Rumen fermentation and microbial biomass synthesis indices of tropical feedstuffs determined by the in vitro gas production technique

This study was conducted to establish rumen fermentation characteristics and microbial biomass synthesis indices for the feed ingredients commonly used in the manufacturing of compounded cattle feeds and/or in feeding large ruminants in Bangalore and surrounding districts of Karnataka state, India. The feed samples selected for the study included 8 protein sources, 9 energy sources, 8 varieties of finger millet straw (FMS) and 12 varieties of rice straw (RS). The fermentation characteristics were studied by measuring the gas production in vitro and by fitting cumulative gas production against time of incubation in the exponential model, Y = D [1 − e−k*t], where Y is gas volume at time t, D is potential gas production (ml/g DM) and k (h−1) is rate at which gas is produced. The D and k for protein sources, energy sources, FMS and RS were 234.7 ± 53.1 and 0.123 ± 0.032, 402.8 ± 51.8 and 0.076 ± 0.021, 261.6 ± 16.4 and 0.064 ± 0.011, 254.8 ± 16.8 and 0.033 ± 0.005, respectively. The feed samples were incubated again to determine the cumulative gas production and true digested organic matter (TDOM) at time of half asymptotic gas production (t1/2). The TDOM was calculated as the difference between OM incubated and the OM residue at t1/2 after refluxing with neutral detergent solution. The ratio of milligram TDOM to milliliter gas produced at t1/2 (partitioning factor (PF)) was regarded as an index of efficiency of microbial biomass synthesis (EMBS) on the assumption: (i) carbon of TDOM is quantitatively distributed between microbial biomass and the sum of fermentative gas and short chain fatty acids (SCFAs); and (ii) gas production is stoichiometrically related to SCFAs. The PF, for the respective group of feedstuffs varied from 3.86 to 6.48, 3.28 to 4.53, 2.73 to 3.72 and 2.55 to 3.23. Using ingredients for which the PF was determined, six mixed diets were formulated to differ in PF with finger millet straw as roughage component (700 g/kg as is) across all the diets. Evaluation of these diets for gas production and PF revealed significant associative effects up to 12 h after incubations. Therefore, it is concluded that the range in the PF of the tested feed ingredients is wide enough to facilitate ingredient selection for higher PF or in other words higher EMBS. However, since the associative effect of ingredients influence gas production and PF of mixed diets, the limitations in using gas production kinetics and PF of the ingredients to achieve a desirable fermentation characteristics of mixed diets should be realized.