Weed emergence for both an aigamo duck farming system and an ordinary farming system using herbicides in paddy fields for 3 years was investigated in Aikawa town, Kanagawa prefecture. Fourteen weed species were identified in the soil surface layers: Echinochloa spp., Monochoria vaginalis (Burm. f.) Presl var. plantaginea (Roxb.) Solms-Laub, Lindernia spp, Eleocharis acicularis (L.) Roem. et Schult., Elatine triandra Schk. var. pedicellata Krylov, Rotala indica (Willd.) Koehne var. uliginosa (Miq.) Koehne, Cyperus spp., Ludwigia epilobioides Maxim., Ammannia multiflora Roxd., Dopatrium junceum (Roxb.) Hamilt., Scirpus juncoides Roxd. var. ohwianus T. Koyama, Sagittaria trifolia L., Oenanthe javanica (Blume) DE. and Cardamine flexuosa With.
In the aigamo duck farming system, weed emergence from the soil surface layers was very low at 0-2cm depth of the outer layer while it was high at the same deeth in the ordinary farming system using herbicides. This is because in the Aigamo duck farming system, the duration of the period of weed control (40-70 days) was 2 or 3 times longer than in the ordinary system using herbicides. The aigamo duck farming system showed a low weed emergence subsequently. In the 2-10cm soil outer layer, there was no appreciable difference between the weed emergence in the two systems. For the 2-10cm soil outer layer, there was no difference between the weed emergence from the soil surface layers in the two systems (Table 2). The emergence of M. Vaginalis, R. indica and E. triandra decreased with continuous aigamo duck farming, while that of L. epilobioides and A. multiflora increased year by year and they became the major species (Figs. 1, 2, Table 2). Based in studies cmducted in 1994 and 1995, compared to the ordinary farming system use of herbicides, the aigamo duck farming system led to a lower weed emergence from the soil surface layers. However, in 1996, due to the predominance of L. epiloboiides, A. multiflora, Cyperus spp. and Dopatrium junceum (Roxb.) Hamilt. in the aigamo duck farming system, the total number of weeds which emerged increased, compared with the ordinary farming system (Fig. 3).