An Ecological Approach to Forage Management

The principles of defoliation management for maximum production of quality herbage are well researched, namely the manipulation of frequency and severity of defoliation, but their impact on the various levels of ecological organisation in pastures is nol fully understood or their interpretation appreciated. Using grazed ryegrass-white clover pasture as; example, detailed sward studies have suggested the following model. Defoliation frequency and severity interact lo produce varying levels of physiological stress and morphological adaptation which affecl the survival rate and size of individual growth units, the lowest level of organisation. Increasing frequency of defoliation reduces organ size but increases survival rate and vice versa. For ryegrass, the dominant component of these pastures, this compensatory relationship results in pastures with marked differences in characteristics, with long-term consequences for pasture stability and persistence, Because stems fragment continually, plant structure and their population distribution is stable, and defoliation management has little influence al this level, but has large effects al the community level of organisation. This fragmentation process results in aggregations of plants into clumps, whose density, while i may restrict individual tiller growth through competition, provides mutual support and protection capable of ameliorating the effects of environmental stress, such as heal stress in drought, or overgrazing. Defoliation management, through tiller survival, has large effects on 1101 only the size of individual clumps, but also the number (density) of clumps which affects the degree of protection for associated species, such as white clover, enhancing their survival. In terms of plant growth processes, both while clover and perennial ryegrass have a period of vulnerability during the re-establishment of lhe basic plant population over the mid-spring 10 early-summer period. The intensive management of grazed improved pastures in spring may be more crucial to the subsequent performance and production of the pasture than at other times of the year. This model suggests that plants with prostrate and/or stoloniferous habit prove to be more adaptable and persislent under stress, and that higher producing improved plants, because of selection for erect growth habit and high harvest index under infrequent defoliation, may be more susceptible to environmental stress owing 10 insufficient plasticity to adapt to slress conditions. With greater awareness of the ecological principles constraining both plants and management, belier design of systems for enhanced pasture productivity and persistence will be possible, and strong directions for plant breeding will become apparent to those organizations concerned with developing cultivars for sustainable pastoral systems.