Downy Brome Control Considerations

Chapter 5 – Weed Control, No. 8, Summer 1987

Roger Veseth

Downy brome, commonly called ”cheatgrass “, has been one of the major grassy weeds for winter wheat producers in the Northwest over the past 60 years. Over 3 million acres of cropland in Idaho, Oregon and Washington are currently infested with downy brome to some degree. Downy brome is a problem weed regardless of tillage system. It can, however, be a particularly troublesome weed under conservation tillage unless producers begin with a control strategy in mind.

Crop Loss Potential

Downy brome competition for soil water and nutrients can seriously reduce winter wheat yields. Dean Swan, Washington State University Extension weed specialist, and Ralph Whitesides, former WSU weed scientist, recently made an estimate of winter wheat yield losses as the result of downy brome. Their estimate was based on 5 years of experiments conducted at various sites across eastern Washington. Downy brome populations of 2 to 11 plants per square foot reduced wheat yields by 2.3 percent to 10 percent for each weed per square foot. On an acre basis, the average yield reduction was estimated to be31 percent. It is not uncommon to fmd similar or higher downy brome populations throughout the region.

Traditional Control Practices

STEEP researcher Alex Ogg, USDA-ARS weed scientist at WSU in Pullman, points out that traditional practices of tillage and delaying seeding date for reducing downy brome have only limited success. Furthermore, the practices themselves can also reduce yield potential.

Using the moldboard plow to bury downy brome seed can also bury nearly all of the surface crop residue. On erosion-prone cropland in the Northwest, an intensive tillage approach to downy brome control, whether it begins with the moldboard plow, is reducing soil productivity because of loss of topsoil by erosion. In addition to erosion, the use of excessive secondary tillage to kill downy brome can also reduce yields by reducing soil water storage through increased evaporation and runoff. A one-half inch loss of soil water with each tillage operation is not uncommon. At 5 to 7 bushels per acre of wheat yield potential per inch of water, yield loss associated with excessive tillage in weed control can be significant.

Delaying the winter wheat seeding date to allow time for fall rains to germinate downy brome also can take its toll on yields. The loss of growing degree days for adequate fall growth can prevent the wheat plant from reaching its full yield potential with the available water. Delayed seeding can also increase the potential losses from winterkill and Pythium root rot damage. Pythium is favored by the cool wet conditions usually associated with late seeding in areas with 16 inches or more precipitation.

Control with Crop Rotation

Ogg emphasizes that one of the most effective controls for downy brome and other winter-annual weeds is crop rotation. Downy brome seed has very low dormancy in the soil, with most seeds germinating during the first year. Consequently, crop rotations and other management practices which help prevent seed production are very effective in controlling the weed. Rotation options which allow the removal of downy brome with timely tillage and/or herbicides before seed set will quickly deplete the soil seed reserve. These include spring crops (cereals or noncereals), fall non-cereal crops with grassy weed herbicide options and summer fallow.

Current research by Ogg shows that growing spring crops in the crop rotation effectively reduces downy brome populations in both conventional tillage and no-till systems. A study is being conducted near LaCrosse, WA, in a traditional winter wheat-fallow area. It was initiated with a spring barley crop in 1985 following winter wheat in 1984.

Soil samples were taken in March 1985 to determine the initial number of viable downy brome seeds present. Two tillage systems are compared: (1) conventional fall plowing and secondary spring tillage before spring seeding; and (2) direct spring seeding with a no-till drill after a preplant application of glyphosate (Roundup). Any downy brome plants which escaped the tillage or herbicide were removed by hand weeding to prevent all seed production.

Soil samples were again taken in the falls of 1985 and 1986 and analyzed for viable downy brome seed. Compared to the spring 1985 samples, the number of viable downy brome seeds decreased about 90 percent after 1 year under both tillage systems. After 2 years of preventing seed production, no viable downy brome seeds were found in either tillage system.

Spring barley was again planted in 1987 using the same two tillage systems. Downy brome control in the crop was rated at 100 percent under both systems. Soil samples in fall 1987 will again be analyzed for viable downy brome seed, Ogg stresses that preventing seed production is critical to the success of crop rotation.

Although three consecutive years of spring barley were used in the study, fallow or other crops could have been used in the rotation to achieve similar downy brome control. Several soil-residual herbicides are registered for control of downy brome in summer fallow when applied in the fall before the fallow season. Options for control of downy brome in the fallow season also include tillage and/or non-selective herbicides for minimum tillage or chemical fallow systems.

Control in Winter Wheat

Growers have limited herbicide options to selectively control downy brome in winter wheat. Two of the currently available herbicides labeled for downy brome control in winter wheat are trifluralin (Treflan) and diclofop methyl (Hoelon). However, both require soil incorporation which restricts their use in conservation tillage systems.

