Controlled grazing studies on Nardus grassland: effects of between-tussock sward height and species of grazer on Nardus utilization and floristic composition in two fields in Scotland.
The utilization of the tussock-forming grass Nardus stricta was investigated in controlled grazing experiments in which various sward heights of between-tussock grasses were maintained throughout the growing season using either sheep, cattle or goats. The utilization of Nardus increased as the height maintained for the between-tussock grasses was reduced. The relationship between sward height and level of utilization of Nardus, however, differed greatly among herbivore species. Under similar sward conditions, cattle and goats utilized more Nardus than did sheep. Utilization of Nardus declined over successive seasons under grazing by sheep but was sustained under grazing by cattle or goats. The severity of defoliation of Nardus achieved by cattle or goats when between-tussock grasses were maintained at 4.5 cm (mean sward surface height above ground) resulted in disappearance of tussocks from the sward and in reduced size, leaf extension growth and nutrient reserves of Nardus tillers. Under cattle grazing, the cover achieved by Nardus decreased from 55.4% to 30.0% over 5 years. Under sheep grazing, both the biomass of individual Nardus tussocks and the cover of Nardus increased. Cover increased from 58% to over 86% at a between-tussock height of 4.5 cm, while at a height of 3.5 cm, after initially declining, it increased to reach 72%. The broad-leaved grasses showed a trend towards increase in cover when between-tussock sward height was maintained at 4.5 cm whether grazing was by cattle or by sheep; the trend was not evident at 3.5 cm (sheep grazing only). Changes in the floristic composition of the sward are discussed in relation to the roles of the inherent growth characteristics of the species present, to selective defoliation and uprooting of the shoots, to altered nutrient cycling pathways under grazing and to the nutrient status of sites.