Diurnal patterns in rumen volume and composition of digesta flowing into the duodenum

Rate of passage of digesta out of the rumen is generally assumed to follow first order kinetics but this assumption may not hold within 24-h feeding c ycles. Four lactating cows offered a mixed forage/concentrate diet ad libit um from 08.00 h to 00.00 h and a protein-rich meal at either 08.30 or 00.30 h were used to investigate diurnal patterns in rumen volume and compositio n of digesta flowing into the duodenum. Cows were fitted with large rumen c annulae to permit complete emptying of rumen digesta and T-piece duodenal c annulae with inserts to enable collection of representative samples of dige sta. Chromium-mordanted lucerne hay manually dosed to the rumen at 2-h inte rvals and a continuous infusion of cobalt-EDTA were used as digesta flow ma rkers. Duodenal digesta was sampled over three alternate days to obtain sam ples representing every hour of the 24-h day, which were analysed individua lly Use of either of the two markers gave similar results for mean daily po w but not for hourly flow and thus hourly flow data were not considered rel iable. The content of crude protein in duodenal digests varied within one f eeding cycle from 200 to 320 g/kg in cows offered protein at 08.30 ('day') and from 240 to 300 in cows offered protein at 00.30 h ('night'). On a dail y basis, fractional passage rates (calcutated from duodenal flow divided by rumen pool size) were significantly (P = 0.016) lower for protein for 'nig ht' (0.0750 per h) versus 'day' (0.0824 per h) cows. The observation that m arked diurnal patterns in the composition of protein flowing into the duode num can apparently be 'smoothed' simply by altering feeding sequence, which was correlated with an increased milk fat yield, suggests there is potenti al for improving production through manipulating the way in which dietary c omponents are offered within a day.