A model of the grazing of hill vegetation by sheep in the UK .2. The prediction of offtake by sheep

1. A model is described which predicts seasonal changes in the quantity and digestibility of offtake by ewes and lambs grazing seven dwarf shrub-dominated and five grass-dominated vegetation types commonly found in the hill areas of the UK. 2. Intake from a given vegetation type is assumed to depend on the area covered by the vegetation type and the potential daily intake of digestible dry matter (DM) available from it, in relation to those for all vegetation types present. Potential daily intake of digestible DM is calculated as the product of diet digestibility and daily DM intake. Daily DM intake is assumed to be limited either by diet digestibility or by a maximum grazing time, whichever is the lower. If the latter, estimates of bite weight and bite rate are used to predict the maximum achievable DM intake. 3. The sensitivity of model outputs to a range of parameters is described. Predictions made by the model of relative grazing times on a range of vegetation types at one site bore a close resemblance to measured values. 4. Analysis of model behaviour is used to consider the factors that influence the behaviour of hill grazing systems and to suggest the circumstances under which the dwarf shrub Calluna vulgaris (heather) is most likely to be at risk from grazing. 5. The model, integrated with one described elsewhere which predicts the seasonal food resource available to herbivores from hill vegetation, represents the first synthesis of the available information on hill grazing systems in the UK into a simulation model. Possible developments and potential uses are discussed.