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Cellulolytic bacteria synthesize branched-chain fatty acids (BCFA) and aldehydes (BCALD), which are vital to cell membrane structure from required branched chain volatile fatty acid (BCVFA) precursors. Our objective was to evaluate changes in BCFA and BCALD production in continuous cultures under varying dietary conditions. We hypothesized that 1) low forage (LF) in the diet would decrease BCFA and BCALD synthesis by lowering abundance of cellulolytic bacteria; 2) adding corn oil (CO) would inhibit cellulolytics and thereby decrease BCFA or BCALD concentrations; and 3) supplemental BCVFA would alleviate adverse effects of CO, recovering BCFA and BCALD production. The study was an incomplete block design with 8 continuous cultures used in 4 periods with treatments (n = 4) arranged as a 2×2×2 factorial. The factors were: high forage (HF) or LF (67 or 33% forage), without or with supplemental CO (3% of dry matter intake, 1.5% polyunsaturated fatty acids), and without or with 2.15 mmol/d each of the BCVFA isovalerate, isobutyrate, and 2-methylbutyrate. Bacterial fatty acids and aldehydes were methylated, separated by thin-layer chromatography, and analyzed by gas chromatography. A mixed model had the random effects of period and fermenter and fixed effects of diet, CO, BCVFA, and their interactions. Total BCFA and BCALD flows (mg/d) did not change with any main effects but shifts in the profiles (% of respective totals) will be discussed hereafter. Total BCFA profile decreased (P<0.01) by 25.3% with LF compared to HF. In contrast, 18:1 FA isomers increased (P<0.01) by 26.3% with LF vs HF. Supplemental CO and BCVFA also increased (P<0.01) 18:1 FA isomers by 25.6% and 15.4%, respectively. Iso even BCFA and BCALD were decreased (P = 0.10, 16.4%) and (P = 0.03, 8.2%), respectively, signifying that CO inhibits iso even BCFA and BCALD incorporation into cell membranes. However, the shifts in anteiso BCFA and BCALD differed, as anteiso BCFA decreased (P = 0.06) by 14.6% with LF vs. HF and anteiso BCALD experienced (P>0.10) no changes, signifying their importance in membrane physiology despite dramatic dietary shifts and changes in bacterial populations. Iso odd BCALD decreased (P<0.01) with LF vs HF by 23.7%. Increased transfer of 18:1 FA isomers with the addition of LF, CO, and BCVFA suggests coordinated responses of BCFA and especially BCALD in plasmalogens to maintain membrane fluidity and therefore optimum fiber degradation and feed efficiency.