A simple procedure using 35S incorporation of the measurement of microbial and undegraded food protein in ruminant digesta.

1. A simple procedure using 35S incorporation for the measurement of microbial protein in absomasal or duodenal digesta of ruminants was developed and tested. 2. Microbial protein synthesized in the rumen was labelled with 35S by intraruminal infusion of 35SO4 and a microbial fraction was isolated by differential centrifugation. 3. 35S not bound by micro-organisms and present as inorganic 35S in whole digesta and in microbial fractions was oxidized to 35SO4 and the samples were brought into solution by acid-hydrolysis. 35SO4 was precipitated as Ba35SO4. 4. The proportion of microbial non-ammonia-nitrogen (NAN) in digesta NAN was determined as 35S: NAN (digesta) divided by 35S: NAN (microbial). 5. In sheep offered grass nuts at maintenance level of feeding, 48% of NAN flowing through the abomasum was of microbial origin. NAN flow to the small intestine was estimated using chronic oxide as a marker and it was calculated that 49% of the grass N was degraded in the rumen.