Comparison of direct and indirect determination of microbial growth in the rumen simulation technique (Rusitec)

Abstract

A study to compare the indirect and direct measurement of microbial protein synthesis in Rusitec using stable isotope nitrogen (15N-urea with 95% enrichment) as a tracer have been conducted. Six type of ration contained 16% CP, 24 - 36% CF, 49 – 62% NDF and 27 – 41% ADF with gradual content of non protein nitrogen (NPN) have been put into nylon bag and fed daily to Rusitec fermentor for 15 days observation. Indirect microbial protein synthesis (MPS) estimated MPS from N-turnover of the ammonia pool, while direct method estimated MPS from microbial isolated. The direct measurement of microbial-N in bacteria isolates resulted in higher microbial protein synthesis compared to the indirect method based. The results obtained are compared and discussed in relation to theoretical stoichiometric data of rumen fermentation.