Evaluation of ruminal nitrogen availability in liquid feeds

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<0.0001), and the response was dependent on the N source. Heating reduced N availability for products made with casein more than for products made with urea. In Experiment 4, the effect of storage (0, 2, or 8 weeks) on N availability was measured for products containing phosphoric acid. Storage reduced N availability for products stored for 2 weeks by 7%, whereas for products stored for 8 weeks the N availability returned to that observed for samples immediately frozen. Experiment 5 was conducted to evaluate the N availability of various liquid ingredients used in feed manufacturing. There were differences among products for N availability; steep liquors averaged 0.38, but cane and beet molasses and separator byproduct were essentially 0. Measuring N availability by using cytosine as a marker for microbial growth in a protein-limited fermentation appears useful. Caution must be taken when processing liquid feeds to ensure ruminal availability of N.