Responses of faba bean ( <i>Vicia faba</i> L.) to sowing rate in south-western Australia II Canopy development, radiation absorption and dry matter partitioning

Sowing rate influences plant density, canopy development, radiation absorption, dry matter production and its partitioning, and seed yield. The canopy development, radiation interception, and dry matter partitioning of faba bean (cv. Fiord) were examined using 6 sowing rate treatments from 70 to 270 kg/ha in field experiments conducted over 3 years at Northam as part of a larger investigation of sowing rate responses in faba bean in south-western Australia. High sowing rates resulted in significantly earlier canopy closure, larger green area indexes, more radiation absorption, more dry matter accumulation particularly during the early vegetative stages, and greater seed yield than treatments where a low plant density was established. The results suggest that further increases in canopy development, radiation absorption, dry matter accumulation, and seed yield are possible by using sowing rates in excess of 270 kg/ha. The rate of node appearance was relatively constant within and across seasons (1 every 65·9 degree-days), whereas the number of branches per plant declined with increasing plant density, and less branches survived through to maturity at high density. The peak photosynthetically active radiation absorption (75-85%) measured at green area index of 2·9-3·8 in the highest sowing rate treatment in this study is similar to previous reports for other crops. The estimated radiation use efflciency (1·30 g/MJ) was constant across sowing rate treatments and seasons. High sowing rates produced tall crops with the lowest pods further from the soil surface than those at low plant density, and hence, mechanical harvesting was easier. The growth of individual plants may have been limited by the low growing season rainfall (266-441 mm) and/or low soil pH (5·0 in CaCl2) at the site, and competition between plants for radiation was probably small even at the highest sowing rate. Early canopy closure and greater dry matter production with high sowing rates may also cause greater suppression of weeds and aphids.