Yield potential is higher when stems are developing, but the nutritive value of mature stems is low. Severity of defoliation affects herbage production and sward vigour. Disadvantages of a low height of defoliation are that only a small photosynthetic surface is left to support regrowth, the intake of grazing animals is restricted and cut crops dry a little less well and are more likely to be contaminated with soil. Selectivity of defoliation by grazing animals affects the quality and quantity of herbage ingested and the botanical composition of the sward. Sheep and cattle given a free choice between perennial ryegrass and white clover tend to select a mixed diet with perhaps two thirds clover.
Cattle avoid grazing herbage growing on cattle dung patches, as far as possible.
Cattle and sheep tend to avoid grazing heather except when food is in short supply and sheep tend to avoid grazing Nardus . Treading by grazing animals and compaction by tractor wheels damages plants directly by bruising and indirectly by compacting the soil. The problem is greatest in wet conditions on silt, clay or peat soils. Penetration of the compacted layer by slitting or spiking can ameliorate the problem. Soil ingestion can have an appreciable effect on the intake of some elements and on the wear of teeth when animals graze short swards in muddy conditions.