Ruminal plant cell wall digestibility estimated from digestion and passage kinetics utilizing mathematical models

Abstract

Methods to describe the digestion and passage kinetics for estimating ruminal neutral detergent fibre digestibility (RNDFD) with simple mathematical models were evaluated in cattle fed a diet of hay, barley and urea (600, 396 and 4 g dry matter (DM) kg −1 total DM) at two levels of intake. The parameters of digestion kinetics were determined by in situ incubation and rumen evacuation technique. Digesta passage kinetics was estimated from duodenal and faecal Cr and Yb concentrations either from exponential decline in marker concentration or by using two-compartmental models with gamma age dependency in the first compartment. Increasing the feeding level from 40 to 80 g DM kg −1 live weight 0.75 decreased ( P < 0.05) the digestibility of DM (0.775 vs. 0.749), DNF (0.758 vs. 0.707) and cell solubles (0.838 vs. 0.824). The amount of total digesta, digesta DM content and the amount of DM in the rumen increased as feed intake increased. The relative increase in rumen pool size of digestible NDF was greater than in that of indigestible NDF (0.72 vs. 0.34). Using in situ digestion kinetics data and the exponential decline in marker concentration in a simple rumen digestion model yielded RNDFD estimates which were much lower than those based on rumen evacuation data, and only about 0.70 of the total in vivo NDF digestibility. This was partly because of the slower ( P < 0.01) rate of digestion observed with the in situ method than estimated from rumen evacuation (0.0458 vs. 0.0607), and partly because of an underestimation of rumen residence time by the passage model. Incorporation of the selective retention of feed particles in the digestion model yielded RNDFD estimates which were 0.13 units higher than those based on exponential marker concentration decline curves. The kinetics parameters for this model were calculated from duodenal marker concentrations using two-compartmental passage models. The RNDFD estimates were 0.94–0.96 of the total in vivo NDF digestibility when the passage kinetics parameters, estimated with the two-compartmental model, were used in combination with the rumen evacuation derived rate of digestion. The values for RNDFD were higher when Cr and faecal sampling were used rather than Yb and duodenal sampling to estimate passage kinetics.