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Diets based on red clover silage (RCS) typically increase the concentration of polyunsaturated fatty acids (PUFA) in ruminant milk and meat compared with grass silages (GS), an effect that has been attributed to higher activity of polyphenol oxidase in red clover, promoting ruminal escape of dietary lipid. Four multiparous Finnish Ayrshire cows in mid lactation fitted with rumen cannulas were used in a 4 x 4 Latin Square design with 21-d experimental periods to evaluate the effects of incremental replacement of GS with RCS on ruminal lipid metabolism, using the omasal sampling technique in combination with Cr-EDTA, Yb acetate, and indigestible neutral detergent fiber as markers. Treatments comprised total mixed rations offered ad libitum containing 600 g of forage/kg of diet dry matter, with RCS replacing GS in a ratio of 0:100, 33:67, 67:33, and 100:0 on a dry matter basis. Silages contained a high proportion of lipid as nonesterified fatty acids (NEFA), with no difference between forage species (75 and 73% for GS and RCS, respectively). Substitution of GS with RCS had no influence on the intakes of NEFA, polar lipid, triacylglycerol, diacylglycerol, monoacylglycerol, or total fatty acids (FA), but altered the ingestion of specific FA. Replacing GS with RCS decreased linearly 18:3n-3 and increased linearly 18:2n-6 intakes. Changes in the proportion of RCS in the diet had no effect on the amounts or on the relative proportions of different lipid fractions at the omasum. On average, NEFA, polar lipid, triacylglycerol, diacylglycerol, and monoacylglycerol accounted for 80, 12, 4.4, 2.4, and 0.8% of total FA in omasal digesta, respectively. Replacement of GS with RCS increased linearly the amount of esterified and nonesterified 18:3n-3 at the omasum. Flows of cis-9 18:1 and 18:2n-6 were also increased linearly in response to RCS in the diet, whereas 3,7,11,15-tetramethyl-16:0 at the omasum was decreased. Replacing GS with RCS in the diet decreased linearly the lipolysis of dietary esterified lipids in the rumen from 85 to 70%. Effects on lipolysis due to forage species were also associated with linear decreases in apparent ruminal 18:3n-3 biohydrogenation from 93 to 85% and a trend toward lowered biohydrogenation of cis-9 18:1 and 18:2n-6 in the rumen. However, forage species had no effect on the flow of bound phenols formed as a consequence of polyphenol oxidase activity at the omasum. In conclusion, despite minimal differences in the extent of lipolysis in silo, lipid and constituent FA in RCS were less susceptible to ruminal lipolysis and biohydrogenation compared with GS. [ABSTRACT FROM AUTHOR]