seed production and, hence, the seed pool and future weed population, suggesting that fertilize r usage can be exploited in an integrated programme of ciopiwecd management. A trend towards lower N fertilizer application owing lo concerns about the environment willlavour most of the weed spedes investigated in these experiments and change the composition of weed populations.The effects of nitrogen fertilizer on the growth and density of natural weed populations io spring barley (Hordeum vuigare L.) and winter wheat (Trlticum aestivum L.) were investigated in the absence of herbicide. An increased level of applied nitrogen did not enhance: weed germination, tended to decrease the total weed biomass and had a differential effect upon the biomass of individual weed spedes ia both wheat and barky. In competition with barley, Chenopodium album L. and Lamlum spp. had lower nitrogen optima ihan the crop, while Urtica wens L. had a higher nitrogen optimum. In competition with wheat, Stellaria media (L.) Vill., Lamium spp. and Veronica spp. had lower nitrogen optima than the crop. The systematic changes in nitrogen effect with time were analysed by fitting orthogonal polynomials to the growth and density curves. The methodology could be recommended for other studies in which time or other systematic factors are included, as it supplies information which a traditional analysis of variance cannot provide. Since seed production is positively correlated with biomass, so nitrogen level affects

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SU MMARYMixed intercropping of spring barley (Hordeum vulgare L.) with field pea (Pisum sativum L.), faba bean (Vicia faba var. minor L.) or narrow-leafed lupin (Lupinus angustifolius L.) was compared with sole cropping in two field experiments at different locations, on a sandy loam soil and a sandy soil, in Denmark in 2001.Grain legumes were dominant in intercrops on the sandy loam soil, except for lupin, whereas barley was dominant in intercrops on the sandy soil site. Combined intercrop grain yields were comparable to grain yields of the respective sole cropped grain legume or sole cropped, fertilized barley on each soil site. On the sandy loam soil, pea-barley and faba bean-barley intercrops increased the proportion of plant N derived from N 2 fixation in grain legumes and increased the barley grain N concentration (from 1 . 7 to 2 . 2 mg/g) compared with sole cropping. However, the later maturity of faba bean compared with barley caused problems at harvest. The grain N concentration of intercropped barley was increased where grain legumes were the dominant intercrops and not on the sandy soil site. Lupinbarley intercrops did not show intercropping advantages to the same degree as faba bean and pea, but lupin constituted a more stable yield proportion of the combined intercrop yield over locations.Furthermore, the study indicated that the natural 15 N abundance at certain locations might not always be sufficient to ensure a reliable estimate of N 2 fixation using the 15 N natural abundance method.

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