Introduction: Dairy Lipid Nutrition and Limited Lipid Utilisation Efficiency

Maximising milk production while maintaining metabolic health in high-yielding lactating dairy cows represents a cornerstone of modern dairy nutrition. Immediately postpartum, dairy cows experience a severe Negative Energy Balance (NEB) due to the sharp increase in milk yield outstripping dry matter intake (DMI). To mitigate NEB, dietary lipid supplementation is widely adopted because lipids supply approximately 2.25 times more gross energy than carbohydrates or protein, making them an efficient energy source to help offset the energy deficit; supplemental fatty acids can also serve as direct precursors for milk fat synthesis.

However, due to the unique gastrointestinal physiology of ruminants, the efficiency with which dietary lipids are digested and absorbed in the small intestine is substantially lower than in monogastric animals such as swine and poultry. Furthermore, excessive dietary fat inclusion (>6% to 7% of dry matter) can suppress ruminal microbial fermentation, potentially impairing fibre digestibility and reducing total feed intake. Consequently, enhancing lipid utilisation efficiency requires nutritional strategies that increase small intestinal solubilisation and absorption without compromising ruminal fermentation stability.

Comparative Lipid Digestion and Absorption in Monogastrics and Ruminants

Monogastric and ruminant animals exhibit fundamental differences in gastrointestinal processing and intestinal absorption mechanisms. In monogastric animals, dietary lipids are supplied predominantly as triacylglycerols (TAG). In the small intestine, TAGs are emulsified by bile and hydrolysed primarily by pancreatic lipase to yield 2-monoacylglycerols (2-MAG) and free fatty acids (FFA). These lipolytic products are incorporated into mixed micelles with bile salts and other amphiphilic components, facilitating delivery to the enterocyte surface and subsequent absorption.

In contrast, dietary lipids consumed by ruminants undergo extensive microbial lipolysis in the rumen before reaching the small intestine. Rumen microbial lipases (e.g., from Anaerovibrio lipolytica and Butyrivibrio spp.) hydrolyse esterified lipids and release non-esterified fatty acids. A substantial proportion of the released unsaturated fatty acids is subsequently subjected to ruminal biohydrogenation, resulting in a digesta lipid fraction enriched in saturated fatty acids, particularly stearic acid (C18:0). Consequently, lipid entering the ruminant small intestine consists predominantly of free fatty acids and microbial lipids, rather than intact dietary TAG and 2-MAG.

Pathway Intermediates: From Fat Digestibility to Milk Performance - The Case for Rumen-Protected Lysophospholipids in Dairy Cows

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