Owing to an increasing demand for food, a constant agricultural production flow must be maintained. Further, for doing so, the use of pesticides is necessary. An alternative that results in less damage to the ecosystem and people themselves may be identified by studies on the allelopathic effect of weeds. Therefore, this work aimed to evaluate the allelopathic action of hexane, dichloromethane, butanol, and ethyl acetate fractions of sorghum (Sorghum bicolor [L.] Moench) on the initial growth of morning glory (Ipomoea grandifolia [Dammer] O’Donell) and slim amaranth (Amaranthus hybridus L.), which was verified by the percentage of germination, speed of germination, seedling length, and fresh and dry biomass weight. The experiments were conducted in an incubation chamber at 25 °C for 7 and 14 days for morning glory and slim amaranth, respectively. The experimental design was completely randomized, with five replicates in Petri dishes. The data were evaluated by analysis of variance, and the averages between each treatment were compared using the Scott Knott test at a 5% significance level. The results indicated that the dichloromethane and ethyl acetate fractions decreased the initial growth of morning glory and slim amaranth more, when compared with the effects of hexane and butanol.

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Soybean (Glycine max (L.) Merrill) is an agricultural crop with a large increase in production in the last three decades and is an essential component in the manufacture of animal and human food. Understanding the interactions between crop and other plant species used as green manure that can improve yield and so reduce environmental damage. Forages can release secondary metabolites in the environment that influence in a beneficial or harmful way to other plants, characterizing the allelopathy process. The Níger (Guizotia abyssinica Cass.) is used in agricultural systems because it releases allelochemicals, especially flavonoids. Therefore, this study is aimed to evaluate the allelopathic effects of the Niger straw on the germination and initial growth of soybean seedlings. The treatments consisted of aqueous extracts of niger stems, leaves and roots in different concentrations (0; 25; 50; 75 and 100%). For each treatment, five replications were performed, each composed by a 25 seeds distributed in gearbox kept in incubator chamber for 7 days at 25ºC and 12 hours photo period. The experimental design was completely randomized and the data were evaluated by analysis of variance and the means of treatments compared by Tukey’s test at 5% significance. The results show a reduction in germination and initial growth of soybean seedlings submitted to aqueous extract of niger stem and roots. In contrast, the aqueous extract of the Niger leaves increased the length of soybean seedlings. The allelopathic effects of the Niger probably occur due to the presence of flavonoids in the tissues of this plant species.