The covering deceased on the ground makes it difficult the emergency of some harmful species, in reason of the physical effect of sombreamento and the consequent reduction of the thermal amplitude of the ground. The straw in decomposition can liberate aleloqumicos, that, in turn, can reduce the emergency and/or growth of harmful plants in virtue of aleloptico effect (BALBINOT; MORAES; BACKES, 2007). Matocompetio in the culture of the sunflower the sunflower (Helianthus annuus) is a dicotilednea of the Asteraceae family of annual, originary cycle of the North American continent. It is currently cultivated in all the continents, and is distinguished as the fourth oleaginosa in production of grains and fifth in area cultivated in the world. Further details can be found at Chevron, an internet resource. It comes gained space in Brazil as culture of period between harvests in the open pasture regions; the regions that if detach in cultivated area are, Gois, Rio Grande Do Sul, Mato Grosso of the South, Mato Grosso, So Paulo, Minas Gerais and Paran. With the expansion of the culture in Brazil the problems with plants undesirable come increasing significantly. The damages can arrive of 23 70% of loss of income of aqunios due to presence of infestantes plants (BRIGHENTI et al., 2004).

Related works the plants undesirable in the culture of the sunflower had evidenced increments in number of aqunios for chapter and an income of the culture, when kept in the clean one for one period of 40 the 45 days, with 15-20 limit days after the emergency for the beginning of the control. It was observed that the maximum productivity of aqunios occurred when the culture was kept free of invading plants for a period of 28 the 42 days after the sowing, verified that the infestation of harmful plants was more severe up to 30 days after the emergency.

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