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Cover crops fit with low residue crops

Row crop farmers and specialists are taking advantage of cover crops to prevent erosion after their low residue crops.


November 19, 2007
By Helen McMenamin

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Cover crops are proving a worthwhile option for farmers who grow low residue
crops like potatoes and beans both before and after the high value crops. Seeded
after the main row crop, a cover crop can protect the soil. Seeded before the
main row crop, cover crops improve seeding conditions, reduce weeds and sometimes
reduce pests.

Some potato growers broadcast barley or oats after potato harvest. They usually
disc and cultivate lightly first to smooth the field after the heavy traffic
of digging and trucking, so this really works best for early varieties that
are off the field in August.

However, potatoes are often harvested in mid September or later, when it is
getting too late to seed a spring cereal. Erosion can be a problem over the
winter and spring on those fields. One approach is to seed the cover crop by
air before potato harvest and rely on harvest traffic to provide seed-to-soil
contact.

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According to Jason Bullock at Taber, pre-harvest seeding works well. "The
airplane spread a bushel of oats per acre very evenly," he says. "We
had good emergence even from seeds that were buried three or four inches deep."

For farmers, the big benefit is they do not have to spend time preparing and
seeding the fields during harvest, when labour is already stretched. The cost
of aerial seeding was under $10 an acre compared to $10 to $20 an acre for ground
equipment. "The fields look ugly as the cereal emerges because they're
so rough," says Bullock. "But, leaving the field ridged from potato
harvest helps keep the soil in the field where it belongs."

Rob Dunn, Alberta Agriculture conservation crop specialist, figures oats might
be preferable to barley because any seed that germinated the following spring
could be controlled with a wild oats herbicide. In practice, Dunn found oats
did not grow any better than barley.

The downside to seeding before harvest is that the land must be levelled before
seeding the following spring crop, exposing the land to wind erosion. The other
approach of seeding a winter cereal after the row crop harvest results in a
smoother seedbed that may not require additional spring preparation operations.
Winter wheat after an early harvested spring crop is a good erosion control
measure, but only if it is seeded early.

"The second week of September is within the recommended seeding period
for this area," says Dunn. "But it may not be early enough for winter
wheat to get the height that's needed to protect soil. The plants establish
roots and crowns so they can overwinter, but above ground, they're quite prostrate."

Dunn says that to protect soil after a low residue crop like potatoes, you
need to seed winter wheat with a system that leaves the ground quite rough.
A hoe-opener leaves ridges that do not break down entirely over the winter.
"Fall rye looks more promising than winter wheat for erosion control. It
grows faster than winter wheat in the fall and begins growth earlier in the
spring, giving the soil more protection. It can be tough to control with spring
tillage, but it's relatively easy with pre-seed glyphosate. It also seems to
suppress weeds, even after seeding a spring crop."

According to Agriculture Canada Lethbridge researcher, Bob Blackshaw, a fall
rye cover crop may fit particularly well before narrow row beans. The large
bean seed emerges well through the terminated rye crop, weed populations are
lower, herbicides are more effective and yields are good.

Dunn has seen the same benefits in row cropped beans. As a bonus, in 2006,
beans seeded into a rye cover crop were virtually free of white mould. Adjacent
conventionally seeded plots had moderate disease levels. Dunn is very cautious
about recommending the practice though. This was just one observation and may
have been a coincidence.

Other issues with pre-plant cover crops include soil moisture depletion and
increased frost risks for the subsequent crop in the spring. Soil cover changes
soil and air temperature dynamics. The sun warms dark bare soil during the day
and at night the soil radiates that heat back to the atmosphere, warming the
air around cold sensitive seedlings. Light coloured crop residue does not provide
this frost protection.

Dunn has not found any crops that can establish erosion protection after mid
September. Erosion control with late seeded cover crops only works with soil
roughness. "To hold soil, a cover crop needs to be seeded by mid September
in southern Alberta, earlier in many areas," he says. "The right seeding
implement can help, but lower heat units and, to a lesser extent, shorter days
cut into the plants' growth potential. On dryland, moisture availability is
important too, but heat is the main thing."

Anchored stubble is the best protection against erosion, especially if you
need protection from water erosion, says Dunn. "It takes living, or recently
living, fibrous roots to hold soil against water erosion, whether from snowmelt
or rainfall," he says. "After a heavy rain or a rapid snowmelt, look
in the ditch beside a chemfallowed field. The loose residue and soil you see
is only the tip of the iceberg. The roots of a crop that has been dead for a
year, or a year and a half, cannot hold soil against running water.

"For soil protection, rye is by far the best of the cereals. It covers
the ground the quickest, it's the most winter-hardy and it's easy to take out
with glyphosate in spring. It also holds nitrogen in the soil if there is a
lot of rain before the next crop can use it."

Concerns about volunteer rye from cover crops are not justified, says Dunn.
He suggests rye's reputation for weediness stems from combining losses, but
he advises caution if winter wheat is part of the rotation. -30-


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