CONTINUOUS SOYBEAN PRICETAG UNCERTAIN

By TOM BUTTON

How badly will soybean yields suffer if farmers
leave corn out of their rotations for another year?
Lots of people are willing to guess, but there's
precious little research to document their gut
belief that continuous soybeans means lost profits.
"All our rotation studies are based on continuous
corn, not continuous soybeans," says University of
Guelph researcher Ken Janovicek. "When the tests
were set up in 1980, nobody ever dreamed we'd have
to face the prospect of continuous soybeans."
Even if researchers started a rotation experiment
today, it would take 10 years to get convincing
results, since continuous soys would have to be
compared to a corn:soy:wheat rotation over three
cycles, Janovicek says.
Still, the limited preliminary results available
suggest that continuous soybean production is
hurting soybean yields more than most growers
realize.
"We don't have a lot of data on yields, but we've
got a lot of observations on soils," says Doug
Young, who conducts rotation research at Ridgetown
agriculture college.
Young says soybeans cause quick physical damage. 
He agrees with cash croppers who complain that
soybeans make the soil 'tighter', and says it's
related to structural changes in the soil,
including a reduction in the stability of
aggregates that makes soils more prone to
compaction.
"As the soil gets degraded, it also loses the
ability to drain itself,"  Young says. "From a crop
point of view, that makes us more vulnerable to
root rots. From the soil point of view, it
increases the amount of field run-off, and that
means more erosion."
Young says the physical breakdown is related to the
lower amount of root growth in soybeans, and also
to the reduced amount of organic matter that a
soybean crop pumps back into the soil. A 150-bushel
corn crop produces four tons of dry stover per
acre. A 40-bushel soybean crop produces just one
ton of dry-matter residue per acre.
Young thinks the damage outweighs the benefits that
come from soybean production, including the fact
that soys are less likely to force farmers to work
wet soils in the spring or churn through muddy
fields at harvest.
"Maybe on the lighter soils you can get away with a
two-year corn:soy rotation," Young says. "But the
heavier the soil gets, the more you need a longer
rotation with wheat underseeded to red clover to
maintain structural stability and internal
drainage."
"There is a yield cost to soybeans after 
soybeans," says Bruce Luzzi, University of Guelph
soybean scientist. "We have to be concerned about
the build up of diseases such as white mould, and
also with the spread of soybean cyst nematode.
"I know we all have to pay attention to short-term
economics, but there are some serious long-term
problems in soybeans that we can avoid if we can
keep in a rotation," Luzzi says. "I for one am
hoping that the corn acreage will stabilize."  

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