SummaryDespite increased concerns regarding the heavy reliance of many cropping systems on chemical weed control, adoption of ecological weed management practices is only steadily progressing. For this reason, this paper reflects on both the possibilities and limitations of cultural weed control practices. Cultural weed control utilises a number of principles, predominantly: (i) a reduced recruitment of weed seedlings from the soil seedbank, (ii) an alteration of crop-weed competitive relations to the benefit of the crop and (iii) a gradual reduction of the size of the weed seedbank. Compared with chemical control, the general applicability, reliability and efficacy of most measures is only moderate, and consequently, cultural control strategies need to consist of a combination of measures, resulting in increased systems complexity. Combined with the trade-offs connected to some of the measures, this hampers large-scale implementation. It is argued that tailoring cultural weed management strategies to the needs and skills of individual farmers would be an important step forward. Research can aid in improving the utilisation of cultural weed control strategies by focussing on a broadening of the range of available measures and by providing clear quantitative insight in efficacy, variability in outcome and trade-offs of these measures.

S U M M A R YWheat, barley and two morphologically contrasting cultivars of peas (leafy and semi-leafless) were grown in pure stands, at standard agricultural densities, and in additive mixtures of cereals with peas. The stands were grown in boxes in the field, and partitions were used to separate the effects of root and shoot interactions. The cereals and peas were either planted at the same time, or one species was planted 10 days before the other. The origin of the N present in each species was determined by applying N fertilizer labelled with 15 N.Both cultivars of peas had greater shoot and root competitive abilities than wheat or barley, probably because of their larger seed size; leafy peas had greater shoot and root competitive abilities than semi-leafless peas. Sowing peas after cereals reduced their competitive ability.The relative yield total (RYT) of cereal-pea mixtures, based on total biomass, averaged 1 -6 when only the root systems interacted, and 1-4 when only the shoot systems interacted, but did not differ significantly from 10 when both root and shoot systems interacted. RYT values were greater when peas were grown with wheat, rather than with barley, and when peas were sown at the same time as the cereals.Shoot competition from peas increased the N % of cereals, but substantially reduced their total N content, because biomass yield was reduced. Shoot competition from cereals had no effect on the N % of peas, and only slightly reduced their total N content. Shoot competition between cereals and peas had no significant effect upon the proportion of N derived from various sources by either cereals or peas.Root competition from peas significantly reduced both the N % and total N content of cereals. Root competition from cereals had little effect on the N % of peas, but significantly reduced their total N content and increased the proportion of N derived from rhizobial fixation from 76 to 94%. Since cereals and peas largely used different sources of N, resource complementarity for N was probably an important component of intercropping advantage, when the roots of cereals and peas shared soil resources.

Growing chick-pea in sustainable systems requires the use and development of more competitive genotypes which can complement the effects of reduced input weed control. A 2-year study assessed the competitive ability of 13 genotypes grown in either the presence or absence of weeds, in a split-plot design including the weeds in pure stands. Crop and weed density, phenology, relative biomass of crop (RB c ) and weeds (RB w ), crop yield characters, crop biometric traits in the absence of weeds, relative biomass total of mixtures (RBT) and crop competitive ability (C b ¼ ln RB c /RB w ) were recorded. Lines C136, C120, C101 and C106, and cultivars Pascia`, Visir and Sultano gave the best seed yield in the absence of weeds (1.8-2.0 t ha )1 DM). Weeds reduced yield by 75% and 83% in C136 and C133 and by 87-97% in the other genotypes. Weed biomass in mixture (mainly Chenopodium album) averaged 4.42 t ha )1 DM. Chickpea genotypes C136 and C133 were the most competitive, but weeds were more competitive than any of the chick-peas. C b was correlated directly to the height of first fertile pod (r 2 ¼ 0.84) and inversely to the insertion angle of primary branches to the vertical (r 2 ¼ 0.77). Intergenotypic variation for competitive ability could be exploited in integrated weed control using more competitive genotypes, or used in breeding programmes aimed to develop highly competitive cultivars on the basis of easily screenable characters.

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