Stimulation of microbial protein synthesis by branched chain volatile fatty acids in dual flow cultures varying in corn oil and forage concentrations

Kelly Mitchell
Graduate (PhD)
Jeff Firkins
Animal Sciences

Branched-chain amino acids (BCAA) are deaminated by amylolytics to BCVFA, which are growth factors for cellulolytic bacteria that cannot degrade or synthesize their own BCAA. Our objective was to assess dietary conditions that increase the uptake of BCVFA. BCVFA should increase stimulation of cellulolytic bacteria in high forage (HF) diets. However, in low forage (LF) diets, amylolytic bacteria should outcompete for BCVFA. Supplemental corn oil (CO) should inhibit cellulolytic bacteria growth, but additional BCVFA should minimize that inhibition. Supplementation of BCVFA could therefore increase efficiency of microbial protein synthesis in LF, HF, and high CO diets. The study was an incomplete block design with 8 dual flow continuous cultures used in 4 periods with treatments (n = 4) arranged as a 2 x 2 x 2 factorial. The factors were: HF or LF (67 or 33% forage), without or with supplemental corn oil (CO, 3% DM), and without or with 2.15 mmol/d each of isovalerate, isobutyrate, and 2-methylbutyrate. Forage was 33:67 alfalfa:orchard grass pellets, concentrate was mostly ground corn and soybean meal, and diets were isonitrogenous. Data were analyzed with PROC MIXED in SAS (v. 9.4, SAS Institute 2015) with random effects of period and fermenter and fixed effects of forage, fat, BCVFA, and their interactions assessed using ANOVA. The main effect of supplementing BCVFA increased (P = 0.10) NDF digestibility (NDFd) by 2.97% units, and CO increased (P = 0.07, Forage Fat) NDFd by 6.46% units in LF diets. BCVFA and HF increased (P = 0.03) bacterial N by 1.54 and 1.84 g/kg OM truly degraded, respectively. Total BCVFA net production (total outflow concentration corrected for dose) were lower (P = 0.01, Forage BCVFA) with HF, HF+BCVFA, and LF+BCVFA compared to LF. Providing supplemental BCVFA improved efficiency of cellular growth of cellulolytics and therefore NDFd, which should improve feed efficiency in dairy cows.