Effects of inorganic nitrogen and amino acids on the degradation of ammonia-treated barley straw and proteosynthesis in a continuous culture of rumen microbes

Using dual outflow continuous fermenters, we have quantified the effects of two nitrogenous supplements on the degradation of ammonia-treated wheat straw by rumen microorganisms, and on the resulting microbial proteosynthesis. The two factors were (1) the amount of inorganic nitrogen supplied as a mixture of urea and ammonium sulfate (from 0 to 47 g CP kg−1 DM), (2) the amount of casein (from 0 to 52 g CP kg−1 DM). A Doehlert experimental design was applied and the responses were modeled using second-order polynomials. NH3–N concentration in the vessels was between 31.8 and 179.8 mg l−1 4 h after the meal, and was strongly increased by both nitrogenous supplements in a linear way. The addition of casein also increased the molar proportions of higher VFA. NDF and ADF degradabilities and OM true degradability averaged 56.4%, 55.9% and 57.7%, respectively, and were not significantly influenced by any experimental factor. The partitioning of OM input into undegraded, fermented and microbial fractions was not significantly changed by experimental treatments. The outflows of non-ammonia N and microbial N were slightly increased by intermediate levels of N supplementation, leading to minor changes in the efficiency of microbial protein synthesis (mean: 21.9 g N kg−1 OMF). As a conclusion, supplementing ammonia-treated wheat straw with inorganic nitrogen or casein up to a level of 50 g CP kg−1 DM did not induce significant changes in the utilisation of straw by rumen micro-organisms.