Invited review: Moving from dietary fat to fatty acids—New insights into how fatty acids affect digestibility, metabolism, and performance in dairy cows

In dairy nutrition, "fat" broadly refers to lipid compounds primarily composed of fatty acids (FA), which play diverse roles in digestion, metabolism, and milk production. The main FA present in dairy cow diets are palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:3) acids. In the rumen, FA are extensively modified, which decreases the toxicity of UFA to rumen bacteria. Rumen bacteria can also synthesize FA from end products of rumen fermentation and AA, primarily producing odd- and branched-chain FA. As FA flow to the intestine, digestibility is influenced by several factors. These include total FA flow, FA profile, the presence of emulsification compounds, and the physical characteristics of fat supplements. The digestibility of FA typically decreases as total FA intake and flow to the intestine increase, especially with 18:0, which exhibits a more pronounced reduction in digestibility than 16:0. Some dietary UFA escape rumen biohydrogenation. Therefore, supplemental blends containing 18:1 can increase its postruminal delivery, improving FA digestibility and absorption. These effects are especially beneficial during early lactation and in high-producing cows. Additionally, the form and purity of supplements influence FA absorption, with highly saturated and pure supplements having lower digestibility. The source of 18-carbon FA in the diet also affects digestibility, with oilseeds being associated with less reduction in digestibility than prilled supplements that provide 18:0. Historically, UFA and medium-chain FA, commonly found in vegetable oils, have been shown to negatively affect NDF digestibility due to various mechanisms, including the coating of fiber particles, microbial toxicity, and reduced cation availability. However, recent studies indicate that FA sources high in 16:0 can enhance NDF digestibility. These improvements are not linked to reduced DMI, suggesting that other mechanisms, such as altered gut peptide activity or microbial community shifts, may be involved. In terms of production responses, 16:0 supplementation consistently improves milk fat yield, ECM, and nutrient utilization across lactation. In contrast, 18:1 enhances BW gain and FA digestibility and is particularly effective in increasing ECM in high-producing cows (over 45 kg/d of milk yield) and cows in early lactation. At the same time, 18:0 often reduces FA digestibility, limiting its production benefits. Across multiple studies, blends containing higher proportions of 16:0, especially in comparison with 18:0, led to linear increases in milk fat yield and ECM, supporting the preferential use of 16:0 for fat synthesis in the mammary gland. Although some earlier work suggested that combining 16:0 and 18:0 in a supplement would optimize FA utilization, this approach has been largely discredited by recent findings. High levels of 18:0 in FA supplements have been shown to reduce FA digestibility and energy intake, limiting their value in most production contexts. Combining blends containing 16:0 and 18:1 resulted in better production responses during early lactation than other supplements containing only 16:0 or 16:0 + 18:0. Overall, our review highlights the complexity of FA digestion and metabolism and underscores the importance of tailoring supplementation strategies to the production stage and physiological status of dairy cows and the outcomes desired. Future research should refine dietary formulations to optimize energy partitioning, enhance productivity, and support metabolic health throughout lactation.