Seasonal dynamics of leaf extension and losses to senescence and herbivory in extensively managed sown ryegrass–white clover swards

Abstract

Tissue flow measurements of leaf material in Lolium perenne (perennial ryegrass), Agrostis capillaris or Poa annua, and Trifolium repens (white clover) were made at three upland sites in Scotland (Hartwood, Sourhope and Fasque) in 1992/93 to determine if there were differences in seasonal growth, senescence and losses to herbivory between species in their response to more extensive sward management. The measurements were made monthly from May until October in established predominantly perennial ryegrass/white clover swards receiving three different treatments. One treatment received a conventional annual fertilizer application of 140 kg N/ha plus maintenance P and K and was grazed by sheep at a sward height of 4 cm (4F), whereas the other two treatments were unfertilized and grazed to maintain a sward height of 4 cm (4U) or 8 cm (8U).

Significant sources of the variation in leaf appearance, increase in green lamina/petiole length (leaf extension), senescence and losses to herbivory were attributable to site, sward, species and date of measurement. The rate of leaf extension for all three measured species was less in 4U than 4F swards, and less in 4U than 8U swards. Leaf extension of L. perenne exceeded that of the other species, even in unfertilized swards, but rates of leaf appearance were less. There was some evidence in spring of a reduction in net growth as a consequence of removing fertilizer inputs and maintaining a sward height of 4 cm. Species differences in the losses of leaf tissue to herbivory were dependent on sward management. In the 4F treatment, leaf loss to herbivory from L. perenne tillers was greater than that from either A. capillaris, P. annua or T. repens in May, June and September. In the 4U treatment more leaf tissue was also lost from L. perenne than from T. repens. In contrast, there was no difference between grass species in losses to herbivory in either unfertilized sward. The responses of species to changes in fertilizer and grazing management were similar at three sites of differing fertility. The results are discussed in relation to plant competition and species dynamics in extensively managed swards.