Rumen protein degradation and biosynthesis. I: A new method for determination of protein degradation in rumen fluid in vitro

1. A method is described for the determination of protein degradation based on measurements of ammonia concentration and gas production (Menke et al. 1979) when a feedingstuff was incubated with rumen fluid in vitro. 2. NH 3 liberated during incubation is in part used for microbial protein synthesis. Production of carbon dioxide and methane can be regarded as a measure of energy available for protein synthesis. The ratio, gas production: incorporation of NH 3 -nitrogen was estimated by addition of starch to the substrate. The response in gas production was linear in the range 0–200 mg starch, when starch was added to 0–200mg feedingstuff dry matter and 30 ml rumen fluid-medium mixture. 3. Linear regression between NH 3 -N concentration ( y , mg) and gas production ( x , ml) yielded an intercept ( b 0 ) representing that amountof NH 3 -N which would be released when no fermentable carbohydrates were available and consequently no bacterial protein synthesis took place. 4. The difference between this intercept b 0 and NH 3 -N content in the blank (rumen fluid without substrate added) indicated the amount of NH 3 liberated from protein and other N-containing compounds of the feedingstuff incubated. In vitro-degradable N (IVDN) was calculated as a proportion of total N by the equation: