AbstractDuring two experimental periods, eight non-lactating, rumen fistulated Simmental cows were given 2 kg/day of a cereal based concentrate and fescue hay ad libitum. The hay differed in neutral-detergent fibre concentration (LNDF: low; HNDF: high) and physical form (L: long; C: coarsely chopped). The rumen degradability of the dietary ingredients and of an extracted soya-bean meal was studied using the polyester-bag method.The estimated effective rumen degradability <DG) of dry matter (DM) was significantly higher for LNDF than for HNDF hay (0·490 v. 0·401; P < 0·01). The DM DG of the extracted soya-bean meal and the dietary concentrate were higher when incubated with LNDF than with HNDF diets (0·630 v. 0·581, P < 0·05, and 0·541 v. 0·514, respectively).Chopping the hay significantly increased the daily DM intake for HNDF diets (from 9·9 to 11·6 kg; P < 0·05), while no statistical differences were found between the other diets (10·2 and 9·6 kg, respectively for L-LNDF and C-LNDF diets). Cows given HNDF hay drank less water (42·4 v. 51·7 I/day) and total water consumption, expressed per unit of DM intake, was lower for the HNDF diets (4·1 v. 5·3 I/kg DM; P < 0·01).Dietary treatments did not affect the estimated rumen turn-over rate of hay (0·027 per h on average, k1), hay post-rumen turn-over rate (0·069 per h on average, k2) or the estimated rumen liquid turn-over rate (0·100 per h on average, kj. The calculated outflow from the rumen was lower for the HNDF diets (159·4 v. 198·7 I/day). The daily amount of estimated saliva secreted did not differ between treatments, although saliva expressed per kg DM intake was significantly lower for the HNDF diets (10·8 v. 14·8 I/kg DM; P < 0·05).The HNDF diets gave a lower post-feeding rumen pH (6·29 v. 6·58; P < 0·01), a higher concentration of n-butyrate (9·58 v. 8·32 mmol/l; P < 0·05) and a lower concentration of ammonia nitrogen (NH3-N) (80·5 v. 128·5 mg/l; P < 0·05). Of the C4 and C5 iso-acids, the rumen liquid from cows given HNDF diets had significantly lower concentrations of isobutyrate, isovalerate and n-valerate (P < 0·01). Hay physical form did not affect rumen pH, NH3-N and volatile fatty acid concentrations.