Estimating Yields Of Stressed Corn
URBANA, ILL.
Even as the Illinois corn crop continues its
rapid development, with 8 percent in dent
stage by July 22, the crop rating continues
to decline; on July 22, only 7 percent was rated
as good, none was rated as excellent, and 66
percent was rated as poor or very poor. It’s of
little comfort, but at least we’re getting close to
the end of the slide in ratings, since they can’t
get much worse.
Having so little of the corn crop rated as good
and so much rated as poor highlights the pressing
question of how much crop there might be
to harvest in 2012. Those who drive past fields
can’t fail to note that, at least in areas where
there has been some rain, and at least in the
morning before leaves wilt in the afternoon heat,
the crop does have some ears and it retains
some green leaf area. Do such fields, many of
which are likely rated as poor, really have little
chance of producing a yield that’s worth harvesting?
Some reports from recent tours and visits to
fields in central Illinois, where good and poor
fields are often close to one another this year,
suggest that the ratings might be lower than the
crop’s potential. It is true that actually achieving
the yield potential that exists today will take
some rain, so to the extent that a poor rating reflects
uncertainty about a return to better conditions,
this might be reasonable. In dry areas,
canopy deterioration continues or is complete,
with leaf area dead or dying as plants deplete
the soil water supply. We already know that
many such fields will produce no grain.
In fields where no green leaf area remains but
some kernels have been set and remain active,
the stalks will be depleted of sugars and nutrients
until the kernels stop growing due to lack
of nutrients or death of conductive tissue. In
most such fields, the stalks have not been able
to accumulate much sugar or nutrients. One
exception might be nitrate, which can remain
as nitrate in stalks because there's not enough
energy to convert it to ammonium and then
amino acids, and there's little demand for
amino acids anyway due to leaf and plant deterioration.
Stalk quality is a concern when stalk nutrients
are depleted to fill kernels, as often happens
under dry conditions. Stalks are likely to
be weak this year, but with small ears, there
won’t be much need for strong stalks in
stressed fields. Even so, lodging may start in
fields that died prematurely, even before grain
has dried down much.
In fields with green leaf area and some kernels
set, where leaves are active (not rolled or wilted)
for half of each day or so, it might be possible to
get a good idea now of how many kernels are
likely to fill. How well they will fill will remain in
doubt, but with growing degree days continuing
to accumulate rapidly, the earliest-planted corn
should now be in dough stage, with some starting
into dent. According to our estimates in the
Illinois Agronomy Handbook, the milk stage (R3)
is reached at about 1,925 GDD, dough stage
(R4) at 2,190 GDD, and dent stage (R5) at 2,450
GDD.
At Urbana, corn planted on April 1 has accumulated
about 2,140 GDD, and since March 15,
when the earliest corn was planted, some 2,350
GDD has accumulated. So while dry soils have
accelerated leaf death, plant drydown, and the
maturation process in general, corn that is now
at R4 in central Illinois is not far ahead of where
GDD totals suggest it should be.
As a general indicator, the more advanced
that kernel development is compared to where
GDD accumulations predict it should be, then
the more stress the plant has endured and the
lower its yield potential. There are already a few
reports of grain harvest having taken place in
southern Illinois, and as we might expect, yields
with such early plant death and drying will be
low. In severely stressed areas, we can see fields
with no green leaf area left. In such fields where
ears are present, either kernel number or kernel
size, or both, will be small.
Estimating yield potential starts, as always,
with an estimation of kernel number. With so
much variability within stressed fields this year
– for example, low areas might have ears and
higher areas might have none – getting an accurate
estimate of ear number is nearly impossible.
While such obviously low yields may not
really need estimating, it may be possible to use
a previous yield or topographic map to estimate
how much of the field might have ears and to
sample only in those with ears, writing off the
rest after confirming the lack of ears there.
To get kernel counts, first count the number of
ears in 1/1000 of an acre (17 feet, 5 inches in
30-inch rows). If most plants have ears but ear
size is highly variable, select five or six ears (instead
of the usual three) to represent the range
of ear sizes from the row section. Count kernels
per ear, average these counts, and multiply by
the number of ears (with kernels) to get number
of kernels per 1/1000 of an acre.
To estimate yield, divide the number of kernels
in 1/1000 of an acre by the number (in
thousands) of kernels expected to be in a bushel
at maturity. In recent years we have been using
the number 80 (thousand) kernels per bushel,
which under good conditions is reasonable. But
this number can range from less than 60 to
more than 120, and it’s almost impossible to
guess what it might be at maturity for a particular
field when we do not know how long stress
will last.
If there's a fair amount of green leaf area and
kernels have already reached dough stage,
using 90 might be reasonable. It typically doesn’t
help much to try to estimate depth of kernels
at dough stage, when kernel depth is
typically rather shallow anyway, especially if
there are 16 or more kernel rows on the ear. If
green leaf area is mostly gone, however, and
kernels look like they may be starting to shrink
a little, kernels may end up very light, and using
120 or even 140 might be more accurate.
While I do think there will be a corn crop in
Illinois – and even a good crop in some areas –
the only way to get an idea of potential yield in
a given field is to visit it to make an objective assessment.
In a field where the crop has dried up
prematurely and has few if any ears, there won’t
be many surprises, and a visit may not be
needed. But in some fields, the ability of the
crop to produce grain even under stressful conditions
might be surprising. Δ
DR. EMERSON NAFZIGER: Extension Specialist,
Crop Production, University of Illinois