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

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 3x3 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 < SP < 1.25 mm) particles, and secondly, by 2-week rumen incubation into potentially degradable and truly undegradable fractions. Dry matter intake and total particulate rumen contents did not differ significantly between rations. Degradation rates of large (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 CrDNF 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. (C) 1998 Elsevier Science B.V.