DISTRIBUTION AND ABUNDANCE OF ANNUAL LEGUME SEEDS IN THE WHEAT-BELT OF WESTERN-AUSTRALIA

The size and composition of pasture legume seedbanks were estimated fr om 2 surveys on a 460-km west-east transect of the wheatbelt of Wester n Australia. Survey 1 (in spring) sampled naturalised legumes, and sur vey 2 (in summer) measured the amount and botanical composition of leg ume seed from selected sites. Seedbanks were examined in greater detai l on 2 farms in the higher rainfall part of the wheatbelt. Survey 2 re vealed that mean seedbank size of the poorest 40% of sites (those with less than or equal to 200 kg seed/ha) was 61 kg/ha, and that 72% of s eeds were naturalised clovers. In contrast, the best 60% of sites (tho se with >200 kg seed/ha) averaged 533 kg seed/ha, of which only 35% wa s naturalised clover seed, the remainder in both surveys being mainly subterranean clover (Trifolium subterraneum). Mean seed bank size (kg/ ha) varied from 359 (survey 2) to 587 (survey 1) and, in both surveys, was poorly correlated with long-term mean annual rainfall and a numbe r of soil parameters. On the 2 farms, seedbank size ranged from 300 to 345 kg/ha (in spring) and from 650 to 740 kg/ha (in summer).Trifolium glomeratum (cluster clover) and subterranean clover were the most wid espread species in both surveys. They were present at 35 and 30 of the 57 survey sites, respectively, and at both farms. Most of the subterr anean clover collected was cv. Geraldton (22 of 30 sites), the next mo st frequent cultivar was Dwalganup (6 sites). The currently recommende d cultivar, Dalkeith, was found at only 5 sites. Several other legumes including T. tomentosum (16 sites), T. suffocatum (8 sites), Medicago truncatula (7 sites), T. hirtum (4 sites), and M. minima (4 sites) we re common, while M. littoralis, M. polymorpha, T. dubium, T. cernuum, T. cherleri, and T. campestre were found at single sites. With few exc eptions, these are naturalised species and were characterised by flowe ring times about 20 days later than sown legume cultivars, and seed si zes less than or equal to 1 mg. The value of these widespread annual l egumes to agricultural productivity and sustainability needs to be qua ntified and their adaptation to wheatbelt farming systems assessed.