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Downy brome control in direct seeding

Research into herbicides and crop rotation provides control recommendations.


November 20, 2007
By Bruce Barker

Topics

16aDowny brome, a winter annual, can cause severe problems in winter annual crops
like winter wheat and fall rye. Research in southern Alberta has found that
densities in the range of 50 to 100 plants per square metre (five to 10 plants
per square foot) reduced winter wheat yield by up to 30 to 40 percent when emerging
within three weeks of the crop in the fall. Downy brome is less competitive
in fall rye than winter wheat, but still can reduce yields by 15 to 30 percent.

"Downy brome was a real issue about 10 years ago when we saw a lot of
continuous cropping of winter wheat, before the acreage dropped off in the 1990s,"
says Don Wentz, Alberta Reduced Tillage Linkages agronomist at Lethbridge. "I
haven't heard of a lot of problems recently, but now that winter wheat acres
are climbing back up, growers need to be vigilant."

Once established in a field, downy brome can be a serious problem. In minimum
tillage systems, seeds can remain viable up to three to four years if they are
deposited on heavy residue. And downy brome is a prolific seed producer as well,
with fall-emerged plants under ideal growing conditions producing 400 pounds
of seed per acre (450kg/ha) with 150,000 seeds per pound.

Lack of herbicides in winter cereals also create problems for growers. Currently,
the only herbicide registered for downy brome control in winter wheat is Sencor
on Norstar. But since few grow Norstar winter wheat any longer, control of downy
brome in-crop is virtually non-existent. Sundance herbicide is used in the US
for control of downy brome in winter wheat, but it is not registered here for
that use.

Start with proper identification
At Agriculture and Agri-Food Canada, research scientist Bob Blackshaw says that
one of the keys to controlling downy brome is proper identification. Downy brome
and Persian darnel are two easily confused weeds. Japanese brome is also starting
to appear in western Canada and is closely related to downy brome.

Downy brome and Japanese brome are winter annuals with optimum germination
at soil temperatures of five to 15 degrees C. They can either germinate in the
fall and overwinter, or germinate in the early spring. Overwintering plants
will head out in late May and produce mature seed by early July.

Persian darnel, on the other hand, is a summer annual that establishes early
in the spring. Plants are lighter green than downy brome or winter wheat. The
leaves have an oil sheen and are not hairy. "It is very difficult to tell
the difference between them until the plants head out," explains Blackshaw.
"Persian darnel heads out much later in the summer."

Another distinguishing feature for downy brome is the hairy leaf, which feels
furry to the touch. Japanese brome leaves are less hairy than downy brome, and
it is not as drought tolerant as downy brome. Blackshaw says that in Alberta
and Saskatchewan, Japanese brome is gaining in prevalence and is a little more
competitive than downy brome.

Control with a combination of cultural practices and
herbicides

Crop rotation plays a very important role in keeping downy brome under control.
Crop rotation prevents the build-up of the seedbank, but also provides opportunities
to control downy brome outside of winter cereal production. Trifluralin effectively
controls downy brome in oilseed and pulse crops, although these products are
out of reach of no-tillers because they require some incorporation.

However, in his research trials, Blackshaw says that some post-emergence grass
herbicides offer control of downy brome in oilseeds and pulses, although these
products do not have downy brome on their label. Blackshaw ranks Assure herbicide
with the best control, followed by Select, Poast and Venture herbicides. Edge
was also found to be effective in these trials, but again, it does require incorporation
and is not registered on downy brome.

Herbicides within herbicide tolerant systems, (glyphosate, Liberty, and Odyssey/Pursuit)
and Clearfield wheat systems (Adrenalin) also control downy brome. Glyphosate
is registered on downy brome. The others were observed to provide control in
Blackshaw's trials, but are not on the labels of those products.

"In areas of the US, where winter wheat is grown more frequently in the
rotation, downy brome is their number one problem," says Blackshaw. "We
recommend three to four years between winter wheat crops, and maintaining an
oilseed or pulse crop in the rotation to help keep downy brome under control."

In spring planted crops, glyphosate also plays an important role in post-harvest
and pre-seed applications. With heavier densities of downy brome, Blackshaw
says 0.5L/ac of glyphosate in the fall reduce the density, making a spring application
of 0.5L/ac to 0.75L/ac more effective. He says the higher rate should be used
if larger, overwintered downy brome is present, growing conditions are poor,
if the infestations include dense, sod-bound patches, or if the plants are covered
with dust.

Wentz says that controlling downy brome in adjacent areas, such as along field
borders and fence lines, also helps to keep it from spreading into fields. Banding
nitrogen, rather than broadcasting, helps to feed the crop instead of the shallow
germinating downy brome. As well, banded nitrogen helps to establish a healthier,
more competitive crop stand.

Thoroughly spreading chaff across the width of the windrow is another useful
cultural practice. This helps to ensure that downy brome weed seeds are also
spread uniformly, preventing the establishment of difficult-to-control sod patches.

Establishing a good crop stand can help to minimize losses due to downy brome.
Blackshaw says research shows that when downy brome emerged three weeks after
winter wheat, yield losses were two to five times greater than when downy brome
emerged six weeks after winter wheat.

"Time of emergence is critical," says Blackshaw. He recommends getting
the fall cereal off to an early, competitive start in the fall by seeding at
optimum seeding dates and using recommended seeding rates. Downy brome emerging
in the spring is less competitive, but can still reduce yields by up to 10 percent
at downy brome densities of five to 10 plants per square foot (50 to 100 plants
per square metre).

Wentz says that winter cereals provide good options for growers to reduce herbicide
use, increase crop diversity, lower production costs, and help protect the land
against wind and water erosion. "Winter cereals can add increased sustainability
to the bottom line, but to keep pests like downy brome from establishing, you
need to maintain diversity in the system." -30-