Effects of dietary C18 PUFA-rich oils on rumen function, growth performance, nutrient digestibility, ruminal lipid metabolism and milk and meat quality in ruminants: a review

Oils rich in linoleic acid (LA) or α-linolenic acid (ALA) are important dietary sources of C18 polyunsaturated fatty acids (C18 PUFAs) for ruminants and can affect nutrient utilization, production performance, and milk and meat quality by modulating ruminal lipid metabolism and microbial fermentation. This review was based on searches of the Web of Science Core Collection, Scopus, and PubMed databases, together with targeted supplementary searches, and ultimately included 39 original studies. It focused on comparing the major response patterns of oils rich in LA or ALA and analyzing the main factors influencing their effects. The effects of C18 PUFA-rich oils on the fatty acid composition of milk and meat were relatively consistent across studies. Specifically, oils rich in LA are more often associated with the enrichment of trans-11 C18:1 and cis-9, trans-11 conjugated linoleic acid (CLA), whereas oils rich in ALA are more favorable for increasing C18:3 n-3 and total n-3 PUFAs. In contrast, their effects on rumen function, nutrient utilization, production performance, and conventional indicators of milk composition and meat quality are not entirely consistent across studies. These differences may be related to the total dietary fatty acid level and composition, basal diet structure, and the supplementation level, processing method, and protection form of the added oils. In practice, oil sources should be selected according to the intended goals for modifying the fatty acid composition of milk and meat. For unprotected oils, approximately 2%–3% of dietary dry matter (DM) may serve as a relatively conservative starting supplementation range. If the supplementation level is further increased toward 4% of dietary DM, the fatty acid supply from the basal diet, effective fiber supply, and the responses of the animals to oil supplementation should be considered comprehensively. Protected products should be evaluated separately according to their actual fatty acid supply, ruminal release characteristics, and post-ruminal utilization. Future research should quantify the transfer efficiency of C18 PUFA at each stage along the pathway from dietary intake, through ruminal microbial transformation, to secretion in milk fat or deposition in muscle, and determine appropriate supplementation levels and potential risks under different dietary conditions and product goals.