In review paper, the latest scientific points of view on the role of protozoa in rumen ecosystem, reasons for total or partial rumen defaunation, as well as nutritional practices aim to defaunation were discussed. The high number of protozoa in rumen (>1×10⁶/ml) decrease the efficiency of energy and protein utilization in rumen ecosystem. Total defaunation of rumen increase significantly bacterial supply to duodenum, as well as raise propionic acid concentration in rumen and simultaneously decrease share of butyric acid in the total of VFA. The defaunation involve decreasing of ammonia level and methane production. However, the defaunation decrease significantly digestibility of cell walls (NDF and ADF) and have negative influence on starch metabolism in the rumen. Partial dafaunation process could be obtained by increasing of concentrate level in ration, as well as by supplement of plant oils or brewers’ yeasts. Intensification of nutrition cause decreasing of protozoa number in rumen to the level 2-4×10⁵/ml.

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