Effects of dietary forage particle size and concentrate level on fermentation profile, in vitro degradation characteristics and concentration of liquid- or solid-associated bacterial mass in the rumen of dairy cows

This study investigated effects of dietary forage particle size (PS) and concentrate level (CL) on fermentation profiles of particle-associated rumen liquid (PARL) and free rumen liquid (FRL), in vitro degradation characteristics and concentration of bacterial mass attached to the solid or fluid rumen digesta phase in dairy cows. The experiment was a 4 x 4 Latin square design with four late-lactation dairy cows in four 23 day periods. Cows were restrictively fed (17 kg dry matter (DM)/d) one of four diets varying in the theoretical PS (6 and 30 mm) of grass hay and in the levels (approximately 200 and 550 g/kg, DM basis) of a cereal-based concentrate. Proportion of large particles (>6 mm) and the content of structural fibre in the diet increased by reducing dietary CL and, particularly, by increasing hay PS. This effect was not reflected by changes in mean total volatile fatty acid concentration or pH in the rumen. However, cows fed high concentrate diets had pH of 5.28 and 5.37 in PARL at 3 h after the last meal, when fine or long chopped bay was offered. The low pH may indicate a depression of the capacity of PARL to degrade fibre in vitro. Gas production in vitro of concentrate increased with the high concentrate diet at 12 h, suggesting that amylolytic capacity was affected only in early phases of fermentation. In addition, elevating dietary CL appeared to shift ruminal fermentation outputs from propionate to butyrate and valerate. Inclusion of coarsely chopped hay to a high concentrate diet does not appear to bring advantages due to increased structure in restrictively fed dairy cows. In addition, results suggest that the response of pH in PARL is more sensitive to dietary changes (i.e., forage PS and CL) than the response in FRL, and so PARL might be better to evaluate the risk of ruminal disfunction in dairy cows. (C) 2007 Elsevier B.V. All rights reserved.