Physiological and morphological responses of tall fescue (Festuca arundinacea Schreb.) and perennial ryegrass (Lolium perenne L.) to defoliation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.) in Plant Ecology and Physiology, Department of Plant Science, Massey University

Tall fescue (Festuca arundinacea Schreb.) has been suggested as an \nalternative to perennial ryegrass, particularly in conditions of moisture limitation, \nbut there is little comparative information on the plant characteristics influencing \nregrowth in the two species, particularly under continuous stocking. The objectives \nof this study were to :i) examine the response of tall fescue to continuous stocking \nin terms of tillering activity, leaf growth and competition, and ii) determine which \nphysiological or morphological factors are important in influencing regrowth after \ndefoliation, using perennial ryegrass as a reference standard. Experiments were \nundertaken in field, glasshouse and controlled environment conditions. \nIn the field sown swards of tall fescue (Festuca arundinacea Schreb. cv. \n'Grassland Roa') and white clover (Trifolium repens L. cv. 'Grassland Tahora'), \nwith volunteer grasses mainly consisting of perennial ryegrass (Lolium perenne L. \ncv. 'Grassland Nui') were continuously stocked with varying numbers of sheep to \nmaintain sward surface heights 90-100 mm (Lax, L), 50-60 mm (Medium, M) and \n30-4 0 mm (Hard, H). Measurements were made on areas with and without clover \nby removal of white clover with clopyralid. Tiller population density, tiller weight, \nand leaf growth and productivity of tall fescue were all reduced under hard grazing. \nTall fescue was susceptible to competition from companion species, particularly \nperennial ryegrass, and tended to be replaced by other species under hard \ngrazing. \nUnder glasshouse conditions individual plants of tall fescue and perennial \nryegrass were defoliated to stubble heights of 100 mm (Lax, L), 60 mm (Medium, \nM), 30 mm (Hard1, H1) and 30 mm (Hard2, H2) 9 times with 5 day intervals over \na period of 4 5 days. Treatments L, M, and H1 were initiated at an average of 1 1 \ntillers per plant for each species; treatment H2 commenced at 6 tillers per plant. \nBoth species showed sensitivity to severe cutting treatments by reduction in tiller \nnumber, tiller weight, leaf growth and less shoot and root growth. Tall fescue \nshowed lower leaf growth, tillering activity and herbage harvested per plant than \nperennial regress, but it produced larger tillers. \nThe comparative response of tall fescue and perennial ryegrass to leaf \ndefoliation was studied under controlled environment conditions at both the \nvegetative and reproductive stages of growth. The oldest leaf lamina was \ndefoliated regularly to maintain four, three, two or only one live leaf per tiller for six \nor seven leaf appearance intervals. In both species repeated removal of older \nleaves had little effect on tiller production, tiller weight, leaf growth rate and \nconsequently total accumulated shoot and root weight, and mean shoot and root \nrelative growth rates. Removal of all fully expanded leaves resulted in significant \nreduction in the above components, though leaf elongation rate was little affected. \nLeaves were shorter, narrower and lighter under hard defoliation, but leaf \nappearance rate was not affected. Hard defoliation affected tall fescue tiller weight \nmore than perennial ryegrass. Water soluble carbohydrate concentrations in stem \nbases of plants decreased with increasing severity of defoliation especially for tall \nfescue. Leaf photosynthetic capacity per unit area was not influenced by \ndefoliation intensity, but photosynthetic capacity per unit leaf weight increased \nunder hard defoliation. Hard defoliation decreased the proportion of root mass to \nshoot mass, and increased the proportion of leaf mass to shoot mass. Plants \nshowed relatively similar sensitivity to defoliation at vegetative and reproductive \nphases of growth. At both phases tall fescue again produced larger tillers with \nlonger leaves and had a longer leaf life-span than perennial ryegrass, but it had \nlower leaf growth and appearance, and produced fewer tillers per plant. \nPhotosynthetic activity per unit leaf area was similar for the two species, but tall \nfescue often had lower photosynthetic rate per unit leaf weight than perennial \nryegrass. \nIn conclusion, hard defoliation intensity reduced both tiller population \ndensity and tiller weight and consequently decreased pasture regrowth through a \nreduction in LAI, life-span of leaf area, photosynthetic efficiency, and shortage of \ncarbohydrate reserves. The factors that resulted in the regrowth of tall fescue \nbeing less responsive to hard defoliation than perennial ryegrass were slower leaf \nturnover, slower leaf appearance rate, lower tillering capacity and longer leaf lifespan. \nThese resulted in lack of plasticity in tiller population density of tall fescue \nin response to hard defoliation.