Chemical Composition of Protozoal and Bacterial Fractions Isolated from Ruminal Contents of Dairy Cows Fed Diets Differing in Nitrogen Supplementation

The objective of this study was to compare the chemical composition of liquid-associated protozoa (LAP), liquid-associated bacteria (LAB) and solid-adherent bacteria (SAB). Four cannulated lactating cows, arranged in a 4×4 Latin square experiment, were fed grass silage supplemented with one of four concentrate mixtures: (1) barley/oats; (2) barley/oats+1% urea; (3) barley/oats+12% soybean meal; or (4) barley/oats+12% soybean meal+1% urea. The LAP contained more organic matter, starch (P<0.05) and N (P<0.1) but fewer lipids (P<0.05) than the bacterial fractions. The LAP had a lower (P<0.05) content of guanine and cytosine than both LAB and SAB, and a lower content of thymine than LAB. In contrast, the LAP contained more (P<0.05) adenine than the bacterial fractions. The SAB contained more organic matter and lipids but fewer (P<0.05) starch and purine and pyrimidic bases than LAB. Microbial N content tended (P=0.08) to be higher when diets were supplemented with soybean meal than with solely barley and oats.