INFLUENCE OF FEED PARTICLE SIZE ON RATE AND EFFICIENCY OF GAIN, CHARACTERISTICS OF RUMEN FLUID AND RUMEN EPITHELIUM, AND NUMBERS OF RUMEN PROTOZOA

Sixty yearling steers were fed five all-concentrate diets differing in feed particle size (range 476–1525 μm geometric mean particle size), prepared by mixing different proportions of a ground and pelleted feed with steam-rolled grain. Average digestible energy content of the diets was 3.093 Mcal/kg. Steers fed the medium particle size feed ate more (P < 0.01) and gained faster (P < 0.05) than those fed the fine or coarse feed. Feed-to-gain ratios did not differ among groups. Rumen wall darkened in color as the feed particle size increased (P < 0.01). The incidence of rumenitis (P < 0.05) and of abnormal papillae (P < 0.01) in steers decreased as the feed particle size increased. However, numbers of protozoa in the rumen fluid increased as feed particle size increased (P < 0.01). At slaughter, the pH increased with increasing feed particle size (P < 0.05), but the viscosity and alkaline phosphatase content of the rumen fluids were similar among groups.