From late August to late September of 2012 and from early September to late September of 2013, the residual weeds survey was conducted to establish a benchmark of the residual weed community in soybean fields in Yamaguchi Prefecture. The surveyed fields were selected from eight regions and included 320 fields in 2012 and 236 fields in 2013. The percentage of residual weeds in a field, calculated by dividing the number of residual weeds in a field by the total field count in the entire prefecture, was the highest for Echinochloa crus-galli (L.) Beauv. var. crus-galli, Persicaria longiseta (De Bruyn) Kitag., Eclipta spp., Digitaria ciliaris (Retz.) Koeler, and Bidens frondosa L., with a percentage of > 30% for 2 years. Amaranthus spp. was observed in the fields at 13–26%, occurred observed in without prefecture throughout bias in the region. In addition, significant weed loss in the fields was also confirmed. On the other hand, Physalis spp. and Solanum nigrum complex were found in less than 5% of the fields, and there was a bias in the affected regions. Ipomoea spp. was observed in 15% of the fields, showing that its distribution has almost expanded within Yamaguchi Prefecture, and some fields had their entire surface covered with this plant. Based on this fact, we have assumed that the intrusion of Ipomoea spp. is progressing in Yamaguchi Prefecture, particularly I. coccinea L. and I. lacunosa L. Even in fields without Ipomoea spp., since in the levee or path between fields it observed, we fear that it will spread into these fields in the future.
The control of Monochoria vaginalis (Burm. f.) Presl var. plantaginea (Roxb.) Solms-Laub. has been difficult in organic rice cultivation. Here, we evaluated the ability of rice to suppress M. vaginalis through light competition by measuring the relative photosynthetic photon flux density (R-PPFD) calculated as a ratio of the photosynthetic photon flux density below or above the rice canopy. Field experiments were carried out to compare R-PPFD and M. vaginalis shoot dry weight and capsules production under different plant densities. We observed that regardless of plant density, R-PPFD began to decline from 27 days after rice transplantation and remained nearly constant after the heading stage (after 82 days). Furthermore, R-PPFD decreased with increasing plant density. A positive relationship was observed between R-PPFD and M. vaginalis shoot dry weight. The number of capsules of M. vaginalis decreased with increasing plant density. Our study suggests that increasing the plant density of rice decreased R-PPFD below the rice canopy and inhibited the growth of M. vaginalis.