Interactions between wheat (Triticum aestivum L.) cultivar, row spacing and density and the effect on weed suppression and crop yield

In 1995/1996 weed suppression ability of two wheat cultivars (Spark and Ton ic) was investigated in conjunction with two row spacings (15 cm and 9 cm) and three sowing densities (125, 250 and 350 plants m(-2)). Weed biomass wa s influenced initially by crop density, and following stem extension by bot h cultivar and density, although these factors did not interact. Tillering capacity was a poor predictor of weed suppression ability and Spark, the mo re profusely tillering cultivar, contained greater weed biomass than Tonic. The earlier stem extension of Tonic increased shading at 0-20 cm above gro und level compared to Spark and this was believed to be the reason for redu ced weed biomass. Row spacing did not influence weed suppression, but grain yield was reduced in 15 cm rows at a density of 350 plants m(-2) compared to 250 plants m(-2). This may be of importance to growers contemplating the use of wider row spacings for mechanical weeding.