Short Corn, Short Yields?
URBANA, ILL.
The Illinois corn crop condition continues to
deteriorate, with less than 40 percent now
rated as good to excellent on June 24, according
to University of Illinois crop sciences
professor Emerson Nafziger.
On the positive side, the current corn crop has
good color, is mostly disease-free, and has uniform
stands with few drowned-out areas. These
factors will contribute to increasing kernel set
in fields pollinating now, at least where there is
enough soil water. Cooler weather this past
week has prolonged the period of adequate
water, and cooler nights reduce respiration,
thus helping the sugar supply.
On the negative side is the lack of rainfall,
with deficits for May and June ranging from 1 to
6 inches in different parts of Illinois. On June
26 the U.S. Drought Monitor showed the entire
state as dry, with most of the state in moderate
to severe drought and the southern counties in
“extreme drought.”
As the crop enters the critical yield-producing
stage, many are wondering about the effects the
lack of soil water has had, and will have, over
the next weeks. “The 2012 corn crop is well rooted,
healthy, and tough, but it’s unrealistic
to expect it to continue to thrive as the soil
water supply continues to decline in dry areas,”
Nafziger said. Plants that are unable to take up
enough water to keep leaves from rolling in the
afternoon are not fully productive, and this
takes a toll on the crop.
“As of June 24, 17 percent of the state’s corn
crop was pollinating, the highest percentage for
this date on record,” he continued. “This week
we would expect most of the crop that was
planted by mid-April in central and southern
Illinois to begin to pollinate, bringing the number
by July 1 to perhaps 40 percent.”
While pollination is the most critical period in
terms of yield potential, breeding for aggressive
emergence of ear shoots and silks has considerably
lessened the likelihood that pollination
will fail completely. However, the number of kernels
set may be lowered on plants that have
been undergoing stress from dry soils, and the
number of fertilized kernels that survive the
weeks after pollination may continue to decline
if the weather stays dry.
Observers in the fields note that corn is entering
or approaching pollination while plants are
shorter than normal. This raises questions
about the connection between plant height and
yield.
“Plant height is the best visible indicator of
how well the plant has been able to take up the
water it needs to expand cells,” Nafiziger explained.
“Cell expansion is sensitive to water
supply, so shortened internodes are one of the
first things we notice on plants that have struggled
to take up enough water to keep growing.”
This year, many fields have plants only 5 to 6
feet tall at tasseling, several feet shorter than
normal. These plants may grow some after tasseling
but will reach full height by the end of
pollination. Some of the fields in the driest areas
have tried to pollinate while the plants were still
very short.
Most of these fields will produce low yields;
some may produce no yield at all. Short plants
may not be able to form the complete canopies
needed for maximum yield because, if they have
had trouble getting enough water to elongate
their stalks, they may have shorter-than normal
leaves.
Even if the leaf area is normal, leaves may be
stacked more closely together on short stalks,
allowing for less interaction among neighboring
plants and less flexibility of leaf movement.
Hence, the plants’ ability to form the complete
canopy that is needed to intercept nearly all of
the sunlight is reduced. This problem is coupled
with ongoing water stress that limits photosynthetic
rates.
Is there anything we can do to help the crop
get through this dry period?
Not much, according to Nafziger. “When water
is clearly the major limitation to plant function,
we would expect little or no response to anything
we can apply that’s not water.”
As an example, he does not think that applying
fungicides to reduce respiration and increase
the plant’s sugar supply is likely to help
much.
“Strobiluron fungicides do act by reducing respiration,
some of which is considered wasteful,”
he explained. “But plants that are not photosynthesizing
well do not have much sugar to
respire away, so reducing respiration probably
won’t do much good.” Moreover, fungal diseases
that would respond to fungicides are not a
threat in most fields today.
By the same token, applying products said to
reduce the “ethylene effects” in stressed plants
is unlikely to have a positive effect when there is
not enough water to keep open the stomata,
which they need to do to allow photosynthesis
to take place.
Protecting the crop from anything that reduces
effective leaf area, such as applying insecticides
if enough insects are present to do
damage, can help the corn to retain its potential
to fill grain if there is rainfall. Foliar nutrients
are unlikely to be of much benefit, and the good
canopy color in most fields indicates adequate
nutrient levels.
While the focus has been on corn, soybean
plants are also showing stress effects. Soybeans
planted around April 20 at Urbana are now
about 24 inches tall and at stage R2 or full
flower. With fair-to-good growth and warm temperatures,
soybeans are moving quickly into
flowering, with 11 percent blooming by June 24.
“An early start to soybean flowering is generally
positive, but we remain concerned about
how water shortages might affect soybean pod
formation,” Nafziger said.
The period over which new flowers appear will
last for up to a month as the soybean plants
continue to increase node numbers and stem
height, and can even recur if stress is relieved
after that. This longer flowering period makes
the soybean crop better able to set pods and to
start filling seeds even if there is some stress
during July.
“But if we continue with little or no rainfall,
abortion of flowers or of pods will likely continue,”
said Nafziger. As with corn, applying materials
promoted to reduce stress in soybean is
not likely to do much good as long as water supply
remains inadequate. Δ