Isolation and characteristics of the protozoal and bacterial fractions from bovine ruminal contents.

Four cows were fed once a day either a Cocksfoot hay diet (H) or a diet consisting of 65% hay and 35% pelleted ground barley (HB). 15(NH4)2SO4 was continuously infused into the rumen as a microbial marker and ruminal digesta samples were collected during the 24-h postprandial period for the isolation of liquid-associated protozoa and bacteria (LAP, LAB) and particle-associated bacteria (PAB). There were marked differences between ruminal pH diurnal variations with diets H and HB. Irrespective of the diet and sampling time, the chemical composition (OM, N, DAPA, 15N) of the protozoa was clearly different from that of the bacteria (P < .001). The LAP contained more OM but less N and 15N than the bacterial fractions. The DAPA used to validate the isolation technique for the mixed ciliate population was not detected in protozoal fractions. The OM content of LAB was lower than that of PAB, whereas the N, DAPA, and 15N contents were higher. The observed effects of diet (P < .01) on LAP mean N contents were due to the different N contents of the LAP samples isolated 23 h after feeding and were correlated with the variation in the number of Endodiniomorphid protozoa (r = .72; P < .05). The N content of LAB was not affected (P > .05) by diet but that of the PAB was increased on diet HB (P < .05). The diet did not affect the 15N content of any of the three microbial populations. However, the 15N content of the bacteria decreased shortly after feeding (P < .001).