Barbour et al. finally proposed the inverse of the variance as an indicator of precision; in this study, we considered a threshold level of “1” as the minimal precision accepted for a reliable sampling. Only dominance of rapeseed in Winter was considered not reliable , being considered with restrictions in the further analysis. The percentage of soil surface covered by straw resulting from the winter crops, as well as crop stubble dry mass, is shown in Figure 1. There was a direct relationship between the volume of dry mass produced by each crop/management and the respective the percentage of soil covered by this stubble On the other hand, there were only weak relationships both between the percentage of soil covered and the number of plants of weed species, and between soil covered and dry mass of weeds, which were not significant. The number of weed individuals and dry mass of weeds per area was lower in areas planted with any oilseed crop compared to fallow areas, for all seasons,cannabis grow tray except dry mass of weeds in crambe at pre-planting . In Winter , about 27 plants∙m−2 were observed as average of the winter crops while 108 plants∙m−2 were observed at the fallow area, immediately after oilseed crops harvest.
The same was observed for dry mass, where 30 g∙m−2 were observed in the oilseed crop areas against 83 g∙m−2 at the fallow area. This translated to about 25% and 36% of the infestation, respectively for number of weed individuals and dry mass accumulated, for areas planted with oilseed crops compared to areas under fallow . At the pre-planting of soybean , infestation was kept at low levels in areas previously planted with radish and rapeseed, but crambe straw failed to suppress weeds at the same level. While 12 and 21 plants∙m−2 were observed for areas previously planted with radish and rapeseed, respectively, at the area with crambe 32 plants∙m−2 were reported, against 71 plants∙m−2 at the fallow area. Dry mass presented similar behavior, except for rapeseed, where plants emerged but did not grow at the same rate of the ones observed at radish and crambe areas . All oilseed crops were able to inhibit weeds emergence and establishment during winter, when crops were present, but at the pre-planting crambe did not perform well. This may be due to two reasons: 1 both soil covered and crop dry mass accumulated at the post-harvest of oilseed crops were smaller for crambe compared to radish or rapeseed, in part because this crop has shorter cycle thus being harvested about 60 days in advance , vertical grow system and 2 the allelopathic effect of radish and rapeseed in suppressing weeds may be more pronounced than crambe. In fact, Uygur et al. reported that extracts of radish completely inhibited the germination of 4 crop plants and 11 weed species.
Although radish accumulate greater dry mass than crambe , its ability to suppress weeds was higher than what would be attributed to the differential dry mass accumulation, being attributed to the allelopathic effect of radish, as previously reported. At post-emergence of soybean , there was still effects of the winter management on the level of weeds infestation, where areas previously planted with oilseed crops were always less infested than the area under fallow, and similar among them in terms of number of weed individuals per area. Dry mass of weeds, however, differed not only from the fallow but also among oilseed crops . Weed dry mass was about 13, 37 and 46 g∙m−2 , respectively at areas previously planted with rapeseed, radish and crambe. Although weed dry mass accumulation was not pronounced at the fallow area by the time of evaluation-probably due to the coupled effect of soybean shading and glyphosate application, the high number of individuals at this treatment indicated that weeds were smaller than the observed at the areas under oilseed crops, but the latent infestation was high. The season-pooled analysis showed that radish and rapeseed performed better than crambe in inhibiting the occurrence of weed species, and winter fallow should not be adopted in areas where soybeans are to be cropped. Although infestation at the area with crambe was higher, this crop was still able to suppress in more than 50% the occurrence of weeds compared to fallow, which also resulted in smaller weed dry mass per area . When number of weeds and its respective dry mass are compared in Pre-Planting and PostEmergence of rapeseed, it is possible to infer that weeds were smaller compared to the ones present at the other areas, thus with delayed emergence. This delayed emergence of weeds in rapeseed is most probably a consequence of the time needed for seeds forced to a quiescent or dormant state to restart their metabolism and allow germination .