SEASONAL-VARIATION IN THE MORPHOLOGY OF PERENNIAL RYEGRASS (LOLIUM-PERENNE) AND COCKSFOOT (DACTYLIS-GLOMERATA) PLANTS AND POPULATIONS IN PASTURES UNDER INTENSIVE SHEEP GRAZING

The morphology of independent plants of 'Grasslands Ruanui' perennial ryegrass (Lolium perenne L.) and three cultivars of cocksfoot (Dactyli s glomerata L.) ('Grasslands Apanui', 'Grasslands Kara' and 'Grassland s Wana') in mixed pastures under intensive sheep grazing was studied a t Palmerston North, New Zealand during 1991/92. Both perennial ryegras s and cocksfoot exhibited a similar pattern of clonal growth and popul ation structure. Distribution of plants among various orders of branch ing showed a relatively stable pattern through most of the year except in spring, when stem decay and plant fragmentation exceeded apical gr owth and regeneration, causing an increase in the proportion of small plants and a corresponding decrease of larger plants. This pattern was similar to that previously reported for white clover (Trifolium repen s L.). Nevertheless, seasonal variation in plant structure (number of leaves, tillers and stems per plant) was small, but variation in organ size (DW or length) was greater. Stolen formation through elongation of internodes occurred throughout the year, but was associated primari ly with flowering. Grazing management caused no differences in plant s tructure between species or among the cocksfoot cultivars, but did aff ect the size of organs, and hence plants. Cocksfoot plants were 50-60% heavier than perennial ryegrass under rotational grazing. Under set s tocking, only perennial ryegrass and Wana cocksfoot exhibited sufficie nt phenotypic plasticity to survive, both Kara and Apanui cocksfoot fa iled to persist. The only consistent difference between the species wa s greater flowering in perennial ryegrass than in cocksfoot, in both t he proportion of plants flowering, and the number of flowering tillers per plant. Both species produced stolons throughout the year, althoug h perennial ryegrass and Wana cocksfoot had a higher proportion of pla nts with stolons than Apanui and Kara cocksfoot. Length and DW of stol ons per plant were similar in both species. As there was little variat ion in plant structure and plant density, length of stolons per unit a rea tended to parallel seasonal changes in pasture tiller density. The role of grazing management in the survival of tillers and plants, and subsequent performance of grass species in pastures is discussed.