Comparative Studies of Diet Selection by Sheep and Cattle: Blanket Bog and Heather Moor

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Abstract
(1) A comparative study was made of the seasonal variation in floristic composition of the diets selected by sheep and cattle grazing together on blanket bog (with Calluna vulgaris and Eriophorum vaginatum as co-dominants) and on species-poor Calluna vulgaris heather moor. The sites were grazed in sequence with the experimental animals moved between sites as appropriate. (2) For each period at each site, sward biomass, species and morphological composition and canopy structure were characterized. Diet samples were collected from three to four oesophageally-fistulated animals of each species in four to five, separate, two-week grazing periods at each site over four years. (3) Sheep diets were slightly more variable than cattle diets; on the blanket bog between-animal variation was similar for sheep and cattle while within-animal variation was greater for sheep; on the Calluna moor, between-animal variation was greater for sheep than cattle whereas within-animal variation was similar for both animal species. (4) Though the proportions of most dietary components differed significantly between sheep and cattle in at least one period, it was clear that there were many components which were selected or avoided in common by sheep and cattle. (5) Species which were avoided included Calluna vulgaris, Erica spp. and Empetrum nigrum: their seasonal patterns of use, mainly outside the growing season, reflected periods of shortage of the preferred species. (6) Species which were selected included Molinia caerulea, other grasses and Carex spp. and Trichophorum cespitosum on the blanket bog and Vaccinium spp., Juncus spp. (mainly J. squarrosus) and grasses and Carex spp. on the Calluna moor. The ability of cattle to match the sheep in the dietary proportions of these preferred components was influenced by their distribution in the sward. (7) On the blanket bog, cattle diets contained more Eriophorum spp. leaf than sheep diets, except in April when the dietary proportions were similar. Sheep diets in April contained very high proportions of the immature flowers of E. vaginatum which were strongly selected. (8) Cattle diets contained more dead components than did sheep diets in every period at both sites. (9) Differences between sheep and cattle were mainly attributable to the greater ability of sheep to select from fine-scale mixtures: differences in grazing height between sheep and cattle were secondary to selection in the horizontal plane. (10) The much lower similarity coefficients between sward composition and diet composition for both sheep and cattle on the blanket bog and Calluna moor compared with the hill grasslands in summer, when the widest range of species was present, indicated that the drive to select was greater on the dwarf shrub communities. (11) The implications of the results for grazing management are discussed briefly.