Effects Of Genetic Selection On Composition And Biosynthesis Of Milk Fats In Early Lactation Of Holstein Cows

Abstract

Genetic selection in dairy cattle has greatly improved the milk yield in the past decades. Contemporary Holsteins (CH) that were selected and maintained by the University of Minnesota produced more than 4,500 kg milk in 305 days than its counterpart unselected Holsteins (UH), which has stable milk yield since 1964. However, the influences of genetic selection on the chemical composition of cow milk were not well characterized. In this study, cows from UH and CH genotypes (CH) (n = 12/genotype) were co-housed and fed the same diet ad libitum 5 weeks prepartum. Weekly milk samples were obtained from the milking on each Tuesday night in the first 9 weeks of lactation. Analysis of macronutrients in milk indicated that CH and UH had comparable levels of lactose (4.75% vs. 4.69%, P=0.08). However, CH had a slight lower level of proteins (2.98% vs. 3.13%, P<0.05) and much higher level of fats than UH (4.33% vs. 3.55%, P<0.01). Lipidomic analysis through high-resolution liquid chromatography-mass spectrometry (LC-MS) analysis, multivariate data analysis (MDA), and MSMS fragmentation further revealed that the TAG profiles of UH and CH milk differed greatly in early weeks of lactation, but became more comparable by week 9. Hierarchical clustering analysis (HCA) of TAGs markers indicated that CH milk were more enriched with preformed fatty acids (FAs) while UH milk had higher abundance of FAs originated from de novo synthesis. This conclusion was further confirmed by quantitative analysis of FAs and organic acid precursors. Overall, these observations suggested that genetic selection increased the contribution of preformed FAs from blood lipids to the biosynthesis of milk TAGs, especially in the early phase of lactation.