Genetic Selection of Holstein Cows Increased Preformed Fatty Acids and Decreased Carnitine Contents in Early-Lactation Milk

Genetic selection of dairy cattle has dramatically increased milk yield of dairy cows, but its influences on the chemical composition of milk, especially during early lactation, are not well characterized. A continuous selective breeding practice in Minnesota generated unselected Holsteins (UH) that represent US Holsteins in the early 1960s and contemporary Holsteins (CH) that produced much more milk and experienced greater negative energy balance than the UH cows. In this study, milk samples were collected weekly in the first 9 weeks of lactation from CH and UH cows (n = 12/genotype), and their metabolite profiles were defined by macronutrient analysis, liquid chromatography-mass spectrometry (LC-MS) analysis, multivariate analysis, structural elucidation, and quantitative analysis. The CH milk contained more triacylglycerols (TAGs) than the UH milk. Lipidomic profiling showed that the CH milk had more TAGs with at least two preformed fatty acid moieties while the UH milk had more TAGs with at least two de novo synthesized fatty acid moieties. Subsequent quantitative fatty acid analysis determined that the increase in preformed fatty acids in CH milk was responsible for different TAG profiles between the two genotypes. Moreover, the analysis of short-chain carboxylic acids in milk showed that episodes of transient and persistent ketosis were more frequent in CH than in UH cows. Interestingly, CH milk contained less free carnitine and short-chain acylcarnitines in the UH milk indicating the greater utilization of mobilized lipids increased the utilization of carnitine. These observations warrant further mechanistic investigations on the metabolic consequences of genetic selection on dairy cows.