Agroecological Influence on Rice Field Weeds in Anosibe Ifanja Commune (Middle West Madagascar)

Weeds, which cause loss of more than half of production, constitute one of obstacles to development of agricultural production in Madagascar. This study aimed to characterize weed flora and to analyze constraint of weeding irrigated rice with a view to improve control strategy. A phytoecological study of weeds was carried out during 2018-2019 season, on 125 rice plots with an average area of 500 m², in Anosibe Ifanja, Middle-West region of Madagascar (18°52′ S and 46°50′ E). Surveys on cropping practices and observations on rice field ecology and the importance of weed cover level were carried out in order to establish relationship between factors affecting diversity, distribution and level of weed infestation in rice field. As a result, 47 weed species grouped into about 20 families were identified. Weed cover quantification in each plot, permits to distinguish three major importance species, including Junglerice - Echinochloa colona (Poaceae), Tail - Leersia hexandra (Oryzoideae), saramollagrass - Ischaemum rugosum (Poaceae) and hemp sesbania - Sesbania rostrata (Fabaceae). Analysis of agro-ecological factors showed that all production techniques and growing conditions have a role to play in variability of plant cover level. Thus, the main production factors that vegetation cover reduce state are ploughing to a depth of 30 cm, practice of ploughing off-season/in-season and good water control. Ecological factors, especially soil type, soil structure and topography, minimize weed cover in rice fields.