protein and fat $/$ lactose ratios were significantly positive, but the prediction of ketosis in individual cows does not seem to be reliable. The level of BHB in milk, which is routinely assessed during milk performance testing, is suggested as a field predictor of clinical or subclinical ketosis in animals.