Diets including combinations of soybean meal and urea effect rumen fermentation, nutrient digestibility, and nitrogen utilisation in dairy cows

Context The presence of chemical residues in foods, particularly in solvent-extracted soybean oil, poses significant food safety concerns. Extrusion offers an alternative extraction process, but the high temperature involved can alter the physicochemical properties of soybean meal and potentially compromise its suitability for use in animal feed. Aims To investigate the rumen degradation of soybean meal extruded at various temperatures and the effects of supplementary urea in diets containing 180°C-extruded meal on rumen fermentation and digestion when fed to Holstein dairy cows. Methods In Experiment 1, the rumen degradation of soybean meal processed at four extrusion temperatures (90, 120, 150 and 180°C) was evaluated using the in situ nylon bag technique to determine rumen degradable protein and rumen undegradable protein (RUP) contents. Experiment 2 used four rumen- and duodenum-cannulated Holstein cows in a 4 × 4 Latin square design to assess the effects of increasing urea supplementation (0, 100, 200 and 300 g/day) in a diet containing soybean meal extruded at 180°C. Key results Increasing extrusion temperature linearly decreased the proportion of rumen degradable protein from 80% to 48%, and increased that of RUP from 20 to 52% (P < 0.05) in soybean meal. Higher extrusion temperatures intensified Maillard reactions, resulting in a denser microstructure and reduced surface area of the meal. Increasing urea supplementation linearly increased acetic, butyric and total volatile fatty acids, and the acetate: propionate ratio (P < 0.05) in rumen fluid. It also improved fibre, organic matter digestibility and microbial crude protein flow to the duodenum (P < 0.05). Conclusions Increasing extrusion temperatures changed soybean meal properties, causing darker color, a tighter structure and reduced protein fraction B. Higher temperatures lowered rumen degradable protein, but increased RUP, although RUP digestibility declined at 180°C. In the low-protein (~12% CP) diet, supplementation with up to 300 g urea/head/day improved feed intake, rumen fermentation, fibre digestion and microbial crude protein supply. Implications High-temperature (~180°C) extrusion of soybean meal increased the proportion of RUP, and in a low-protein diet, supplementation with urea was needed to optimize nutrient utilization.