Effect of defaunation on the metabolism of rumen micro-organisms.

1. Rumen contents of a fasted fistulated wether, obtained in a faunated, defaunated and refaunated period were incubated in vitro with a mixture of cellobiose and maltose, in the presence of ammonium bicarbonate and 32PO43-. Total synthesis of microbial N (Nt) was calculated from 32P incorporation and N:P determined in microbial matter. The N:P value was not affected by defaunation. Net synthesis of microbial N (Nn) was calculated from ammonia-N incorporation. An estimate of degradation of microbial N was calculated as Nt-Nn. Energetic efficiency of synthesis was calculated from the volatile fatty acids produced during incubation, as g N incorporated per kg organic matter fermented (g N/kg OMf). 2. Defaunation decreased the proportions of acetate, butyrate and methane and increased those of propionate in fermentation end-products. Fermentation rate when expressed per mg microbial N was not affected by defaunation. 3. Expressed per unit volume of rumen contents, Nn was increased by defaunation whereas Nt remained unchanged. Thus, a decrease in degradation can be calculated. Energetic efficiences of total and net synthesis were increased from 35 and 13 to 47 and 30 g N/kg OMf respectively. 4. Specific rates of both total and net synthesis of microbial N were significantly increased by defaunation whereas the specific rate of degradation was not affected.