Occurrence of weeds in soybean fields of Oita Prefecture

To determine the distribution pattern of weeds in soybean field of Oita prefecture, field surveys were conducted from mid-August to September (the growing stage of soybean) and November (the maturing stage of soybean) in 2015 and 2016, respectively. The dominance of each weed in different fields was evaluated at six levels based on the observations made in the fields. Moreover, the percentage of occurrence of each weed was calculated from the data obtained. The weed species with high percentage of occurrence (occurring in more than 30% fields) were Echinochloa spp., Digitaria ciliaris (Retz.) Koeler, Amaranthus hybridus L., Physalis angulata L. var. angulata and Commelina caroliniana Walter. Among the invasive weeds with high percentage of occurrence, A. hybridus L. and P. angulata L. var. angulata occurred over the entire Oita Prefecture, but C. caroliniana Walter occurred mainly in the northern region. This is probably because C. caroliniana Walter invaded the soybean field later than did the other two species. The weeds with medium occurrence percentage in the fields were those found in the northern region; C. caroliniana Walter was more abundant than A. hybridus L. and P. angulata L. var. angulata . From these results, it can be said that C. caroliniana Walter might spread quickly after invading the soybean fields. In addition, four invasive Ipomoea spp. and Sicyos angulatus L. were confirmed to be present in the soybean field. This is first time that the presence of S. angulatus L. in the soybean fields of Kyushu region is being confirmed.