A new simple empirical model for early prediction of crop losses by weed competition was introduced. This mo~el relates yield loss to relative leaf area of the weeds shortly after crop emergence using the relative damage coefficient q as the single model parameter. The model is derived from the hyperbolic yield density relationship and therefore accounts for the effects of weed density. It is shown that the model also accounts for the effect of different relative times of weed emergence. A strong advantage of the approach is that it can be used when weeds emerge in separate flushes.The regression model described experimental data on sugar-beet -lambsquarters (Beta vulgaris L. -Chenopodium album L.) and maize-barnyard grass (Zea mays L. Echinochloa crus-galli L.) competition precisely. The model describes a single relationship between crop yield loss and relative leaf area of the weeds over a wide range of weed densities and relative times of weed emergence. Possibilities for scientific and practical application of the model are discussed.

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A simple model is introduced to estimate the degree of intra-specific and inter-specific competition and niche differentiation from final biomass data of a set of populations varying in spp. composition and total density. Competition effects can be estimated using addition series. The model is illustrated with results from mixed cropping of maize and groundnut. (Abstract retrieved from CAB Abstracts by CABI’s permission)

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Breeding strategies for drought tolerance in potato were evaluated by means of a crop growth model, in which seasonal courses of crop dry matter accumulation and soil moisture availability were simulated in dependence of plant characteristics and weather and soil data.Several plant characteristics substantially influenced the simulated instantaneous water consumption of the genotype. However, effects of genotypic differences on final tuber yield were much smaller because of the close relationship between transpiration and growth. Hence, a lower water consumption not only saved water for later use, but was also at the expense of the actual growth rate. Selection for low-transpiration types, at unchanged water use efficiency, would result in lower yields under optimum conditions. Short periods of drought, in general, reduced tuber yield of late genotypes less than that of early genotypes. Late genotypes had a surplus of leaf area for full light interception giving a lower impact of leaf area reduction. Late drought affected early genotypes less because of escape.The simulation results emphasized the complexity of selection for drought tolerance caused by the many plant processes involved, the contrast between instantaneous and cumulative reactions and the strong genotype x environment interaction for drought tolerance.