Within the South African feedlot industry, there are currently several different roughage sources available for use in feedlot finishing diets. To evaluate the influence of dietary energy and fibre source on rumen function in feedlot animals, four roughage sources (wheat straw, Eragrostis curvula hay, cottonseed hulls and maize silage) were used in combination with hominy chop or dry rolled maize in two experiments. Four Beefmaster steers (270kg ±15kg) fitted with ruminal cannulae were used in two separate experiments in a 4 x 4 Latin square design. Experiment 1 was conducted to evaluate each roughage source in combination with hominy chop namely: wheat straw, hominy chop (WSHC); Eragrostis hay, hominy chop (EHHC); cottonseed hulls, hominy chop (CHHC); and maize silage, hominy chop (MSHC). Experiment 2 was conducted to evaluate each roughage source in combination with dry rolled maize (DRM) namely: wheat straw, dry rolled maize (WSDRM); Eragrostis curvula hay, dry rolled maize (EHDRM); cottonseed hulls, dry rolled maize (CHDRM); and maize silage, dry rolled maize (MSDRM). Diets were designed to contain equal amounts of energy, starch, crude protein, neutral detergent fibre (NDF) and 7.5% roughage source in both experiments. All diets were evaluated for particle size distribution through the Penn State Forage Particle Separator (PSPS) and ruminal fermentation parameters (volatile fatty acid composition, VFA; rumen ammonia nitrogen, NH3-N; lactate and ruminal pH) were compared for each experiment. Results from experiment 1 showed that animals fed the MSHC had the lowest (P<0.05) concentration of VFA while animals fed the CHHC diet produced the highest (P<0.05) ruminal propionate concentration, lowest (P<0.05) acetate: propionate ratio and had the lowest (P<0.05) ruminal pH during the 24h observation period. Time intervals below pH 5.6 and pH5.2 for CHHC was 940 minutes (P<0.05) and 388.75 minutes respectively. Measurements for rumen NH3-N concentrations and lactate did not differ between treatments. Results from experiment 2 revealed that animals fed WSDRM had numerically the lowest concentration of VFA and differed (P<0.05) from CHDRM and MSDRM diets. Propionate and acetate as well as A:P ratios for CHDRM were numerically higher than other treatments but differed (P<0.05) from the WSDRM diet. Rumen NH3-N concentrations did not differ but lactate concentrations were higher for EHDRM when compared to the MSDRM and WSDRM diets (P>0.05). Ruminal pH observations showed steers consuming the MSDRM diet to have the lowest mean ruminal pH of 5.53 which differed (P<0.05) from the WSDRM diet with a mean ruminal pH of 6.1. Time periods spent below pH 5.6 and 5.2 for steers consuming the MSDRM diet was highest at 703.75 and 306 minutes respectively and differed from steers consuming the WSDRM diet. Results from these experiments indicated that different roughage sources in combination with specific energy sources resulted in different rumen fermentation characteristics. Evaluation of particle size distribution from the roughage source, particularly the large pool (upper and middle sieve sizes on PSPS) further revealed that particle size alone does not explain all variation in fermentation patterns alone but the digestible NDF as percentage of total NDF for these fractions could be a valuable predictor for chewing and rumination activity to ultimately establish a more optimal ruminal pH. / Dissertation (MSc(Agric))--University of Pretoria, 2011. / Animal and Wildlife Sciences / unrestricted