Evaluation of ruminal nitrogen availability in liquid feeds

Abstract A method to assess N availability in liquid feeds was evaluated. Microbial protein mass produced in vitro , estimated using cytosine as a marker, was assumed to be a function of available N. Samples were incubated with rumen fluid, McDougall's buffer, and an excess amount of starch for 12 h. In Experiment 1, graded levels of casein, urea, and a mix of casein and urea were added to fermentation tubes to assess the effect of protein source. Microbial mass was linearly related to added protein, but it was independent of protein source. In Experiments 2–4, products were manufactured from different base ingredients (cane molasses; steep liquor; distiller's solubles; concentrated separator byproduct; sucrose; a mixture of sucrose, glucose, and fructose; soluble starch; hydrolyzed starch) by adding either casein or urea to study the effect of different processes on ruminal N availability. In Experiment 2, the effects on ruminal N availability of adding 20 g/kg NaCl or 40 g/kg phosphoric acid to the aforementioned liquid feeds were measured. Addition of NaCl reduced N availability by 21%, and phosphoric acid reduced N availability by 50%. In Experiment 3, the effect of heating was measured in products containing 20 g/kg NaCl. Heating at 100 °C for 15 min reduced N availability (P