The effects of quantity of dietary starch and type of dietary protein on hindgut fermentation were evaluated. Thirty-two Holstein bull calves were fed diets containing variable amounts of orchardgrass hay and a grain mixture. The amount of starch and types of protein were: [L1] low starch, soybean meal (SBM); [L2] low starch, fishmeal plus dried brewers' grains (FBG); [Hl] high starch and SBM; [H2] high starch, FBG. The percentages of acid detergent fiber (ADF) and crude protein were: [L1] 19.2%, 15.1%; [L2] 18.0%, 15.6%; [H1] 9.5%, 14.9%; [H2] 9.6%, 15.4%. After calves were fed the diets for 17 days, they were slaughtered to obtain their intestinal tracts. Ileal, cecal, and colonic digesta and feces of calves fed Hl and H2 versus Ll and L2 contained less water and ADF. Concentration of nitrogen in digesta and feces did not differ. Ileal, cecal, and colonic digesta from calves fed H1 and H2 had significantly greater numbers of viable anaerobic bacteria and lower pH._ Cecal digesta from calves fed high fiber diets (L1 and L2) had lower total VFA, propionate, and buytrate concentrations than calves fed high starch diets. Colonic and cecal digesta of calves fed diets H1 and H2 contained less ammonia. Acetate and propionate flux across cecal epithelium ro vrtro was faster for diets H1 and H2. Results indicate that high dietary starch stimulated anaerobic bacterial growth and fermentation in the hindgut, and enhanced acetate and propionate flux across the cecal epithelium. Acetate and propionate transport across the cecal wall probably is not due solely to passive diffusion, but it may involve a carrier. Replacement of SBM by FBG also altered cecal fermentation to a lesser extent. Calves fed H2 had significantly greater numbers of viable anaerobic bacteria in cecal and ileal digesta and 2 to 10 times the number of bacteria associated with cecal epithelium than calves fed the other diets. Butyrate cecal concentration and production was significantly increased when calves were fed diets containing FBG. Cecal VFA production may account for approximately 3 to 5% of digestible energy intake. / Ph. D. / incomplete_metadata