Diurnal patterns of intake rate by sheep grazing monocultures of ryegrass or white clover

The effects of time of day on the ingestive behaviour of groups of five sheep (Ovis aries L.), grazing monocultures of ryegrass (Lolium perenne cultivar 'Parcour') or white clover (Trifolium repens cultivar 'Kent Wild White'), were measured over periods of 1 h starting at 07:30, 11:30, 15:30 and 19:30 h in August. Grazing behaviour was monitored with an automatic recording system and intake rate was measured by weighing animals before and after grazing. Over the day, for grass and clover respectively, prehension biting rates decreased linearly from 85 to 68 and from 62 to 57 +/- 7.4 (SED) bites min(-1) and masticating rate increased from 71 to 90 and from 78 to 91 +/- 5.6 (SED) chews min(-1). Total jaw movements remained constant over the day but were greater on grass than clover (P < 0.01) at 155 and 145 +/- 3.4 (SED) jaw movements min(-1). Bite masses also increased linearly from 30 to 51 for grass and from 56 to 107 for clover +/- 15.2 (SED) mg dry matter (DM) per bite and differences between grass and clover were significant (P < 0.001). Intake rates were significantly lower for grass than for clover (2.8 vs. 4.5 +/- 0.18 (SED) g DM min(-1); P < 0.001). There was an interaction between time of day and herbage species with intake rate increasing over the day from 2.5 to 3.2 and from 3.5 to 5.5 +/- 0.67 (SED) g DM min(-1) for grass and clover respectively. These changes in ingestive behaviour were associated with diurnal changes in the chemical composition of the two herbages, Herbage DM percentage increased from 07:30 to 19:30 h to a greater extent on grass (15-24%) than clover (12-18%, P < 0.001). WSC increased linearly from 15.6 to 18.3% for grass and from 5.4 to 7.3% of DM for clover (P < 0.05). Starch content of the herbage also increased linearly from 3.0 to 4.14% for grass and 3.6-8.7% of DM for clover. (C) 1997 Elsevier Science B.V.