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Weed communities in inland valley rice-based cropping systems of the southern Guinea savannah

Touré A., Marnotte P., Huat J., Mariko M., Kossou D.. 2013. In : La science rizicole pour la sécurité alimentaire à travers le renforcement de l'agriculture familiale et l'agro-industrie en Afrique : 3ème Congrès du riz en Afrique 2013, 21-24 octobre 2013, Yaoundé, Cameroun. Programme et résumés. Cotonou : ADRAO [Centre du Riz pour l'Afrique], p. 237-237. Africa Rice Congress. 3, 2013-10-21/2013-10-24, Yaoundé (Cameroun).

In order to determine effective integrated strategies against weed infestation in inland valley rice-based cropping systems, it is first necessary to identify the dominant weeds that compete with and reduce the yields of the crops. The objective was to relate weed species to environmental factors and cropping systems along the catena. Weeds species and percentage of soil coverage were determined along the catena using the quadrat sampling method, in 46 inland valley farms of three villages in the departments of Mono-Couffo in Benin during the wet and dry seasons. The ecological profile, used to differentiate the weed communities, represented the ratio between the local coverage of a species within a particular ecological (or environmental) class and the local coverage of a species for all classes combined. The first three dominant weed species for each ecological situation were determined. For the catena positions, the first three dominant weed species were Dactyloctenium aegyptium, Commelina benghalensis and Digitaria horizontalis on the inland valley crests; Ludwigia hyssopifolia, Corchorus aestuans and Ludwigia octovalvis on the sloping hydromorphic fringe; and Leersia hexandra, Ipomoea aquatica and Fimbristylis ferruginea in the valley bottom. According to the seasons, the first three dominant weeds were Echinochloa colona, Cleome viscosa and Talinum triangulare in the dry season crops, and Leersia hexandra, Ipomoea aquatica and Sphenoclea zeylanica in the wet season rice. For the cropping systems that are largely influenced by the moisture level along the catena, Ipomoea aquatica, Fimbristylis littoralis and Leersia hexandra were common in rice-jute mallow systems. In rice-okra systems, Brachiaria spp., Commelina benghalensis and Digitaria horizontalis were dominant. In rice-maize systems, Ludwigia hyssopifolia, Panicum laxum and Corchorus aestuans were common. For rice-fallow systems, Bacopa decumbens, Heteranthera callifolia and Imperata cylindrica were common. Certain upland weed species normally occurring on the valley crest and during the dry season (i.e. with the vegetables crops) are increasing in populations at the valley bottom in the wet season (i.e. with the lowland rice), apparently caused by insufficient flooding during the rice rotation and enhanced by the alternate wet-dry annual rotation pattern. Over time, this could result in dominant species shifts for inland valley weed communities, not only in the Benin sites but also in other inland valley areas in West Africa with similar cropping patterns and weed populations. This information provides guidance for weed-control technology development and transfer that could benefit inland valley rice-based cropping systems of the southern Guinea savannah of Africa.
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