In vitro method for determining the ruminal degradation rate of rapeseed meal protein using N isotope labelled ammonia nitrogen

An in vitro method based on observations of ¹⁴N and ¹⁵N isotope fluxes between ammonia N and non-ammonia (NAN) pools was established to study the ruminal degradation rate of rapeseed meal protein. Feed protein equal to 125 mg of N/l was incubated in the presence of rumen fluid, mineral buffer, and a carbohydrate mixture formulated to provide a constant supply of fermentable energy over the entire incubation period. The ammonia N was labelled with the ¹⁵N isotope, and the incubations were carried out for 0, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 8, and 10 h. A model with six pools was used to estimate the rate of protein degradation to ammonia N and the rate of microbial N synthesis from ammonia N. The parameter values were adjusted based on the sizes of the ammonia ¹⁴N, ammonia ¹⁵N, ¹⁴NAN, and ¹⁵NAN pools observed at different time points over the incubation period. The rate of rapeseed meal N degradation was 0.06/h (0.028 standard deviation between runs), and the predicted effective protein degradability was 0.38 (0.122 standard deviation between runs). The current approach seemed appropriate for determining microbial N synthesis from ammonia N, but measurement of the direct incorporation of amino acids into microbial N may be required to adequately characterize the metabolic events involved in ruminal protein degradation.