Bite dimensions and grazing movements by sheep and cattle grazing homogeneous perennial ryegrass swards

Pairs of ewes or heifers were allowed to graze for short periods from previously prepared 'homogeneous' areas of ryegrass, in order to study their bite dimensions and movement patterns and to investigate the role of different species in initiating spatial heterogeneity. Apparent bite area was calculated from number of bites and total bitten area when ewes or heifers took 10-15 bites from an 80 cm x 80 cm area in a homogeneous Lolium perenne sward. Apparent bite depth was calculated from sward surface height (SSH) within and adjacent to bitten areas. Apparent bite mass was calculated from grazed stratum bulk density. Incisor arcade breadth was measured. Animals also grazed 20 m x 40 m plots for 45 min. Position and activity were recorded and distance travelled calculated. Number of bites and steps, number and duration of grazing bouts, feeding station dimensions and horizontal head sweep were recorded. Bites per bout, per feeding station and per m forward movement, bite rate, inter-bout interval, and distance travelled, speed of movement, number of steps, step length and step rate within and between bouts were calculated. Distribution of grazing bout and inter-bout durations and the animals' foraging paths were tested for randomness. Apparent bite area for heifers was 2.2 times that for ewes and incisor arcade breadth 1.8 times that for ewes. Apparent bite depths were similar at 0.35 of SSH. SSH distribution within bitten areas was non-normal and differed between animal species. Heifers' bite mass was 2.1 times that for ewes. Heifers had longer grazing bouts, more feeding stations per bout, moved farther per bout but had similar movement speed while grazing. Distance between feeding stations was similar between species but heifers' feeding station residence time was twice as long. Heifers' biting rate was lower but they took more bites per bout, per feeding station and per metre of forward movement. Mean head sweep distance was greater for heifers. Both species moved a similar distance between grazing bouts but ewes moved more slowly with more, shorter steps. For both species, grazing bout and inter-bout durations were not randomly distributed. At a 1 m(2) scale for both species and a 5 m(2) scale for heifers, movement was non-random with a strong propensity to walk in a straight line but at a 5 m(2) scale sheep movement was uniformly distributed on the circle. (C) 2004 Elsevier B.V. All rights reserved.