Influence of sampling site on concentrations and carbohydrate-degrading enzyme activities of protozoa and bacteria in the rumen

Four ruminally cannulated cows were used to assess the distribution of the concentrations and carbohydrate-degrading enzyme activities of the liquid-a ssociated protozoa (LAP) and solid-associated bacteria (SAB) in the rumen. The cows were fed diets (7 kg of DM/d) of 100% hay (Diet F) or 60% barley p lus 40% hay (Diet C) in a 2 x 2 crossover experimental design. Samples of r uminal digesta were collected successively from the dorsal (DS), ventral (V S), and anterior (AS) sacs Ih before and 3 h after the morning feeding on t wo sampling days with a 2-d interval. Irrespective of diet and sampling tim e, the greatest proportion of entodiniomorphs, representing the main popula tion of protozoa, was found in the DS (40% mean; P < .05). Low pH values ob served in the DS (P < .05) indicated higher fermentative activity in this s ite than in the other parts of the rumen. Protozoa may contribute more than previously thought to the high digestive potential present at the top of t he rumen that has classically been attributed to bacteria. The specific act ivity of plant cell wall polysaccharide-degrading enzymes in the LAP was co rrespondingly greater (P < .05) for DS than for VS or AS. For the two diets and sampling times, specific activity of fibrolytic enzymes in the SAB ten ded to be less in the upper than in the lower parts of the rumen, and less in the VS than in the AS. This tendency became significant (P < .05) for to tal fibrolytic enzyme activity. Differences in bacterial colonization of pa rticles among the three sampling sites may explain such differences in fibr olytic activity of the SAB. Data suggest a spatio-temporal complementary ac tion of the bacteria and protozoa in ruminal plant cell wall degradation, a t least with the barley diet, for which the number of protozoa was highest.