RUMEN DIGESTA KINETICS IN DAIRY-COWS FED GRASS, MAIZE AND ALFALFA SILAGE - 1 - COMPARISON OF CONVENTIONAL, STEADY-STATE AND DYNAMIC METHODSTO ESTIMATE MICROBIAL-DEGRADATION, COMMINUTION AND PASSAGE OF PARTICLES

The rate constants of dry matter in rumen digesta in lactating rumen-f istulated Holstein-Frisian cows were determined using a four pool mode l with linear kinetics. The rate constants were derived from either ar bitrary 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) sil ages ad libitum twice daily for a limited period of time, supplemented with mixed concentrates (7 kg d(-1)). The ingested feed and rumen con tents (evacuation) were subdivided by wet sieving into large (LP > 1.2 5 mm) and small (0.04 mm < SP < 1.25 mm) particles, and secondly, by 2 -week rumen incubation into potentially degradable and truly undegrada ble fractions. Dry matter intake and total particulate rumen contents did not differ significantly between rations. Degradation rates of lar ge (k(dLP)) and small (k(dSP)) rumen particles estimated by in sacco r umen incubation were significantly lower than those derived with the s teady-state and dynamic methods and are likely to have been a result o f inadequate recognition of a fast degradable fraction. Passage rates of SP (k(pSP)) from the rumen estimated by CrDNF disappearance were si milar to those derived from the steady-state and dynamic methods. The steady-state method predicted non-soluble dry matter intake was most s ensitive 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 c hanges in k(c) and k(pSP) for rations AS, to changes in k(dSP) for rat ion MS and to changes in k(dLP), k(dSP) and k(c) for ration GS. (C) 19 98 Elsevier Science B.V.