Periods of competition between weeds and soybean crop in Cerrado

This paper aimed to identify the periods of weed interference in soybean cultivar P98Y12, in the 2011/2012 harvest. The importance of this paper is that soybean is a commodity of great participation of the Brazilian economy and one of the largest contributions in the balance of exports. The assay was carried out in Oxisol, in the town of Campo Grande, Mato Grosso do Sul state, Brazil. The experimental design was a randomized block with four replications and fifteen treatments that consisted of periods of coexistence between the weeds and the crop. Additionally, phytosociological research was performed among the weeds. Under the conditions of the assay the Period Previous to Interference (PPI) was recorded at 7 DAE, the Critical Period for Prevention of Interference (CPPI) between 7 and 42 DAE and the Total Period for Prevention of Interference (TPPI) between emergence and 42 DAE. The infestation that occurred in the experimental area caused 90.42% of losses in grain yield. The weed species that showed the highest Importance Value Index (IV) were Digitaria horizontalis (94.2%) and Ipomoea grandifolia (71.9%).   Key words: Glycine max, grain production, productivity, weed competition, weed control, Brazil.