Resurrected As Way To Drop Costs And Increase Yields
When commodity prices slide
downward, a farmer is faced with
two alternatives if he plans to
maintain his profits reduce
production costs or increase
yields.
By using parts of a program that
was standard practice nearly 40
years ago, a number of producers
have found a way to do both.
“Not long ago, crop rotations
were the cornerstone of any crop
production program,” says Garren
Benson, professor and Extension
agronomist at lowa State
University. “Any textbook dealing
with crop and soil management
dealt with the subject in great
length and extolled the benefits of
rotation.”
Among these benefits were the
ability of one crop to provide
nitrogen for the following crop,
improved insect, weed and disease
control and improved soil struc
ture and tilth. Crop rotations also
offered the potential to reduce
erosion and spread labor demands
in a time when farms were ex
tremely labor intensive.
Because forage-consuming
cattle and horses were part of the
scene, hay and pasture played an
important role.
However, as chemical fer
tilizers, insecticides and her
bicides became available at
reasonable prices, the yield
benefits of rotation became less
important. It appeared that
chemicals could be used as a
substitute for a rotation program.
Now evidence partly to the
contrary is piling up as
agronomists and soil scientists
prove that crop rotation still can
increase yields even if all other
factors are equal.
Crop Rotation Changes
“During the 50s and early 60s,
farmers were asking if crop
rotations were really necessary,”
Benson notes. “But in reality,
there were two parts to the
question. One was the question of
yield and profitability, and the
other was the long-term effect on
the soil.”
In answer to those questions,
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1950’s studies showed that a wide
range of cropping systems could
produce high corn yields, but
warned continuous com should
only be grown on land where
erosion could be controlled. In fact,
studies showed continuous com on
coarse and medium-textured soils
could be maintained at 95 to 100
percent of the yield of com in
rotation. Thus, the opinion was
that as long as adequate nitrogen
was supplied and com rootworm
was controlled, com yields were
nearly identical, regardless of the
previous crop.
“In some tests this was true,”
Benson adds. “But in others, it was
not. Also, most of the earlier
studies did not include soybeans,
which has since become the major
crop with which com is rotated.”
Whether the difference between
then and now is attributable to
cultural and cropping practices or
years of repetition, no one is really
sure. What is certain, according to
Benson, is evidence that rotation
does offer the majority of farmers
a yield advantage.
Advantages
“If one were to average studies
in the Midwest in the last few
years, a typical com yield
reduction for com following com
versus com following some other
crop would be about 10 percent,
with a five to 15 percent range
being typical,” he says. “This
assumes recommended rates of
nitrogen were used and other
management practices were at a
high level.”
Although the exact reasons for
yield increases associated with
crop rotation are hard to pinpoint,
there appear to be several factors
which interact, including soil
nutrient differences.
For example, soybeans are
considered to be worth the
equivalent of between 20 and 60
pounds of nitrogen per acre,
depending on the location, soil
type, yield, etc. Some experts
simply recommend reducing the
nitrogen application by one pound
for every bushel of soybeans
produced the previous fall.
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The crop rotation concept is regaining its former popularity, helping producers reduce
costs and increase yields.
Naturally, the equivalent value
of nitrogen provided by a forage
legume crop is even greater up
to 140 pounds per acre.
But recent studies also
demonstrate high rates of nitrogen
alone can not overcome the yield
differential caused by rotation.
Crop Benefits
“It’s interesting to note that com
isn’t the only crop to benefit from
rotation,” says Kent Crookston,
professor of agronomy at the
University of Minnesota.
In most cases, he explains, every
crop in the rotation program
produces higher yields. In other
words, com yields better when it is
rotated with soybeans, and
soybeans yield better when they
are rotated with com. Moreover,
the phenomenon is not limited just
to com and soybeans. Com yields
also improve when the crop is
preceded by sorghum, and
soybeans yield better when they
follow sunflowers.
“We’ve also just completed a
study which shows com will do as
well when it is rotated with fallow
as it will when rotated with
soybeans,” Crookston continues.
“So we feel the yield increases
have to be credited to something
besides the nitrogen contribution
of the previous crop. Com is ob
viously not contributing any
nitrogen to soybeans.”
As Crookston points out, the
theory for higher yields has
changed directions. Instead of
believing rotation produces
positive influences, researchers
now are theorizing that a change in
crops simply prevents the negative
influences which exist when a crop
is repeated year after year.
“A few years ago, we were
thinking soybeans were good for
com,” he explains. “Well, in a
sense they are. But to be more
realistic, com is bad for com, or
any crop is bad for itself.”
Rotation Mystery
The mystery surrounding yield
differences runs even deeper some
years, according to lowa State’s
Benson. Quoting research con
ducted at the University of Min
nesota, he says corn yields were
reduced by 39 percent in 1975 and
1976 for com following com versus
com following soybeans. In con
trast, the yield difference for the
same tests averaged only 11
percent in 1977 and 1978. Similar
differences have been observed at
the University of Illinois and at
lowa State University.
“Even though we might suspect
the benefits of nitrogen, soil
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moisture, reduced disease and
insect pressure for the typical
yield increases, we’re still not sure
how important each factor is to the
large yield differences," he says.
As a case in point, he notes that a
high percentage of the severe com
following com yield reductions
came from areas where com was
stressed due to lack of moisture in
July or August.
“Problems may be even worse
when com doesn’t have an ex
tensive root system when com
bined with low rainfall and high
temperature,” Benson says.
Soil Properties Change
“If root expanse is a key, then
anything that restricts roots must
be considered,” Benson says,
referring to changes in soil tilth
and the degree of compaction.
In addition to affecting the
amount of moisture and nitrogen
available to the plant, soil con
ditions appear to also affect the
way the plant meets its car
bohydrate needs. As a result, the
plant may be stressed to the point
it becomes more susceptible to
stalk and root rots.
Another soil property which
continues to be elusive to soil
scientists is the possible toxic
effects of leftover crop residue.
As Minnesota’s Crookston ex
plains, agronomists slowly are
discovering both the residue and
roots of a crop contain chemicals
which either inhibit or stimulate
seedling vigor. For instance,
researchers at lowa State
University have shown soybean
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