Effects of defaunation on microbial activities in the rumen of rams consuming a mixed diet (fresh Digitaria decumbens grass and concentrate)

The effects of defaunation on microbial activities in the rumen of sheep given mixed diets (fresh Digitaria decumbens and concentrate) were studied. Eight faunated rams and eight defaunated rams, with ruminal cannula were used. Each animal was fed with 4 different mixed diets, Digitaria decumbens grass supplemented with 4 mixtures of Soya meal and ground maize. The protein digestible in the intestine (French feeding system, PDI) to energy (French feeding system, forage unit UFL) levels (P/E) in the diets were 80 (D1) 100 (D2), 120 (D3) and 140 (D4). The major cellulolytic bacteria were enumerated by specific 16S rRNA targeted hybridisation probes, some microbial activities of the rumen were estimated by enzymatic assays and by the in sacco degradation technique. The diet (P/E ratio) and defaunation had no effect on the percentage of the cellulolytic bacteria, represented by three species: Ruminococcus albus, Ruminococcus flavefasciens and Fibrobacter succinogenes. The diet had a significant effect (P < 0.05) on the total activity of the fibrolytic enzymes (Carboxymethyl cellulase, xylanase) and glycosidases (beta-D glucosidase, beta-D xylosidase). Defaunation decreased (P < 0.05) the total activity of fibrolytic polysaccharidase (Carboxymethyl cellulase, xylanase) and glycosidase (beta-D glucosidase, beta-D xylosidase) enzymes. Neither defaunation nor the diet had any effect on alpha-amylase activities. Defaunation decreased (P < 0.05) total alpha-D-glucosidase activity. The decrease of the total activity of carboxymethyl cellulase after defaunation was more marked (P < 0.05) with the D3 diet. The diet had a significant effect (P < 0.01) on the in sacco DM degradation (96 h). Defaunation decreased the in sacco DM degradation (96 h, P < 0.05) and this decrease was negatively correlated with the P/E ratio. Defaunation significantly decreased the total volatile fatty acid concentration (P < 0.05) and the molar proportion of butyrate (P<0.01). From these results, despite reduced fibrolytic and amylolytic activities, a lower proteolysis and a more advantageous ruminal VFA profile occurred in defaunated animals. As a consequence, a better use of nutrients (protein, energy) can be expected by defaunted animals.