The influence of nitrogen level, rainfall, seed pools, and pasture biomass on the botanical composition of annual pastures

Abstract

The influence of nitrogen level, rainfall, seed pools and pasture biomass on the botanical composition of annual pastures was examined over five years at two sites at Baker's Hill in the south-west of Western Australia. At the first site a pasture that had declined in content of Trifolium subterraneum to 5% remained at this level over five years when receiving 100 kg ha-1 of nitrogen fertilizer. On plots not receiving nitrogen fertilizer, but grazed to the same level, Trifolium seedling numbers in autumn increased from 5 dm-2 to a maximum of 60 dm-2, but Trifolium content of the pastures in October only increased to 15%. At the second site, established pasture was resown to different annual pasture species. Plots resown to Trifolium had 30% of Trifolium in October of the reseeding year, but only 5% three years later. Trifolium seedling numbers in autumn were >70 dm-2 in three of four years. The Trifolium was replaced by Arctotheca calendula. Plots sown to Arctotheca maintained c. 50% of this species, whilst plots sown to Erodium botrys also maintained a high level of this species but had very little Arctotheca. Plots sown to the grass Lasiochloa echinata developed a progressively higher content of Arctotheca. Annual grasses (Bromus diandrus, Vulpia myuros, Hordeum leporinum and Lolium rigidum) occurred in all plots, but their contents showed no specific trend with time. The major species were found to have different adaptive characteristics. Whilst seedling number in autumn was a major determinant of the content of a species in the pastures in August, Trifolium content was enhanced by high autumn rainfall and low winter biomass. The contribution of each Trifolium seedling to clover content in August was much less than that of each Arctotheca seedling to the capeweed content at that time. The pasture composition in October was mainly determined by cornposirim in August, although heavy rain in October favoured Arctotheca and light rain favoured the grass species.