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Tessema, Taye; Tanner, D.G. & Hassena, Mohammed Résumé Des etudes durant deux annees etaient conduites en Ethiopie pour examiner les effets de competition par especes predominantes des adventices sur le ble (Triticum aestivum L.). Dans le premier essai, les effets de quatre especes d'adventices (Avena abyssinica Hoechst, Lolium temulentum L., Snowdenia polystachya Fresca (Pilg), et Phalaris paradoxa L.) a huit densites de semis (de 0 a 320 m-2) sur le rendement en grain de ble etaient etuies. A. abyssinica et S. polystachya etaient hautement competitifs, resultant en une pins grande reduction du rendement en grain de ble (de 85 et 86% respectivement a 320 plants m-2) que L. temulentum et P. paradoxa. L temulentum exercait un plus grand effet competitif que P. paradoxa. Le niveau de competition des adventices variait avec le taux de germination des adventices. Le second essai etudiait lea interactions competitives de Avena fatua L. seme a quatre densites (de 0 a 90 m-2) en competition avec quatre varietes de ble communement cultivees par les exploitants fermiers en Ethiopie. La variete seminaine Dashen emit la plus sensible a la competition de A. fatua, exhibant une perte de rendement de 63% a la densite maximale de 90 plantes d'avoine sauvage m-2 . Les indices de competition etaient derives de chaque essai pour faciliter la prediction en perte de rendement en grain de ble et pour deriver les seuils economiques pour l'intervention d'herbicide en relation avec le taux de germination des adventices. Mots Clés Adventices, ble, competition, herbicide, perte du rendement

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Grass weed competition with bread wheat in Ethiopia: II. Prediction of grain yield loss and implications for economic weed control Tessema, Taye; Tanner, D.G. & Hassena, Mohammed Abstract Two multi-year studies were conducted in Ethiopia to examine the effects of competition by the predominant grass weed species on bread wheat (Triticum aestivum L.). In the first experiment, the effects of four grass weed species (Avena abyssinica Hoechst, Lolium temulentum L., Snowdenia polystachya Fresen (Pilg), and Phalaris paradoxa L.) at eight seedling densities (from 0 to 320 m2) on wheat grain yield were studied. A. abyssinica and S. polystachya were highly competitive, resulting in a greater reduction of wheat grain yield (i.e., 85 and 86%, respectively, at 320 seedlings m-2) than L temulentum and P. paradoxa. L temulentum exerted a greater competitive effect than P. paradoxa. Competitiveness of the grass weeds varied with weed seedling density. The second experiment studied the competitive interactions of Avena fatua L. sown at four seedling densities (from 0 to 90 m-2) in competition with four bread wheat cultivars commonly grown by peasant farmers in Ethiopia. The semi dwarf cultivar Dashen was the most sensitive to A. fatua competition, exhibiting a yield loss of 63% at the maximum density of 90 wild oat seedlings m-2. Competitive indices were derived from each experiment to facilitate the prediction of wheat grain yield loss, and to derive economic thresholds for herbicidal intervention in relation to weed seedling density. Keywords Competition, grass weeds, herbicide, wheat, yield loss

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