INFLUENCE OF ROTATION AND TIME OF GERMINATING RAINS ON THE PRODUCTIVITY AND COMPOSITION OF ANNUAL PASTURES IN WESTERN-AUSTRALIA

A long-term rotation experiment located in south-western Australia was used to measure the effect of rotation and time of germinating rains on the productivity and botanical composition of grazed annual pasture s in 2 contrasting seasons in an environment with an average annual ra infall of 325 mm. The density of self-regenerating seedlings of subter ranean clover (Trifolium subterraneum), capeweed (Arctotheca calendula ), and grasses (Lolium rigidum, Hordeum leporinum, Bromus diandrus) wa s greatly increased (approx. 3 times the density) when there was a sec ond year of pasture after crop compared with the first year after crop . The lower plant density resulted in first-year pastures having only about 33% of the autumn biomass accumulation of second-year pastures. This difference in early pasture growth had no effect on total pasture production in 1992, but in 1993 total pasture production was 30% grea ter in second-year pastures compared with first-year pastures. Botanic al composition varied between and within seasons with the percentage o f subterranean clover increasing throughout the season and the percent age of capeweed decreasing throughout the season. Grasses comprised <2 0% of the biomass in all seasons and treatments. Production of subterr anean clover seed in 1993 was higher in a 1:2 crop-pasture rotation th an in a 1:1 crop-pasture rotation and direct drilling in the cropping phase increased seed set compared with conventional tillage in both 1: 1 and 1:2 crop-pasture rotations. Capeweed seedlings emerged in large numbers after rainfall between February and May and subsequently showe d a relative growth rate twice that of subterranean clover and the gra sses, but exclusion of rainfall until June resulted in a significant r eduction in the emergence of capeweed seedlings. Additionally, capewee d had a lower rate of seedling survival compared with other pasture sp ecies, and this is contrary to observations by other researchers that capeweed is highly resistant to moisture stress during early growth.