Milk fatty acids: Mammary synthesis could limit transfer from duodenum in cows

Abstract

AbstractThis study reports a quantitative analysis of two experiments comparing the effects of different dietary forage/concentrate ratios and lipid supplements on cow lipid digestion and milk fatty acid (FA) secretion. We studied, on an individual cow basis, the relationship between digestive variables [fiber intake and duodenal 18‐carbon (C18) FA flows], plasma FA content, milk FA yield and composition. Milk short‐ and medium‐chain FA (C4–C16) secretion depended mainly on additive effects of dietary fiber intake and lipid supplementation level. For high‐lipid diets (3–5% oil in dry matter intake), milk C18 secretion was proportional to C4–C16 secretion, independently of C18 dietary supply. For the low‐forage high‐lipid diet, C4–C16 secretion apparently limited C18 secretion, which represented only 36% of absorbed C18, compared to 79% with a high‐forage high‐lipid diet (higher C4–C16 secretion). Despite these high variations in total C18 secretion, the composition of milk C18 was not significantly different, within cows, from the duodenal C18 composition, when substrates and products of Δ‐9 desaturase were summed. Mammary desaturation activity (expressed in mol/day) was a linear function of the estimated mammary uptake of the substrates, and this endogenous production represented more than 75% of the milk secretion of the Δ‐9 desaturated products.