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Cheatgrass Infestations Worst in Modern Memory

PULLMAN, Wash. — Winter wheat fields in eastern Washington are plagued with the worst infestation of cheatgrass in recent memory.

Currently, downy brome infestations in many eastern Washington winter wheat fields are nothing short of disastrous.

It’s too late to do much to help this year’s crops, but farmers and ranchers should act now to minimize infestations next year.

Downy brome (the approved name for cheatgrass) is well suited to the climate and cropping systems of eastern Washington. In fact, it may be better adapted to the winter wheat/summer fallow rotation than the winter wheat.

The large amounts of seed produced this year will increase future infestations if current production practices are continued.

Reducing the size and severity of future infestations requires changes in production practices to make the environment less favorable to downy brome.

Chemical control options will increase with the introduction of new herbicides within the next few years. University studies in the Pacific Northwest have shown that these herbicides consistently provide greater than 85 percent control of downy brome. “This summer, downy brome infestations are so severe in some fields that some yield loss would occur even if 85 percent of the weeds were removed.

Although new herbicides being tested effectively control downy brome, nearly all weed scientists agree that herbicides are not a stand- alone management practice. These herbicides will be most effective when integrated into a total package to control downy brome. Cultural practices must be used to reduce the downy brome seed bank in the soil.

Severe downy brome infestations this year are due, in part, to fall rains that promoted early emergence and fall growth of the weed. Some of the most severe infestations are in early planted wheat fields. Infestations may be lighter in fields where planting was delayed until fall rains brought on the first flush of downy brome.

This first flush of weeds were then killed with tillage or non-selective herbicides prior to seeding. Downy brome emerging ahead of, or along with, the wheat are the most competitive and cause the greatest yield loss in winter wheat.

Downy brome develops and matures more rapidly than the winter wheat and continues its growth and development at temperatures that induce dormancy in winter wheat. This let downy brome increase its competitive hold both late last year and early this year while the wheat was relatively noncompetitive.

Some of the most severe infestations may be attributed to late winter or early spring top dressing with nitrogen fertilizer. Spring top dressing in particular can benefit downy brome growth and increase winter wheat yield loss due to increased competition. Deep placement of nitrogen during the summer fallow period would have given the more deeply rooted wheat a competitive advantage. With moderate to severe infestations of downy brome, it is advisable to forgo top dressing.

Farmers can minimize future infestations from this year’s “bumper crop” of downy brome by reducing the amount of viable weed seed before winter wheat is sewn for next year’s crop. This will minimize infestations and provide better return to investment of herbicide and other production expenses.

Use cultural or mechanical methods to enhance germination of downy brome this fall, after which tillage or non-selective herbicides can be used to control the weed.

Germination is enhanced by even distribution and greater soil contact of the downy brome seed. Even-seed distribution can be accomplished through the use of a combine-mounted straw chopper or chaff spreader.

Light tillage following wheat harvest will increase contact between soil and seed. USDA-ARS research has shown light disking or cultivation to most effectively reduce the bank of downy brome seed in the soil when combined with fall applications of non-selective herbicides.

Skewtredding is also effective, particularly where it is desirable to leave more surface residue than is left by disking or sweep cultivation. Even post harvest harrowing is more effective than applying no tillage, although effectiveness will largely depend on the timeliness of fall rains, the relative effective ranking of disk, sweep, cultivator, skewtredder, harrow, no treatment management has been shown in both dry and moist years.

Managing downy brome soil seed banks is particularly effective because of downy brome’s short seed life. Typically, the downy brome soil seed bank can be nearly eliminated if no influx of seed occurs over two years. Rotating out of winter wheat for two years can effectively reclaim fields from downy brome infestations. Following the winter wheat crop with two years of spring crops or following the fallow year with a spring crop, will greatly reduce re-infestation potential before rotating back to winter wheat.

With the current low cost of wheat, opportunity cost of complementary crops may be reduced, thereby favoring the use of these alternatives.

Opportunity cost is the difference in profit potential between the profit made from wheat and profits made from the alternative. The time may be ripe for finding a profitable crop that is less environmentally suited to perpetuating downy brome infestations.

Select alternative crops that have recognizable markets and options for weed or other pest control. It’s also important to select crops that will not require large capital investments in specialized equipment.

Once fields are reclaimed from severe downy brome infestations, it is important to continue management under a maintenance strategy. This could include a spring crop in the rotation, but it must rely on harvest, post harvest tillage, and optimal use of other production inputs along with the use of new herbicide resources.

However, understanding why the problem occurred, adjusting current production practices, and using new tools for management will result in a more desired integrated approach to downy brome. The results of the integrated approach will minimize downy brome infestations even if future weather patterns are favorable for infestations.

Information on downy brome and its management is explained in greater detail in “Managing Downy Brome Under Conservation Tillage Systems in the Inland Northwest Cropping Region,” extension publication PNW0509. The publication should be ready for distribution by August 1998.

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