In vitro estimation of rumen protein degradability using 35S to label the bacterial mass

An experiment was carried out in order to simplify a previously developed 15N-method for in vitro estimation of rumen protein degradability. Casein (Cas), whole soybeans (Sb) heated at 120oC for 20 min (SbTherm) and sunflower (Sfl) were incubated at 39oC for 4 hours in a water bathshaker with the following media: McDougall's buffer, strained and enriched with particle associated bacteria rumen fluid (2:1), rapidly (maltose, sucrose, glucose) and more slowly (pectin, soluble starch) degradable carbohydrates with final concentration of 815 mg/100 ml and 21.7 μCi/100 ml of35S (from Na235SO4). After the incubation had been ceased, a bacterial fraction was isolated through differential centrifugation and specific activity of bacterial (Bac) and high speed total solids (TS) nitrogen was measured. The ratio was used to calculate bacterial mass in TS and through the Kjeldahl nitrogen concentration in TS - the net bacterial growth (against control vessels without protein). The level of ammonia-N in the supernate after blank correction was used to find the ammonia-N released from protein degradation. The data showed that the rate (and extend) of degradation for the Cas (as a standard protein) was lower compared to those obtained through the 15N-method but it was higher than the rate derived through another in vitro method. The Cas equivalent of the Sb was higher than the figure we found in a previous experiment with solvent extracted soybean meal suggesting that the 35S-method underestimated the degradability of the Cas. After being tested on a wider range of foodstuffs, the proposed 35S-method might be considered as an alternative procedure which is less laborous than the 15N-method. (author)