Correlation of Milk Fat with Dietary and Metabolic Factors in Cows Fed Restricted-Roughage Rations Supplemented with Magnesium Oxide or Sodium Bicarbonate

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Abstract
Twelve cows were fed restricted-roughage rations in a Latin square trial with NaHCO3 (0.8 lb/day) and MgO (0.4 lb/day) as supplemental treatments. Both supplements prevented the depression in milk fat concentration caused by roughage restriction. Milk fat concentration was studied as a function of plasma fat fractions, plasma acetate, ruminal metabolites, and mammary gland uptake of these metabolites. Sodium bicarbonate affected milk fat through depression of ruminal molar percent propionate, whereas MgO affected milk fat through increased mammary uptake of plasma acetate and triglycerides. Low-density lipoprotein concentrations were uniquely low in cow plasma compared to other species, but still accounted for a major portion of the arteriovenous difference for plasma lipids across the mammary gland.