Effects of November–April grazing pressure on hill country pastures 2. Pasture species composition

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Abstract
Species composition of pasture located on a steep (32°), NW-facing hillside was measured for 3 years under contrasting treatments: hard or lax grazing during November–January (pre-dry); hard grazing or spelling during January–March (dry); and hard grazing, lax grazing, or spelling during March–April (post-dry). Measurements included growth unit (tiller, stolon, crown) density, % dry weight composition, and % frequency occurrence of species. Annual species were most responsive to differences in grazing pressure, being influenced by reseeding and the opportunity for re-establishment. On flat areas, Poa spp. increased where pastures opened up from spelling during the dry phase, where pastures were kept short during the post-dry phase, and where ryegrass density declined after a dry summer. Adventive grasses (such as Bromus mollis L. and Anthoxanthum odoratum L.) increased where lax grazing during the pre-dry phase allowed reseeding, and where spelling during the dry phase reduced pasture density. Trifolium subterraneum L. content was also highest where lax grazing during the pre-dry phase encouraged reseeding; however, the herbage thatch associated with summer spelling reduced successful seedling establishment. Spelling during the dry phase allowed Lotus spp. to reseed and increased their content. Trifolium repens L. was reduced where grass dominance occurred with lax grazing during the pre-dry phase, particularly when followed by hard grazing during the dry phase. Lax grazing and spelling decreased Lolium perenne L. tiller density in the short term, and spelling during the dry phase eventually led to longer term decreases in abundance. Agrostis tenuis Sibth. was less responsive to grazing pressure, as were low fertility demanding grasses such as Notodanthonia spp. and Festuca rubra L. Flat weeds (Hypochaeris radicata L., Rumex acetosella L.) were strongly encouraged by spelling during the dry phase. The significance of these changes depends on pasture composition, as determined by long-term fertility and climatic limitations (e.g., annual: perennial ratio), and the degree of pasture improvement being sought (e.g., legume content). In this experiment, the impact of grazing pressure on annual pasture species was of greatest significance.

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