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 (C₁₈) FA flows], plasma FA content, milk FA yield and composition. Milk short- and medium-chain FA (C₄-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 C₁₈ secretion was proportional to C ₄-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 C ₁₈ 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.