The germination of a population of seeds was modelled using the concept of hydrotime or hydrothermal time. Typically, a Normal distribution for base water potential (Ψ b(g) ) was used within these models to relate variation in Ψ b(g) to the variation in time to germination of a given fraction of seeds. We sought to examine empirically the validity of this assumption, to compare the fit of alternative distributions and make recommendations for improved germination modelling procedures. Eight statistical distributions (Gumbel, Weibull, Normal, Log-Normal, Logistic, Loglogistic, Inverse Normal and Gamma) were fitted to data for four weed species Hordeum spontaneum, Phalaris minor, Heliotropium europaeum and Raphanus raphanistrum. Methods for incorporating each of these distributions into hydrotime are presented. For three species (H. spontaneum, P. minor and H. europaeum), the Normal distribution gave the worst fit (with AIC values: À124.2, À296.9 and À264.5, respectively) and would lead to biased predictions, whereas the Loglogistic distribution consistently provided the best explanation of Ψ b(g) variation in these species (with AIC values: À188.6, À326.2 and À272.1 respectively). All distributions failed to provide an unbiased description of the observed distribution of Ψ b(g) in R. raphanistrum. The Normal distribution is not necessarily the best function for base water potential in hydrothermal models and, indeed, may give much more biased predictions than alternative functions. The 'best' distribution may vary with species. The distribution of Ψ b(g) within a seed sample should therefore be examined and an appropriate equation selected, before using a model to make predictions.

Summary Determination of weed seeds in the soil is tedious and time consuming. To evaluate the different seed extraction methods to improve seedbank estimations, three extraction methods (sieving, cloth bag and flotation) were compared, based on their accuracy and time needed for separation processes and enumeration. Seeds of Datura stramonium,Amaranthus retroflexus,Portulaca oleracea and Plantago major were used to artificially infest soil samples of four textures, namely clay, clay loam, loam and sandy loam containing 19%, 27%, 38% and 65.5% sand respectively. Soil textures had a significant effect on counting time in all extraction methods. In flotation, four submethods involving different solutions and centrifuge rotation speed were examined, but showed no differences, so data were pooled. Counting time in flotation, cloth bag and sieving methods was 9, 16 and 30 min respectively. However, when the time needed for other processes was taken into consideration, flotation and bag methods did not differ significantly. Species‐wise seed recovery was not affected by soil texture in the bag method, suggesting an advantage for this approach as its accuracy is not soil texture‐dependent. Total seed recovery for flotation, sieving and bag methods was 61%, 67% and 75%, respectively, and was not significantly different. Considering the efficiency of methods, the cloth bag technique could be recommended, because it was as time consuming as the flotation method, but required the same minimum equipments and costs as the sieving method.

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Xanthium strumarium and Datura stramonium are two of the most important weeds of maize in Iran. Their growth was compared in a field study conducted over 2 years. Relationships were derived between weed density and crop yield, yield components and various growth parameters. Yield, grain number ear )1 and grain weight were affected more by X. strumarium than by D. stramonium. Xanthium strumarium followed a pattern of growth resembling that of the maize, whereas D. stramonium continued growing for a longer period, mainly affecting maize during the grain-filling period. A large portion of the loss in maize yield caused by X. strumarium could be explained by a reduction in grain number ear )1 , whereas a greater portion of the maize yield loss from D. stramonium was explained by a reduction in grain weight. Weed biomass was greater for X. strumarium than for D. stramonium, whereas seed production was higher for D. stramonium. It is concluded that although X. strumarium should be given the highest priority for control in maize on the basis of its superior competitive ability, control of D. stramonium is also essential because of both its considerable competitiveness in the current crop and its long-term effects resulting from its greater seed production.Comparisons between years followed by ** indicate differences (P < 0.01). NA indicates that no comparison was made because of using different models. For abbreviations see Fig. 1.Weed species competition with maize 257

Yousefi AR, Gonzalez‐Andujar JL, Alizadeh H, Baghestani MA, Rahimian H & Karimmojeni H (2012). Interactions between reduced rate of imazethapyr and multiple weed species–soyabean interference in a semi‐arid environment. Weed Research.52, 242–251. Summary Field experiments were conducted to evaluate Xanthium strumarium and Amaranthus retroflexus competitive ability with soyabean under unsprayed and sprayed (half the recommended rate of imazethapyr) conditions. Weed density combinations of 0, 4, 8 or 12 plants m−1 of row for A. retroflexus and 0, 2, 4 or 8 plants m−1 of row for X. strumarium were established alone, and in all possible combinations. When X. strumarium and A. retroflexus were grown at the same density, X. strumarium contributed 60–69% of the total weed biomass depending on weed density. The half rate of imazethapyr reduced the biomass of both weed species by 90% as compared with untreated plots. Untreated A. retroflexus and X. strumarium reduced soyabean yields to 1911 kg and 1656 kg ha−1, respectively, while with the reduced rate of herbicide, the yield only decreased to 3612 kg and 3453 kg ha−1 respectively. The total weed density required to cause a 5% soyabean yield loss was 0.16 and 0.32 plants m−1 of row with no‐herbicide treatment, while with the half rate of imazethapyr, it increased up to 2.78 and 4.32 plants m−1 in 2007 and 2008 respectively. Owing to the higher competitive ability of X. strumarium, it should be given a greater priority for control than A. retroflexus in soyabean grown in a semi‐arid environment. The half rate of herbicide significantly decreased the competitive ability of X. strumarium and A. retroflexus indicating that, in soyabean, application of this rate might be more cost‐effective and more environment‐friendly than the recommended ones.