Fractionation of Substrate as an Intrinsic Characteristic of Feedstuffs Fed to Ruminants

An experiment was conducted to detennine whether fractionation of substrate into a soluble fraction, a potentially digestible fraction, and an indigestible fraction is an intrinsic characteristic of substrate and, thus, not affected by the ruminal environment in which it is incubated. The soluble fraction disappears during washing and is, by definition, an intrinsic fraction. Ten substrates were incubated for 4 or 42 d in the rumen of steers fed either mature alfalfa hay or a concentrate diet. The ADF residue remaining after 42 d of incubation was consistently higher for steers fed the concentrate diet but was not affected by animal, indicating that the indigestible ADF fraction, per se, is not an intrinsic fraction of substrate. This implied that certain microbial digestive activity could be not only reduced, but even inhibited, by changing the ruminal environment. A data analysis method was proposed that accounted for existence of intrinsic fractions by simultaneously analyzing digestion profiles of a single substrate, incubated in different ruminal environments. These profiles shared a common beginning point, whereas asymptotic endpoints depended only on dietary treatment. In general, parameter estimates were similar to those obtained from individual analysis of the digestion profiles, but asymptotic standard errors were smaller.