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Two alfalfa varieties, namely Rasen and Yuba, and rice by‐products of Koshihikari (Oryza sativa L. cv. Japonica), including hull and bran were used for weed control in rice. Rasen and Yuba were sequentially evaluated to have the strongest allelopathic potential among eight common alfalfa varieties in Japan. Rasen, Yuba and rice by‐products in a laboratory experiment exhibited suppression of emergence of lettuce (Lactuca sativa L.), barnyardgrass (Echinochloa oryzicola Vasing.), and monochoria (Monochoria vaginalis Presl var. plantaginea Solms‐Laub.). In a paddy field experiment, at 15 days after application, a dose of 1 ton ha−1 of alfalfa, rice by‐products and the incorporated alfalfa‐rice by‐products significantly inhibited weed growth and reduced weed species, except for rice bran. However, at 40 days after application, the greatest weed control was sequentially recorded with Yuba (95.2%), Rasen (90.5%) and the incorporated Rasen‐hull (88.3%). Rice hull exhibited stronger (51.7%) weed control than the negligible weed control of rice bran (25.1%). Inhibition of both Rasen and Yuba on weed emergence after application became more vigorous, whereas those of rice by‐products were gradually devitalized. Rasen had a maximum increase of rice yield (80.6%) compared to the control and it was slightly higher (9.6%) than the herbicide treatment. Findings showed that Rasen was the most promising material for weed control among those studied. Results denoted that weed control capability of allelopathic materials had an impetuous effect on rice growth and yield.

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