Effects of grazing on sward condition

Grazing differs fundamentally from cutting and burning in that it removes the vegetation piecemeal, and more selectively, at least at low to medium grazing intensities. Grazing also produces dung, and its associated invertebrates can be important in the diet of some birds, notably Red-billed Choughs Pyrrhocorax pyrrhocorax (Roberts 1982). Trampling by stock can create a continuity of bare

Fig. 14.3 Grazing is important in certain habitats to prevent vegetation succession and maintain suitable vegetation structure for birds. A simple but effective way to monitor its effects is to erect grazing exclosures and compare the vegetation inside and outside of them. Yellowstone National Park. (William J. Sutherland)

Fig. 14.3 Grazing is important in certain habitats to prevent vegetation succession and maintain suitable vegetation structure for birds. A simple but effective way to monitor its effects is to erect grazing exclosures and compare the vegetation inside and outside of them. Yellowstone National Park. (William J. Sutherland)

and disturbed ground, particularly under wet conditions and thereby increase birds' access to soil invertebrates.

The effects of grazing vary depending on the vegetation already present, the densities, and type of stock and the timing and frequency of its access. Grazing at medium stocking levels will selectively remove only a moderate proportion of the sward, and tends to create variation in the vegetation structure. Very high or very low levels of grazing will in time remove, respectively, virtually all, or almost none, of the vegetation, and thereby produce little variation in sward structure. High densities of small mammals can survive under light cattle grazing regimes, which preserve tussocks and maintain a dense litter layer. Grazing creates coarse scale variation in sward structure and composition by accentuating existing variation in plant composition resulting from differences in topography and previous management. If such variation is not present, then grazing is unlikely to create it. The effects of prescribed grazing levels on the sward vary from year to year, primarily due to weather-related differences in vegetation growth. Thus stocking levels often have to be adjusted by eye to achieve the desired conditions. Nevertheless, it is still useful to measure the height and variation in structure of the sward at key times of year, for example, the beginning of the breeding season, to help interpret changes in bird numbers. This can be done by taking measurements of sward height against the graduated side of a Wellington boot as you walk across the field or by using a sward stick (see Chapter 11).

Three types of domestic animal are commonly used to graze grasslands: cattle, equines (horses, ponies, and donkeys), and sheep. Cattle feed by ripping off tufts of vegetation and are the first choice for producing fine-scale variation in sward structure and patches of bare ground. Equines and sheep nibble the vegetation and are more selective in the plants they remove. If they like the vegetation, they nibble it uniformly short, but if they do not, they will ignore it. They therefore produce little fine-scale variation in the sward, but at moderate grazing densities can create coarse-scale variation comprising short, uniform lawns and dense, rank areas. Judicious grazing, particularly by cattle, is therefore better than cutting or burning in providing suitable conditions for birds requiring a close juxtaposition of suitable nest-sites and ranges of feeding conditions. Grazing influences vegetation composition by encouraging unpalatable and low growing plants that can tolerate repeated defoliation, particularly grasses and rosette-forming species.

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