**3. Critical period of weed interference**

The critical period of weed control (CPWC) has been defined by Silva et al. [51] as a window in the crop growth cycle during which weeds must be controlled to prevent quantitative and qualitative yield losses. In essence, the CPWC represents the time interval between two separately measured crop-weed competition components: (1) the critical timing of weed removal (CTWR) or the maximum amount of time early-season weed competition can be tolerated by the crop before it suffers irrevocable yield reduction, and (2) the critical weed-free period (CWFP) or the minimum weed-free period required from the moment of planting, to prevent unacceptable yield reductions [52]. The former component is estimated to determine the beginning of the CPWC, whereas the latter determines its end. Results from both compo‐ nents are combined to determine the CPWC. Theoretically, weed control before and after the CPWC may not contribute to the conservation of the crop yield potential.

The beginning and end of the CPWC determined using the functional approach will depend on the level of acceptable yield loss (AYL) used to predict its beginning and end. Many studies report 5% as the maximum AYL. But it can be adjusted depending on the cost of weed control and the anticipated financial gain [52].

Silva et al. [53], evaluated the CTWR in soybean, cv. BRS-244 RR in low, medium and high weed density and observed that the CTWR was 17 days after emergence (DAE) in low infestation area and 11 DAE in medium and high infestation area, considering 5% of tolerance of crop yield decrease. According to the authors, weed interference during the full crop cycle reduced soybean grain yield in 73%, 82% and 92%, for low, medium and high weed density, respectively. Meschede et al. [54], evaluated the CPWC of *Euphorbia heterophylla* in soybean crop, cv BRS 133, under low seeding rate, and observed that the presence of weeds caused daily yield loss of 5.15 kg ha-1, whereas their absence provided a daily yield gain of 7.27 kg ha-1. According to the authors, weed-crop coexistence for up to 17 DAE did not cause any negative effect on crop yield, and the maximum length of time in which weeds had to be controlled to prevent crop yield losses was 44 DAE; the CPWC was, therefore, from 17 to 44 DAE. Carvalho and Velini [55], observed that weeds germinated 20 days after the emergence of soybean, cv. IAC-11, did not affect crop yield.

Different results of CPWC showed that the degree of weed interference on crops depends on the infesting plant community (species, density and population), on the crop (cultivar, spacing and density) and environment (soil, climate and management). Thus, it is necessary a greater number of studies to create a data base and in the future create models to predict the adequate moment of weed control for each situation.
