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Downy Brome Competition and Control in Dryland Wheat¹

Competition between downy brome (Bromus tectorum) and winter wheat causes serious economic losses to growers in the Pacific northwest. On a site which received 25 cm average annual rainfall and had a light soil (very fine sandy loam) winter wheat yields were reduced 28% by fewer than 54 downy brome plants per m² (5 plants/ft²). On a site which received 55 cm average annual precipitation and had a silt loam soil, winter wheat yields were reduced 92% by more than 538 downy brome plants per m² (50 plants/ft²). Downy brome seed-to-straw ratios were indicative of a tremendous yield potential. Seed-straw ratios of 1:4 are probably an exception but ratios of 1:1 and 1:2 were common.

Chemical fallow studies at three sites showed that nonselective downy brome control can be obtained by the use of herbicide combinations such as 3-amino-l-2, 4-triazole-ammonium thiocyanate (amitrole-T), plus chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) or amitrole-T plus 2,4-dichlorophenoxyacetic acid (2,4-D). Amitrole-T plus atrazine was more effective when used as a fall treatment. Amitrole-T plus 2,4-D was more effective in the spring. These chemicals reduced tillage requirements and decreased downy brome growth by 90%.

Key Words: Bromus tectorum • downy brome densities • chemical fallow • yield reductions • seed-to-straw ratios • weed-crop competition

² Formerly Junior Agronomist (now Assistant Agronomist, Oregon State University) and Agronomist, Agricultural Experiment Station, Washington State University, Pullman, Wash. 97801.

Received for publication November 20, 1967.