Relationship between Gas Production, True Degraded Organic Matter and Microbial Biomass Synthesis for Protein Feedstuffs as Influenced by Incubation Time in the In vitro Gas Production

Five protein feedstuffs, namely ambadi (Hibiscus cannabinus) cake, cottonseed meal, groundnut meal, rapeseed meal, and sesame meal were evaluated using the in vitro gas production technique, for fermentation stoichiometry, gas production, true digested organic matter (TDOM) and microbial biomass (MB) synthesis at incubation time of half asymptotic gas production (t_1/2) and 24 h. The mean molar ratio of short chain fatty acids (c2:c3:c4) for the protein supplements at t_1/2 and 24 h were 0.71:0.20:0.09 and 0.72:0.19:0.09, respectively. The mean gas production (ml/g DM) at t_1/2 and 24 h were 101.2 and 196.3 respectively, whereas the TDOM (mg/g DM) at the corresponding incubations were 578.6 and 657.5. Similarly, mean partitioning factor (PF) at t_1/2 (5.87) was significantly (P<0.0001) higher than at 24 h (3.37). The MB (mg/g DM) in the apparent undigested residue in the incubations was 172.8 and 100.0 at t_1/2 and 24 h, whereas the RNA equivalents (mg/g DM) for the corresponding incubations were 9.69 and 7.45, respectively. However, the ratio of RNA: MB at t_1/2 and 24 h were 0.0581 and 0.0808, respectively (P=0.0257). The disproportionate increase in gas production against the increase in TDOM at 24 h compared to at t_1/2 with similar fermentation stoichiometry at both incubations is explainable by the unfermented OM at t_1/2 that is solubilized by the ND extraction, thereby contributing to TDOM. Thus the artifact caused by solubilization of unfermented OM at t_1/2 could equal to or greater than the artifact caused by microbial lysis up to 24 h incubation. It is concluded that, for protein feedstuffs, the t_1/2 may not be the most appropriate time for determination of PF as an index of microbial biomass synthesis efficiency.