Submitted to: Animal Feed Science And Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/9/2005
Publication Date: 9/30/2005
Citation: Grings, E.E., Blummel, M., Sudekum, K.H. 2005. Methodological considerations in using gas production techniques for estimating ruminal microbial effciencies for silage-based diets. Animal Feed Science And Technology 123-124:527-545. Interpretive Summary: Microbial biomass production provides the major source of protein for the ruminant animal and the prediction of the effciency of microbial production can be crucial in ruminant nutrition. It has been proposed that efficiency of microbial production can be detected by a combination of in vitro gas volume measurements with a concomitant determination of the truly undegraded feed residue to calculate true substrate degradability. Using this concept, the in vitro efficiency of microbial production of four silage-based diets was estimated at the time when half of the maximal gas production had been produced the present work had three objectives: 1) to examine the relationship between in vivo and in vitro microbial efficiency of silage-based diets when the in vitro measures are made at substrate-specific incubation times, 2) to compare these in vitro estimates of the efficiency of microbial biomass production with direct determinations of microbial nitrogen, and 3) to examine the effect of in vitro nitrogen supplementation on these relationships. Efficiency of microbial biomass production estimated from true substrate degradability and gas production measures at substrate specific times was well related to in vivo measures of microbial efficiency. Diet rankings were dependent on nitrogen-supplementation of the incubation media. Estimates of microbial nitrogen were better related to in vivo microbial efficiency when estimated using nitrogen balance that for determinations based on residue nitrogen concentrations. This research indicates that the best microbial efficiency value of silage-based diets is that of combined gas volume and true substrate degradability measures. Further research is varrented on the impact of nitrogen content of the incubation media and the impact of silage fermentation acids on gas production profiles.
Technical Abstract: In four silage-based diets, efficiency of microbial production (EMP) was estimated through renal allantion excretion in steers and compared with in vitro estimates of EMP. In vitro estimates were conducted at times when half of asymptotic gas production of a given substrate was reached and consisted of combined gas volume and true substrate degradability measurements and two methods of microbial nitrogen determination. These in vitro estimates were conducted in both nitrogen supplemented (N-rich) and un-supplemented (N-low) incubation medium. In vivo microbial crude protein production ranged from approximately 8 to 12 gram per MJ metabloizable energy intake with a mean value of 10 gram. In vitro estimates of EMP derived from combined gas volume and true substrate degradability measurements were significantly related to in vivo EMP in both N-rich (R2 = 0.94, P = 0.03) and N-low (r2 = 0.91, P = 0.04)incubation medium. In vitro microbial nitrogen production relative to true substrate degradability were not related (P>0.05) to EMP in vivo. Stoichiometrical considerations suggested that silage-fermentation acids interfere with in vitro substrate degradability measurements, where they are removed from the incubation residue without having contributed to fermentation. In addition, incubation of the silages with autoclaved rumen liquor showed that immediate gas volumes were produced from buffering the silage fermentation acids, and these gas volumes accounted for 6.5 and 14.8% of the gas volumes produced during actual in vitro fermentations. It is concluded that in silage analysis by in vitro gas production techniques, more attention should be given to effects of silage fermentation acids. Ranking of silage-based diets for microbial efficiency may be best estimated by combined gas volume and true substrate degradability measures.