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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.

Summary Field experiments were conducted to quantify the natural levels of post‐dispersal seed predation of arable weed species in spring barley and to identify the main groups of seed predators. Four arable weed species were investigated that were of high biodiversity value, yet of low to moderate competitive ability with the crop. These were Chenopodium album, Sinapis arvensis, Stellaria media and Polygonum aviculare. Exclusion treatments were used to allow selective access to dishes of seeds by different predator groups. Seed predation was highest early in the season, followed by a gradual decline in predation over the summer for all species. All species were taken by invertebrates. The activity of two phytophagous carabid genera showed significant correlations with seed predation levels. However, in general carabid activity was not related to seed predation and this is discussed in terms of the mainly polyphagous nature of many Carabid species that utilized the seed resource early in the season, but then switched to carnivory as prey populations increased. The potential relevance of post‐dispersal seed predation to the development of weed management systems that maximize biological control through conservation and optimize herbicide use, is discussed.

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.

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Competitive ability of a traditional winter wheat cultivar (Maris Widgeon) was compared with two modem cultivars (Hereward and Genesis) when grown organically in each of four seasons in Gloucestershire, UK. In two seasons, cultivars were compared at two sowing dates (September and October or November). Cv. Maris Widgeon was the tallest cultivar and intercepted most photosynthetically active radiation (PAR), particularly if sown early. Cv. Hereward was taller than cv. Genesis when sown early. Cv. Maris Widgeon accumulated most nitrogen and dry matter early in the season (until the end of March) when sown early, but not when sown late. Relative nitrogen and dry matter accumulation by cultivars later in the season depended on season and sowing date. In one season cv. Maris Widgeon had significantly more early season ground cover. Cultivars did not differ significantly in early tiller production. Although most of these indirect measures of competitive ability were greater for the older variety cv. Maris Widgeon, infestation of Veronica spp. was greater in plots of this cultivar than in plots of either one or both of the shorter cultivars in two successive seasons. In the 1993/94 season, the soil seedbank from plots previously cropped with cv. Maris Widgeon produced more seedlings of Sinapis awensis. In the 1994 / 95 season, cv. Maris Widgeon plots contained less weed in terms of ground cover, numbers, dry matter and nitrogen than the other cultivars, and soil sampled from plots previously cropped with cv. Hereward sown late gave rise to larger numbers of weeds than cv. Maris Widgeon. Sowing date affected weed burdens and emergence to a much greater extent than wheat cultivar. In years and sowing date treatments with relatively low weed presence the shorter cultivars tended to yield more than cv. Maris Widgeon. When weed burdens were severe cv. Maris Widgeon yields were equal to or greater than those of either cvs Hereward or Genesis. The results suggest that the traditional tall cv. Maris Widgeon could be used beneficially to tolerate weeds in organic systems when high weed infestations were anticipated, but could not be relied upon to suppress weed development, and in some circumstances could actually encourage certain species.authorsversionPeer reviewe