Quantitative meta-analysis on the effects of defaunation of the rumen on growth, intake and digestion in ruminants

A quantitative meta-analysis was applied on 90 publications and 169 comparisons dealing with defaunation of the rumen (removal of protozoa from the rumen) in order to point out the major quantitative effects of defaunation and identify interfering factors. Generally speaking defaunation significantly (P < 0.01) increased average daily gain (11% on average, 64 trials) but did not affect dry matter intake. As a consequence, defaunation improved feed conversion efficiency (P < 0.05). These different effects were especially obvious as diets were both high in forage and low in nitrogen and as animals had a low growth potential. Defaunation significantly (P < 0.001) decreased organic matter digestibility (-1.7 units, 52 trials) and cell wall carbohydrate digestibility (-5.7 units, 15 trials). The same trend was observed for rumen digestibility. On the other hand, duodenal nitrogen flow, expressed as a ratio of nitrogen intake, was enhanced by defaunation (P < 0.001). Duodenal microbial nitrogen flow, expressed as the percentage of live-weight, increased by 21% after defaunation (P < 0.05). The latter two results may account for the lower ammonia concentration in the rumen (-50.3 mgNNH(3)/1, 75 trials) and the higher microbial synthesis efficiency (11.8 g microbial N/kg OMDr 19 trials) observed for the ciliate-free animals (P < 0.001). The level of concentrate in the diet strongly interfered with the effect of defaunation on the pH in the rumen. pH in the rumen was increased by defaunation with diets containing a high level of concentrate (PCO greater than or equal to 50%), whereas it decreased with diets containing a low level of concentrate. The molar proportion of propionic acid was enhanced by defaunation, whereas the molar proportion of butyric acid was lowered (P < 0.01) by defaunation, in the ruminal pool of volatile fatty acids (VFA). Finally, defaunation increased the ruminal volume and the liquid phase outflow rate (P < 0.05), but to a lesser extent. We consider that these results strongly suggest a more efficient use of nutrients in ciliate-free animals, especially when they are given poor diets limiting animal production. (C) 2003 Elsevier B.V. All rights reserved.