Diurnal variation in rumen fill of dairy cows grazing Persian clover at different pasture allowances

Abstract

In this experiment 12 rumen-fistulated cows in late lactation, grazing Persian clover (Trifolium resupinatum) at four pasture allowances [9, 16, 32 and 53 kg dry matter (DM)/cow.day], had their entire rumen contents removed and sampled six times over 6 days, with the time of sampling varied between days. These rumen pool size measurements occurred at 0645 hours, 0915 hours, 1045 hours, 1345 hours, 1530 hours and 1900 hours. The cows were offered new allocations of pasture twice daily at 0715 hours and 1530 hours. Herbage intake increased (P < 0.001) from 5.6 to 20.4 kg DM as pasture allowance increased. Pasture allowance had no effects on rumen pool sizes of wet matter (WM) (P = 0.95), DM (P = 0.914), neutral detergent fibre (NDF, P = 0.499) or the DM% of the digesta (P = 0.078). Rumen pool sizes of WM, DM and NDF varied throughout the day. Pool sizes of WM and DM were lowest at 0645 hours and highest at 1900 hours for all allowances (P < 0.001) and along with pool sizes of NDF, were also lower at 0645 hours compared with 0915 hours and at 1530 hours compared with 1900 hours. The lack of increase in rumen pool sizes as pasture allowance increased when cows grazed Persian clover suggests that outflow rates increased with the increasing intakes. The low, and similar, rumination times between allowances also suggests that there is little restriction to the ability of outflow rates to increase when cows graze Persian clover at increasing allowances. The changes in rumen pool sizes over the day were consistent with the twice daily allocation of fresh pasture to the cows. The diurnal rumen pool size information obtained from this experiment will assist with the development of mechanistic rumen models that are capable of simulating the discontinuous intake and varying rumen pool sizes of cows in grazing systems.