Rumen digesta kinetics in dairy cows fed grass, maize and alfalfa silage. 1. Comparison of conventional, steady-state and dynamic methods to estimate microbial degradation, comminution and passage of particles.

Abstract The rate constants of dry matter in rumen digesta in lactating rumen-fistulated Holstein–Frisian cows were determined using a four pool model with linear kinetics. The rate constants were derived from either arbitrary named `conventional' methods (i.e. in sacco incubation, marker passage), or average pool sizes (steady-state method), or changes in pool sizes with time (dynamic method). According to a 3×3 Latin square design, the cows were fed grass (GS), maize (MS) and alfalfa (AS) silages ad libitum twice daily for a limited period of time, supplemented with mixed concentrates (7 kg d −1 ). The ingested feed and rumen contents (evacuation) were subdivided by wet sieving into large (LP>1.25 mm) and small (0.04 mm k dLP ) and small ( k dSP ) rumen particles estimated by in sacco rumen incubation were significantly lower than those derived with the steady-state and dynamic methods and are likely to have been a result of inadequate recognition of a fast degradable fraction. Passage rates of SP ( k pSP ) from the rumen estimated by CrNDF disappearance were similar to those derived from the steady-state and dynamic methods. The steady-state method predicted non-soluble dry matter intake was most sensitive to changes in k pSP and the rate of comminution of LP ( k c ). The non-soluble rumen dry matter degradation was most sensitive to changes in k c and k pSP for rations AS, to changes in k dSP for ration MS and to changes in k dLP , k dSP and k c for ration GS.