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Summary: The effects of reduced‐cultivation Systems on weeds in cereals are reviewed. Increasing dependence of these Systems upon Chemical weed control together with changes in the soil physical environment are expected to modify existing weed floras. Annual‐grass weeds are likely to remain a problem with the use of minimal cultivations, particularly when early drilling is practised, while hitherto unimportant species may become more prevalent, e.g. Bromus spp. Furthermore, reduced cultivations may encourage the establishment of wind‐disseminated species. However, annual dicotyledonous species characteristic of arable land are expected to continue to decline.

1 SummaryIn 19954996 weed suppression ability of two wheat cultivars (Spark and Tonic) was investigated in conjunction with two row spacings (15 cm and 9 cm) and three sowing densities (125, 250 and 350 plants mP2). Weed biomass was influenced initially by crop density, and following stem extension by both cultivar and density, although these factors did not interact. Tillering capacity was a poor predictor of weed suppression ability and Spark, the more profusely tillering cultivar, contained greater weed biomass than Tonic. The earlier stem extension of Tonic increased shading at 0-20 cm above ground level compared to Spark and this was believed to be the reason for reduced weed biomass. Row spacing did not influence weed suppression, but grain yield was reduced in 15 cm rows at a density of 350 plants mP2 compared to 250 plants m-2. This may be of importance to growers contemplating the use of wider row spacings for mechanical weeding.

Differential competitive ability of six winter wheat cultivars and traits that confer such attributes were investigated for a range of seed rates in the presence or absence of weeds for a naturally occurring weed flora in two successive years in split-plot field experiments. Crop height and tillering capacity were considered suitable attributes for weed suppression, although competitiveness is a relative rather than an absolute characteristic. Maris Huntsman and Maris Widgeon were the most competitive cultivars whereas Fresco was the least competitive. Manipulation of seed rate was a more reliable factor than cultivar selection for enhancement of weed suppression, although competitiveness of cultivars Buster, Riband and Maris Widgeon was not enhanced by increased seed rate. Crop densities ranging between 125 and 270 plants m )2 were found to offer adequate weed suppression. Linear relationships were observed between individual and total weed species dry weight and reproductive structures per unit area.