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

This 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 (C 18 ) FA flows], plasma FA content, milk FA yield and composition. Milk short‐ and medium‐chain FA (C 4 –C 16 ) 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 C 18 secretion was proportional to C 4 –C 16 secretion, independently of C 18 dietary supply. For the low‐forage high‐lipid diet, C 4 –C 16 secretion apparently limited C 18 secretion, which represented only 36% of absorbed C 18 , compared to 79% with a high‐forage high‐lipid diet (higher C 4 –C 16 secretion). Despite these high variations in total C 18 secretion, the composition of milk C 18 was not significantly different, within cows, from the duodenal C 18 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.