RESUMO -O levantamento fitossociológico da comunidade de plantas daninhas na cultura do girassol foi realizado em duas épocas distintas: no desenvolvimento inicial da cultura (entre 20 e 40 dias após a semeadura) e na pré-colheita do girassol. As espécies de plantas daninhas foram identificadas e quantificadas pelo método do quadrado inventário (1,0 x 1,0 m), com amostragem de 12 m 2 por área. Os levantamentos foram realizados em 54 propriedades de seis municípios da região do cerrado e em 38 propriedades de oito municípios da região dos pampas, que são as duas principais regiões produtoras brasileiras. , para o Rio Grande do Sul. O índice de similaridade dentro das regiões foi de 0,91 para os levantamentos do cerrado e de 0,79 para os do Rio Grande do Sul. Entretanto, entre as regiões, os índices ficaram abaixo de 0,5, mostrando similaridade mediana entre a flora daninha do cerrado e a do Rio Grande do Sul, na cultura do girassol, nas duas épocas estudadas.Palavras-chave: competição, fitossociologia, Helianthus annuus, infestação.ABSTRACT -A phytosociological survey of the weed communities present in sunflower crop was carried out during two distinct crop stages: early development and pre-harvest. Weed species were identified and quantified according to the inventory square method (1.0 x 1.0 m), using 12 m 2 of sampling per area surveyed. At each stage, the survey was carried out on 54 farms of six counties of the "cerrado" (savanna region, Central Brazil) and 38 farms of eight counties of the "Pampas" (region in the State of Rio Grande do Sul, the Southernmost state in Brazil), which are the two main Brazilian sunflower production regions. Frequency, relative frequency, density, relative density, abundance, relative abundance, relative importance index, and similarity index were computed. A total of 60 weed species were identified, 17 of which were present in both regions. Asteraceae and Poaceae were the two main families among the 16 found. The main species present in the "cerrado" were Euphorbia heterophylla, Chamaesyce hirta, Ageratum conyzoides, Commelina benghalensis, Zea mays and Bidens sp. The most abundant species present in the State of Rio Grande do Sul were Bidens sp., Raphanus raphanistrum, Lolium multiflorum, Gnaphalium spicatum, Sonchus oleraceus, Euphorbia heterophylla, Sida rhombifolia, Digitaria sp. and Ipomea sp. Weed density was higher at pre-harvest than at the early stage of the crop, in both

This study was carried out to categorize coefficients of variation (CV's) associated with important traits of routine use in the sunflower breeding programs. Separate categories were obtained for the August/September and February/March sowings. Data were used from the intermediate and final sunflower experiments carried out by Embrapa Soybean and other collaborator institutions of the Network of Official Sunflower Trials. Taking into account the mean and the standard deviation of the CV's obtained from the analyses of variance of the experiments, they were fitted into the low, medium, high and very high categories. An additional classification was made using the median and pseudo-sigma, in place of the mean and standard deviation, respectively. In this study it was ascertained that the CV's categorization depended on the trait studied. Sowing date effects were more pronounced for seed and oil yield and plant height. For all assessed traits in February/March, the methodologies adopted were similar (regardless of the CV's distribution) and satisfactory to assess the experimental accuracy of the trials. In the August/September trials there was good agreement between the results of the methodologies adopted and that of Gomes (1985) for seed and oil yield. For the other traits, the former methodologies were shown to be more adequate. The proposed CV's categorizations for the sunflower traits use as maximum acceptable limits the values of 23.5% (August/September sowing) and 31.5% (February/ March sowing) for seed and oil yield, 6.0% for oil content (August/September and February/March) and plant height (August/September), 9.5% for plant height (February/March) and 4.5% for flowering and physiological maturity (August/September and February/March).

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