RESPONSES OF FABA BEAN (VICIA-FABA L.) TO SOWING RATE IN SOUTH-WESTERN AUSTRALIA I - SEED YIELD AND ECONOMIC OPTIMUM PLANT-DENSITY

Sowing rate influences plant establishment, growth, seed yield, and th e profitability of a crop. However, there is limited published informa tion on the optimum sowing rate and plant density for faba bean in Aus tralia. The response of the growth and seed yield of faba bean (cv. Fi ord) to sowing rate (70-270 kg/ha) was examined in 19 field experiment s conducted over 3 years in south-western Australia. The economic opti mum plant density was estimated at each site by fitting an asymptotic model to the data and calculating the point where the cost of extra se ed equalled the return from additional seed yield, allowing a 10% oppo rtunity cost for the extra investment. On average across all sites and seasons, only 71% of the seeds sown emerged. Increasing sowing rate r esulted in more dry matter production at first flower and at maturity, and at about half of the sites there was a small trend of reduced har vest index. In general, the mean number of seeds per pod (1.8-2.6) and mean seed weight (32-45 g/100 seeds) were unaffected by sowing rate. As sowing rate increased, the number of pods per plant (5-35) generall y decreased, but this was compensated by the large plant population an d more pods per unit area. The asymptotic models fitted to the seed yi eld data accounted for 15-81% of the variance. In 8 experiments, the m odels indicated that yield was continuing to increase substantially as sowing rate increased at the largest sowing rate treatment. The estim ated optimum plant densities in these experiments were beyond the rang e of the data or had large standard errors and, hence, were excluded f rom any further consideration. Among the remaining 11 experiments, the estimated optimum plant densities varied from 31 to 63 plants/m(2) wi th a mean of 45 plants/m(2). This study demonstrates that targeting so wing rates greater than the current commercial practice for faba bean in southern Australia of 15-30 plants/m(2) results in more yield and p rofit. Additional experiments are required with sowing rates in excess of 270 kg/ha to estimate accurately the optimum plant density for fab a bean. Fungal diseases were either absent or controlled with fungicid es in these experiments but the interactions between disease, time of sowing, and sowing rates also deserve further attention.