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This paper examines the effects of sheep and insect grazing on the generation of natural gaps in turf and the establishment of seedlings therein. The site is on an old field on limestone. There were five sheep grazing treatments: ungrazed, and grazed briefly in spring, briefly in autumn, in autumn or in summer and autumn. The first three of these treatments also contained plots in which insecticide was applied regularly during the growing season. Pins of increasing size (1.6 m m -12.8 mm diameter) were lowered at grid intersections in permanent quadrats to record gap type and the occurrence of seedlings on seven dates over 15 months.Heavily-grazed plots had more bare ground in spring and more herb seedlings established in them. Seedlings of annual species of Bromus were able to establish in gaps (bare ground and litter) of smaller size than was required by herb seedlings (which had much smaller seeds). The use of an insecticide increased seedling establishment in October in ungrazed and in spring-grazed paddocks but decreased it in autumn-grazed paddocks. This may indicate the greater importance of insect herbivory in making gaps in the matted sward of autumngrazed plots. Seedlings which established in October 1986 and March 1987 were more likely to die than those establishing earlier in 1986. This may be because of winter-kill and especially vigorous spring vegetation growth respectively. Seedling mortality was not affected by gap size or grazing treatment (unless such effects operated before we detected the seedlings).

Abstract. The effects of grazing upon the composition of a grassland were examined in a replicated grazing experiment with sheep over eight years. Grazing was controlled to produce two levels of grazing in each of winter, spring and summer, combined in a 2×2×2 factorial experimental design. Particular species were either increased or decreased by winter grazing while, in general, dicotyledonous species were favoured by grazing in both winter and spring. A model fitting the main effects of grazing in three seasons showed the importance of grazing in winter and spring to achieve sward diversification.

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