Decline in subterranean clover (Trifolium subterraneum L.)-based pasture systems: causes and solutions

[Truncated abstract] Subterranean clover (Trifolium subterraneum) (subclover) is the dominant pasture legume in southern Australian pastures. In the south-west of Western Australia (WA), there has been a perceived decline in the productivity and persistence of subclover. However, there is a lack of quantitative data on this decline and the possible causes. In this thesis I have reported upon an investigation into the limitations to subclover composition and the pasture growth rates of subclover-based pastures. Farmer perceptions of pasture decline in WA were also assessed. In the growing season of 2004, 65 farmers were surveyed to assess the performance of subclover-based pastures in WA from 1974-2004. Increased cropping frequency and longer cropping phases (phase-farming rather than ley-farming) was prevalent even in those enterprises dominated by grazing and pasture production. The majority of farmers had not perceived that pasture productivity, or subterranean clover persistence, had declined. However, most survey respondents felt that subterranean clover pasture productivity was suboptimal (59%). Farmers cited competition from weeds (29.5%), poor seed set (34.4%) and seasonal variability (36.1%) as contributing factors to suboptimal productivity. Farmers’ estimates of pasture composition differed from measured values, with the average proportion of subterranean clover in pasture estimated at 38.5%, with the actual value at 18.0%. This study found through regression tree analysis that the ideal pasture composition to maximise growth rate while maintaining a subterranean clover content (to ensure nitrogen fixation) was approximately 40% subterranean clover and 40% grass as the best performing pastures across the state.