Seasonal variation in the morphology of perennial ryegrass (Lolium perenne) and cocksfoot (Dactylis glomerata) plants and populations in pastures under intensive sheep grazing

Abstract

SUMMARYThe morphology of independent plants of ‘Grasslands Ruanui’ perennial ryegrass (Lolium perenneL.) and three cultivars of cocksfoot (Dactylis glomerataL.) (‘Grasslands Apanui’, ‘Grasslands Kara’ and ‘Grasslands Wana’) in mixed pastures under intensive sheep grazing was studied at Palmerston North, New Zealand during 1991/92.Both perennial ryegrass and cocksfoot exhibited a similar pattern of clonal growth and population structure. Distribution of plants among various orders of branching showed a relatively stable pattern through most of the year except in spring, when stem decay and plant fragmentation exceeded apical growth 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 repensL.). 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. Stolon formation through elongation of internodes occurred throughout the year, but was associated primarily with flowering.Grazing management caused no differences in plant structure between species or among the cocksfoot cultivars, but did affect the size of organs, and hence plants. Cocksfoot plants were 50–60% heavier than perennial ryegrass under rotational grazing. Under set stocking, only perennial ryegrass and Wana cocksfoot exhibited sufficient phenotypic plasticity to survive, both Kara and Apanui cocksfoot failed to persist. The only consistent difference between the species was greater flowering in perennial ryegrass than in cocksfoot, in both the proportion of plants flowering, and the number of flowering tillers per plant. Both species produced stolons throughout the year, although perennial ryegrass and Wana cocksfoot had a higher proportion of plants with stolons than Apanui and Kara cocksfoot. Length and DW of stolons per plant were similar in both species.As there was little variation in plant structure and plant density, length of stolons per unit area 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.