Estimating ruminal microbial efficiencies in silage-fed cattle:: comparison of an in vitro method with a combination of in situ and in vivo measurements

This study was conducted to detect possible differences in microbial efficiencies in cattle receiving four based diets. In vivo/in situ methods for the estimation of microbial efficiencies were compared in vitro method based on a modification of an in vitro gas test. Perennial rye-grass (Lolium perenne L.) was harvested at four stages of maturity during primary spring growth and ensiled after a short period of wilting. Silages deficient in crude protein (CP) were balanced to 13 % CP in the dry matter of the diet with soya-bean meal. All silages were supplemented with a barley-based pre-mix (0.49 kg per day) and fed to four Angler Rotvieh steers in a 4 x 4 Latin square design. Organic matter intake (OMI) was restricted to about 85 % of the voluntary feed intake and in vivo organic matter digestibilities were calculated from faecal marker concentration of titanium(IV)-oxide during a 14 day collection period. Estimates of the effectively degradable organic matter intake (EDOMI) were obtained from in situ incubations. In vivo microbial efficiencies were calculated on the basis of measurements of renal exogenous allantoin excretions and related to digestible OMI and EDOMI. Silages and mixed diets were incubated in vitro in buffered rumen fluid to determine true OM degradation and gas production, rn vitro microbial efficiencies were estimated from the partitioning factor, i.e. the ratio of organic matter degraded after 24 h of incubation to gas volume produced by it. As a general observation, the different methods gave similar rankings of silages and diets with respect to OM digestion and microbial biomass yield. The silages from advanced stages of maturity were ruminally less degradable and digestible in vivo, in situ and in vitro. However, ranking by in vitro gas production alone, gave different results. In addition to being of lower degradability/digestibility, silages from advanced stages of maturity were also less efficient, i.e. less of the degraded substrate was converted into microbial biomass. In vivo/in situ and in vitro estimates of microbial efficiencies ranked the silages similar.