The effect of the spatial distribution of plant species within the sward on the searching success of sheep when grazing

The ability of grazing sheep to exploit preferred patches of grass species according to their spatial distribution was studied. The choices made by groups of four ewes in three cocksfoot ( Dactylis glomerata ) plots (80 × 80 m), in which 136 patches (30 × 30 cm) of perennial ryegrass ( Lolium perenne ) were either randomly distributed (plot Rand ) or grouped (plots Ag.1 and Ag.2 ) to create preferred feeding sites (four rich sites of twenty‐five patches and four poorer sites of nine patches) were compared. On measurement days, each group was allocated to one of the three plots, and grazed it in at least four successive 30‐min tests. At the end of the experiment the ewes were allowed to graze the plots continuously for 3 months to assess the medium‐term effect of grazing sheep on the depletion rate of patches. Behavioural observations showed that, once the ewes had discovered a preferred feeding site in plots Ag.1 or Ag.2 , they frequently returned to it within a test and between tests, whereas they failed to learn the precise location of many isolated patches in plot Rand . Thus in 2 h of grazing, the ewes spent 0·40 more time feeding on perennial ryegrass when the patches were grouped than when they were randomly distributed, and perennial ryegrass depletion was greater in plots Ag.1 and Ag.2 than in plot Rand (0·193 vs. 0·105 of initial height). The richer sites were also more depleted than the poorer ones (0·209 vs. 0·152 of initial height). When the plots were continuously grazed, perennial ryegrass depletion remained greater in plots Ag.1 and Ag.2 than in plot Rand for at least 3 weeks. Hence the spatial distribution of patches can affect diet selection by sheep, and so probably their impact on grasslands.