Graduate Thesis Or Dissertation
 

Interference of annual weeds in seedling alfalfa

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9p290c82w

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  • The interference of barnyardgrass (Echinochloa crusgalli), pigweed (Amaranthus retroflexus),downy brome (Bromus tectorum),and tumble mustard (Sisymbrium altissimum) with alfalfa was studied. Alfalfa was seeded with each of the four weeds on August 14 and September 15, 1981, and on April 13, 1982. Alfalfa also was seeded on August 27, 1981, with a mixture of the four weed species, and the weeds were allowed to grow with the crop during several intervals. The presence of weeds emerging with the crop in the fall reduced alfalfa yields in the first two cuttings in the spring. When alfalfa and each of the weeds were planted on August 14 (early seeding), barnyardgrass and downy brome were the most interfering weeds. In a later seeding (Sept. 15), the summer annual weeds (barnyardgrass and pigweed) were killed by frost in the fall and did not compete with the crop; downy brome and tumble mustard (winter annuals), however, became more aggressive than if allowed to emerge with the crop a month earlier. When alfalfa seeded on August 27 was kept weed free for approximately 65 days after emergence, no further weed control was needed for maximum forage yields in the spring. Conversely, the crop tolerated weeds that emerged with it if they were eliminated before the onset of winter. In the spring seedings, the four weeds interfered equally with the crop. Alfalfa needed to grow free of weeds from approximately the 17th to the 39th day after emergence to avoid forage yield and quality reductions. Weeds that emerged early with the crop reduced forage yields more than later-germinating weeds. Once the alfalfa developed a canopy capable of covering the soil, weeds emerging thereafter were suppressed by shading. The initial tolerance to the presence of weeds allows the farmer flexibility in the timing of the first weed control measure. This initial tolerance seems to indicate that interference is not significant until competition for light begins.
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