An experiment was conducted with three spring barley (Hordeum vulgare ssp. vulgare L.) cultivars of contrasting competitiveness to identify variations in their early competitive response to weed competition and to study canopy architecture and growth development at the early stages of cultivar growth. The aim was also to investigate whether differences in shoot morphology, growth development and competitive response could explain the differences in competitive ability against weeds. The barley cultivars were grown outdoors in boxes either in monoculture or in a mixture with white mustard (Sinapis alba L.) as a model weed. The experiment was run until the barley cultivars were in the stem elongation stage. The varieties with strong to medium competitive ability against weeds shortened the time of emergence in the presence of S. alba in contrast to the least competitive cultivar. The results also indicated that spring barley cultivars with strong competitive ability against weeds have an early stem extension as a response to weed competition and a low competitive response. Morphological traits, namely large length of the two first internodes, long main shoot in the tillering stage and a small leaf angle, may be important traits in competition for light.

The aim was to study the growth and development of six spring barley (Hordeum vulgare ssp. vulgare L.) cultivars as a response to a model weed population of Sinapis alba L. The development of light interception profiles over time was characterized for each cultivar in weed-free stands. The cultivars were chosen such that they represent a range of weed-suppressive abilities based on previously performed trials. One field experiment each was conducted in 1996 and 1997 at a site SE of Uppsala, Sweden. The two cultivars with low competitive ability against weeds, Etna and Blenheim, allowed the highest weed biomass and the lowest crop biomass in competition with the weeds. These two cultivars transmitted more photosynthetic active radiation through the canopy down to 20 and 40 cm height than did the other cultivars. Although the biomass of cv. Etna was low, the grain yield was higher than that of the other cultivars when grown in competition with weeds. In 1997, cv. Svani with good competitive ability against weeds transmitted least light and had greater grain yield than most other cultivars. The absence of a relationship between high grain yield and low weed suppressive ability in the present study indicates that it should be possible for plant breeders to combine high grain yielding capacity with approved weed-competitive ability.

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There is an increasing interest in the use of cover crops in agriculture, in Sweden mainly for the use as catch crops to reduce nitrogen leakage. Some of these crops are known for their allelopathic abilities, which may play a role in the control of weeds and contribute to reduced herbicide use. This study aimed to explore the possible suppressive effect of the cover crop species white mustard, fodder radish, rye, and annual ryegrass on the early growth of the weed species silky windgrass, shepherd's-purse, and scentless false mayweed. In a greenhouse experiment using fresh cover crop residues, white mustard was the only crop that showed an effect. It reduced both seedling establishment, by 51 to 73%, and biomass, by 59 to 86%, of shepherd's-purse and scentless false mayweed. In contrast, in a growth chamber experiment using frozen material, mean germination time of silky windgrass was extended by 20 to 66% by all cover crops. Also, three out of four cover crops reduced root growth in scentless false mayweed by 40 to 46%, and two out of four cover crops reduced root growth in shepherd's-purse by 13 to 61%. However, considering seedling survival, white mustard was the most prominent cover crop, reducing survival by 21 to 57% in shepherd's-purse and scentless false mayweed. In this paper we provide evidence that different weed species show different response to different cover crops under climatic conditions prevailing in Scandinavia. Such results emphasize the importance of understanding weed–cover crop interactions as necessary for developing cropping systems that can utilize cover crops to suppress local weed flora.