Summary The objectives of this study were to identify traits in spring barley (Hordeum vulgare) and spring wheat (Triticum aestivum) related to the competitive ability of the crop and to determine their importance. Weed biomass 1–2 weeks prior to ear emergence was used as a measure of genotypic differences in competitiveness against weeds. Crop trait measurements comprised early crop biomass, early shoot height, straw length, heading and maturity. Early crop biomass 1–2 weeks prior to ear emergence was used as an overall trait for the various characters related to early vigour. In addition to these morphological traits, the potential allelopathic activity of the tested cultivars was determined using an agar‐based bioassay with Lolium perenne (ryegrass). In both barley and wheat, multiple regression analysis revealed that early crop biomass and potential allelopathic activity were the only parameters that significantly contributed to competitiveness. In barley, early crop biomass explained 24–57% of the observed genotypic variance across 4 years, allelopathic activity explained 7–58% and combined they explained 44–69% of the observed genotypic variance. In wheat, the corresponding figures were lower: 14–21% for early biomass, 0–21% for allelopathic activity and 27–37% when combined. Model predictions suggested that new cultivars with increased early vigour and allelopathic activity offer a potential to further reduce weed interference.

It is becoming apparent that allelopathy plays a significant role in the competitive ability of cereals against weeds; barley, wheat and rice cultivars with high allelopathic activity have been identified. However, direct breeding efforts to improve allelopathy have to date only been reported in rice. In this study, a breeding programme in spring wheat (Triticum aestivum) was evaluated to determine the efficiency of selection of allelopathy and the ability of breeding lines to suppress weeds in the field. The material used originated from a cross between a Swedish cultivar with low allelopathic activity and a Tunisian cultivar with high allelopathic activity. Allelopathic activity was measured as growth inhibition of perennial ryegrass (Lolium perenne) roots when grown together with the wheat cultivars on agar. For screening of F 2 populations, a single plant bioassay was used for the first time. In the F 6 and F 7 generations, three breeding lines with an average improved allelop-athy of 20% and one line with an unimproved allelopathy activity, but with the same phenotype as the high allelopathic lines, were tested together with the low allelopathic Swedish parent. The main result from the field study was a 19% average reduction of weed biomass for the high allelopathic lines, but no significant reduction of the low allelopathic breeding line. Early shoot length and early crop biomass (stages 37-39, Zadoks scale) and straw length of the high allelopathic lines were not significantly different from the Swedish parent. A negative effect was that grain yield was reduced by 9% in the high allelopathic lines. It is suggested that the reduced biomass of weeds in plots planted with the highly allelopathic wheat lines is related to differences in allelopathic activity and not differences in plant growth.

There is an increasing interest in an integrated, less herbicide-dependent approach to weed management, based on combinations of cultivation practices and competitive crops. More competitive cultivars are essential to this approach and breeders need to know how to breed such. It is known that early vigour and extensive early season vegetative growth are important traits. The questions are, which is most efficient to use in breeding and if allelopathy should be bred for? Weed suppressive ability (WSA) of 12 winter wheat, two rye and two triticale cultivars was studied in a 2-year field trial with natural weeds and under-sown Apera spica-venti and rapeseed (Brassica napus). Crop biomass, shoot length, straw length and plantlet root length and root weight were analysed. Biomass growth in autumn and spring was studied in weed-free plots using multispectral analysis. Grain yield, winter hardiness and maturity data were collected from official variety tests in Sweden. Allelopathic activity was assessed with use of a bioassay. Factor analysis showed that early season crop biomass and allelopathy were two traits explaining most of the variance in weed biomass. It was predicted by partial least square regression models that weed biomass could be reduced by 60%, if both crop biomass and allelopathy are improved to the levels of triticale. An improvement of allelopathy alone should reduce weed biomass by 18-28%. The factor analysis and multivariate models showed that WSA could be improved without a negative trade-off in grain yield or winter hardiness.

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