Field studies were conducted to determine if varying the time N fertilizer was applied would affect downy brome interference in winter wheat. Five treatments were compared: four broadcast application times of NH4NO3 at 56 kg N ha-1 during the fallow-crop cycle: 1) during fallow; 2) at planting; 3) during crop dormancy; 4) before winter wheat jointing; and 5) a control where no N was applied. Downy brome was least responsive to N applied during fallow. All N applications during the growing season of winter wheat increased downy brome biomass and culms m-2. Downy brome interference prevented winter wheat from responding positively to N. For example, when crop season precipitation was only 70% of normal, applying N reduced grain yield of winter wheat infested with downy brome from 12 to 28%. This study indicates that N manipulation offers potential for reducing downy brome interference in winter wheat.

A 9-yr large-scale integrated pest management (IPM) study was initiated in 1985 to develop and refine profitable conservation cropping systems in the Palouse wheat-growing region of the Pacific Northwest. Weed scientists from the USDA-ARS and the land-grant universities of ID and WA led a team of researchers and extension personnel from eight disciplines to investigate the interactions of crop systems, tillage systems, and weed management levels (WML) on crop production. Ineffective weed control has been a major deterrent to the adoption of conservation tillage by wheat growers. With this in mind, the primary focus of the scientists on the IPM project was integrated weed management (IWM) in conservation crop production systems for highly erodible land. For the first time in the Pacific Northwest, systems research developed a conservation production system using a 3-yr crop rotation that controlled weeds effectively, reduced erosion, was less risky than traditional farming systems, and was profitable. Broadleaf weeds were more prevalent in the 3-yr rotation of winter wheat-spring barley-spring pea compared to continuous wheat in both conservation and conventional tillage systems. In conservation tillage, troublesome grass weeds included wild oat and downy brome. Wild oat was controlled effectively at the moderate and maximum weed management levels under conservation tillage in the 3-yr rotation. Two years out of winter wheat (such as in the 3-yr rotation) reduced downy brome populations. In contrast, growing a spring crop 1 yr, followed by 2 yr of winter wheat was not effective for controlling downy brome. Effective weed control was instrumental in developing successful conservation IPM cropping systems, and education and technology transfer were important in helping action agencies assist growers in adopting these systems.

Field studies were conducted to determine the most effective rate of several herbicides applied at various growth stages to control downy brome in conservation fallow programs. Downy brome growth stage affected the efficacy of all herbicides. All herbicides were less effective when application was delayed until the boot stage of downy brome. Fluazifop-P and sethoxydim must be applied prior to tillering to effectively control downy brome. Glyphosate, the commercial mixture of glyphosate plus 2,4-D, paraquat, and HOE-39866 consistently controlled downy brome up to the 3- to 5-tiller stage. Glyphosate at 180 to 200 g ha-1, paraquat at 250 to 300 g ha-1, and the commercial mixture of glyphosate plus 2,4-D at 600 to 660 g ha-1 controlled downy brome 80 to 90%. The effective rates were lower than rates currently registered for downy brome control in western Canada, and thus there is potential for making conservation fallow programs more economical when downy brome is the predominant weed.

Downy brome control in winter wheat is often inadequate. The effects of three crop rotations and two tillage intensities on downy brome populations and associated crop yields were determined in an experiment at Lethbridge, Alberta from 1987 to 1993. Downy brome densities in continuous winter wheat increased from 24 to 970 plants/m2 between 1988 and 1993; and were often higher with zero tillage. Inclusion of fallow or spring canola in rotation with winter wheat suppressed downy brome densities to less than 55 and 100 plants/m2, respectively, over the six years. In continuous winter wheat, yields decreased as downy brome densities increased progressively over years, indicating that monoculture winter wheat production will not be viable in regions where downy brome is prevalent unless effective herbicides are developed. In the more arid areas of the Canadian prairies, a winter wheat-fallow rotation may be most suitable but in higher precipitation areas, a winter wheat-canola rotation is a viable alternative. Crop rotation is a key component of an improved management system for control of downy brome.

Densities up to 100 downy brome m2 were established in winter wheat in southeastern Wyoming and west-central Kansas to quantify wheat yield loss from downy brome interference and to approximate economic threshold levels. A quadratic equation best described wheat yield loss as a function of weed density when downy brome emerged within 14 days after wheat emergence. Densities of 24, 40, and 65 downy brome m2 reduced wheat yield by 10, 15, and 20%, respectively. Wheat yield was not reduced when downy brome emerged 21 or more days later than wheat. Economic thresholds varied with changes in downy brome density, cost of control, wheat price, and potential wheat yield. In a greenhouse experiment, dry weight of 72-day-old wheat plants grown in association with downy brome was not affected by the distance between the weeds and wheat, whereas downy brome plant dry weight increased with increasing distance between the weeds and wheat.