The modification of an in vitro gas production test to detect roughage related differences in in vivo microbial protein synthesis as estimated by theexcretion of purine derivatives

The relationship between variations in in vitro microbial biomass productio n and microbial protein supply to the host animal was examined in 13 Malawi an goats fed on stover leaves from two varieties of maize at Bunda College, Malawi, in 1995. The in vitro parameters were analysed based on the concep t of the partitioning factor (PF) which is obtained by the combination of g as volume measurements with determinations of the amount of substrate truly degraded. The PF reflects substrate-dependent variation in the hi vitro pa rtitioning of degraded substrate between short chain fatty acids (SCFA), ga ses and microbial biomass. The in vivo microbial protein synthesis was esti mated by the urinary excretion of the purine derivatives (PD) allantoin, ur ic acid and xanthine + hypoxanthine. For the two types of stover leaves exa mined, the higher microbial efficiency of one variety in vitro was reflecte d by different PD excretions on practically identical digestible dry matter intakes in vivo. It is concluded that substrate-dependent variations in mi crobial efficiencies as detected by the PF are also evident and relevant fo r in vivo microbial protein synthesis.