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Cheatgrass Infestations Disastrous for Some

PULLMAN, Wash. — Heavy fall rains combined with cultural practices have resulted in the worst infestation of cheatgrass in recent memory.

“Downy brome infestations in many eastern Washington winter wheat fields are nothing short of disastrous,” says Washington State University extension weed scientist Joe Yenish. (Downy brome is the approved common name for cheatgrass.)

It’s too late to do much to help this year’s crops, Yenish says, but farmers must act now to minimize infestations next year.

“Downy brome 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,” Yenish says.

“The large amounts of seed produced this year will increase future infestations if current production practices are continued,” he warns.

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

New herbicides for cheatgrass control may be released within a few years. Meanwhile, farmers must make the most of management practices. Although new herbicides being tested effectively control downy brome, nearly all weed scientists agree that herbicides are not a stand-alone management practice.

Herbicides are most effective when integrated into a total package of control measures. “Cultural practices must be used to reduce the downy brome seed bank,” Yenish said.

Last fall’s rains were timely for early emergence and growth of the cheatgrass. 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, said Yenish.

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,” says Yenish. 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, Yenish said. With moderate to severe infestations of downy brome it is advisable to forgo top dressing.

Yenish urges farmers to minimize future infestations from this year’s “bumper crop” of downy brome by reducing the amount of viable seed before winter wheat is sewn for next year’s crop.

He recommends cultural or mechanical methods to enhance germination of downy brome this fall, after which the weed can be controlled with tillage or non-selective herbicides.

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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