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The competitive threshold of Iamb's-quarters (Chenopodium album L.) in alfalfa and oats was studied in the greenhouse and in the field. In oats, the competitive threshold of Iamb's-quarters was reached at a density of 1000 plants/m2 in the field. In the greenhouse, the weed could not compete with oats when both were sown simultaneously. When Iamb's-quarters was sown 15 days earlier than oats, crop yields were significanty reduced under a density of 800 plants/m2. The competitive threshold of Iamb's-quarters in alfalfa was reached at a density of 200 plants/m2 in the greenhouse and 1000 plants/m2 in the field. The difference between the results of the two experiments was mainly caused by the growth conditions (soil type, fertility, humidity) which were quite different and influenced both the weed and the crop. Two harvest treatments were compared and it seems preferable to take the first cut of alfalfa at the 10% bloom stage rather than at the vegetative stage. A cut taken at the vegetative stage had a negative effect on alfalfa. This effect did not seem related to the weed but rather to a weakening of the legume. A close relationship existed between the number of individuals per square metre and the biomass of Iamb's-quarters, on one hand, and the yield of alfalfa or oats, on the other hand. The classes of Iamb's-quarters with the largest densities, or the highest biomass, were the most competitive. |