The only postemergence herbicide options are metribuzin (Lexone or Sencor) or metribuzin plus terbutryn (Igran). Although good control can be achieved, the difficulty in optimum timing of growth stages of the winter wheat and downy brome often limits effectiveness, Wheat must be in the tillering stage with at least 2-inch long secondary (crown) roots to avoid significant injury from metribuzin. On the other hand, control of downy brome is most effective at earlier pre-tillering stages.

Fall Postemergence Application

In much of the Northwest cropping region receiving 16 inches or more annual precipitation, a delayed seeding date is commonly used to reduce the incidence of soilborne diseases such as Cephalosporium stripe and Pseudocercosporella (strawbreaker) foot rot. Consequently, winter wheat is often too small to safely apply these postemergence herbicides in the fall. Late seeding can also delay spring spray applications past the time of optimum downy brome control because of the delayed wheat development.

Ogg points out that removal of winter annual weed competition from winter wheat in the fall will typically increase yields compared to controlling the weeds in the spring. Downy brome is also much easier to kill in the fall than when it is tillering in the spring, Consequently, he encourages growers to consider management options to allow fall weed control measures where possible.

The advantage of fall downy brome control is shown in a research summary by Swan and Whitesides. Seven downy brome experiments in winter wheat were conducted at diverse locations in eastern Washington from 1981 through 1985. Their studies compared fall and spring postemergence applications of metribuzin at labeled rates with and without terbutryn.

Crop damage symptoms of reduced stand, suppressed growth and discoloration averaged 10 percent when the herbicides were fall-applied and 4 percent when spring applied. Fall herbicide applications resulted in 30 percent higher average downy brome control than spring applications (Fig. 1). Fall-applied metribuzin plus terbutryn gave the best downy brome control and resulted in yields significantly higher than the check at all locations for all years (Fig, 2). Other treatments resulted in significantly higher yields only at some locations or in certain years.

Integrated Management Approach

These scientists stress that growers need an integrated management approach for effective control of the unique complex of weed and disease problems they encounter. They suggest using a combination of crop rotation with herbicides and/or tillage. A 3-year rotation with 2 consecutive years of any combination of spring crops (cereals or non-cereals), fall non-cereals (with selective grassy herbicide options) and/or fallow will reduce downy brome seed production and deplete soil seed reserves.

The 2 years out of winter cereals also control Cephalosporium stripe and reduces the incidence of Pseudocercosporella (strawbreaker) foot rot, Fungicides are also available for strawbreaker foot rot if needed. Consequently, there is less need for delayed seeding of winter wheat. An earlier seeding date in the 3-year rotation also reduces Pythium damage and provides more time for sufficient wheat plant development. This may allow fall application of metribuzin or metribuzin plus terbutryn for more effective downy brome control. In areas where barley yellow dwarf is a problem the risk of this disease must also be considered when selecting an earlier seeding date.

Experimental Herbicides

A promising experimental herbicide for selective downy brome control in winter wheat is ethyl metribuzin. Developed under the trade names of Tycor by Mobay and Seige by DuPont, the herbicide has been under extensive testing in the Northwest since 1984. Although DuPont recently withdrew from the registration effort, Mobay is continuing.

Ethyl metribuzin can be safely applied to winter wheat in the fall from the 1-leaf stage through tillering, which is much earlier than with metribuzin. This allows the herbicide to be applied when downy brome plants are small for improved control. Ethyl metribuzin is less active and has a shorter soil residual than metribuzin, so higher rates have been required. Research in the Northwest has shown that fall-applied ethyl metribuzin commonly gives 90 to 100 percent control of downy brome in winter wheat. Registration date for Tycor is uncertain, possibly in the fall 1988 or 1989.

An experimental granular herbicide produced by Monsanto with the trade name of Buckle has also shown promise for downy brome control in no-till winter wheat. In a 1986-87 study at Pullman, Ogg applied Buckle using three methods of application in no-till winter wheat after spring barley. The USDA-IN (Yielder) no-till drill (5: 15 inch paired-row spacings) was used in the experiment. Application methods included: (1) broadcast at seeding ahead of the drill, (2) broadcast in 14-inch wide bands between the pairs of rows; and (3) broadcast in 10-inch wide bands between the pairs of rows. Downy brome control in 1987 was estimated at 93 percent for the broadcast and 14-inch bands and 70 percent for the 10-inch bands.

Monsanto is planning to expand the granular triallate (Fargo) label in northern Idaho, and eastern Oregon and eastern Washington to include suppression of downy brome in winter wheat, If approved, the herbicide label would allow preplant-incorporated applications in conventional tillage systems and preplant surface applications in no-till (direct-drill) systems when the Yielder no-till drill is used.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.