Digestion kinetics of alfalfa and wheat straw assuming heterogeneity of the potentially digestible fraction

Abstract

Although it is recognized that the potentially digestible fraction of substrates in the rumen is heterogeneous in composition, most deterministic models that describe ruminal in situ digestion assume existence of only one or two homogeneous fractions of potentially digestible material. Alternatively, a stochastic model based on heterogeneity of substrate has been developed, although its validity has not been determined. To evaluate such a model, alfalfa hay and wheat straw were physically fractionated by dry sieving. Dacron bags containing these fractions were incubated in the rumen of a steer and the residue that remained was analyzed sequentially for DM, NDF, ADF, ADL, and acid detergent insoluble ash. Digestion kinetics of chemical fractions were determined by simultaneous analysis of the digestion profiles of DM, NDF, ADF, ADL, and acid detergent insoluble ash. The weighted mean and variance of the fractional digestion rate constant were .083 h-1 and 4.7 x 10(-4) h-2 for alfalfa and .021 h-1 and 2.4 x 10(-5) h-2 for wheat straw, respectively. In contrast, fitting DM data to a stochastic model assuming heterogeneous rates, the mean and variance of the fractional digestion rate were .078 h-1 and 8.3 x 10(-11) h-2 for alfalfa and .018 h-1 and 5.3 x 10(-12) h-2 for wheat straw, respectively. Because the estimates of the variance approached zero for the stochastic model, it is implied that this model reduced to a model involving a single fractional digestion rate. It is concluded that parameter estimates obtained from a stochastic model do not necessarily have a biological meaning.