After the rain
This article was originally published on September 5, 2012 and expired on November 30, 2012. It is provided here for archival purposes and may contain dated information.
Between August 31 and Sept. 2, the remnants of
Hurricane Isaac dropped 2 to 6 inches of rain on the southern two-thirds of
Illinois. Rainfall over the past 6 weeks in over much of the southern half of
the state has ranged from one to several inches above normal.
University of Illinois crop sciences professor Emerson
Nafziger said that, "While the rain came too late to help corn yields and did
little to help soybean yields, it fell very nicely over 3 days. Most of it
disappeared without a trace, with creeks still dry, and no tile flow for the
moment. Moistened surface soil is the only sign that there has been rain."
In the driest parts of the state, the recent rainfall has
helped to restore soil water to more normal levels. At the U of I South Farms,
depending on rotation, soil moisture in the top 40 inches is now close to the
levels found in late May. Corn following corn appears to have taken in more
water, possibly due to larger, deeper cracks. This return to favorable soil
moisture lessens concerns about the drought carrying over to next year, though
as happened in 2012, rainfall during the season will still be the critical
According to the latest NASS report, as of Sept. 2, 63
percent of the corn crop was "mature" and 12 percent harvested while 41 percent
of the soybean crop was listed as "turning yellow" and 7 percent as "dropping
leaves." These numbers will continue to rise in the coming weeks, and both
crops have nearly finished adding yield, with corn a little ahead of soybeans.
Growing degree day (GDD) accumulations since May 1, when
planting was nearly complete, range from approximately 2,700 in northern
Illinois to 3,000 in the southern part of the state. GDD requirements for
hybrids grown in Illinois range from approximately 2,500 for very early hybrids
to 2,800 for late-maturing hybrids. Thus, GDDs in nearly every field have been
sufficient to mature the corn. Most producers are waiting for the corn to dry
The drought caused some cornfields to die, but those that
still have some green should be close to maturity (black layer). Nafziger said
that, as corn-grain moisture approaches 20 percent, the advantages to getting
the crop harvested tend to outweigh the disadvantages.
"Crop-insurance considerations have to be taken into
account, but low-cost drying offered by some elevators, weak stalks that may
fall in the next moderate wind, grain quality that can get worse as the crop
stands in the field, and the increase in harvest loss as grain dries below 18 to
20 percent moisture, all weigh on the side of early harvest," he
Reasons for delaying the harvest include the fact that
elevator shrink (typically 1.4 percent per point of moisture loss) exceeds
actual shrink (about 1.2 percent per point), meaning fewer bushels are sold
than are actually harvested. This loss, about 1 percent at 20 percent moisture,
diminishes as grain dries. Last, with soils near field capacity and hence
highly compactible in areas where there has been recent rain, it may pay to
wait for soils to dry a little before driving a heavy combine on them.
In most areas, soybean harvest has not begun. Some fields
are showing a lot of green color, in some cases because pods are still filling,
but in other cases the plants may be staying green because they have few pods
to fill. "If a soybean field was under a lot of stress early in the season,
check to see if it is filling a good number of pods," Nafziger advised.
Drought during the growing season can cause early senescence
even when there are not many pods, but the factors that determine when a crop
with a green canopy will end the seed-filling process are not well understood.
The cool nights in mid-August, the shortening days, and the the slowing of
sugar movement into the pods when seeds reach a certain size could also send a
Nafziger noted that the beginning of soybean yellowing was
fairly uniform across the state this year. As of Sept. 2, 40 to 50 percent of
the crop in the north and 50 to 60 percent of the crop in the south had turned
yellow. The fact that these amounts had no correlation with the percentage of
corn rated as mature implies that soybean senescence and maturity is affected
more by weather or day length than by stress levels.
While the official estimates and observations during crop
tours in recent weeks indicate that soybean yields will be relatively low, it
is difficult to make accurate projections. Samples are usually taken in one or
a few places in the field because soybeans tend to look uniform.
"In fact, yield monitors show that yields are often not as uniform
as the crop appearance would suggest," Nafziger noted. "Samples tend to be
small – only a few feet of row for plant counts, then a few plants of those for
pod counts – and may not be accurate, but taking enough samples to improve
accuracy may often not be worth the time and effort."
Another reason that it is difficult to make accurate yield
estimates for soybeans is that pod counts, and hence seeds per plant, vary
considerably across plants. "My only advice is that, if your counts and
calculations indicate the field will yield 70 bushels and you think from
experience that it will yield only 50, trust your experience," said Nafziger.
"It may make more sense to guess than to try to do pod counts and
Estimating pod numbers once leaves drop is usually a better
method than a lot of pod-counting when leaves are still green. "There's an art
to this, and experience counts a lot, but in general, stems that look as if
they are bristling with pods -- three to five pods per node on six to eight nodes
with pods sticking out away from stems -- and with pods filled at the top of
the stem indicate good yields, while two or three pods per node that do not
stand out away from the stem (hence aren't as well filled) and on fewer nodes
will mean lower yields," Nafziger said.
He added that a good place to practice on this is in strip
trials where information about actual yields after harvest is collected.
Questions remain about wheat planting following failed corn
fields and the need for fertilizer, especially nitrogen, in such fields.
Rainfall over southern Illinois has certainly moved much of the soil nitrate to
below the zone where the wheat roots would reach it early. Many producers are
chopping, tilling in, or rolling down corn residue, putting it in contact with
the surface soil. With warm soils and a month still before planting is expected
to begin in southern Illinois, it is likely that the levels of nitrogen tied up
in the surface soil will increase.
"Our earlier suggestion that wheat following corn shouldn't
need nitrogen at planting in dry areas this year is probably no longer a good
one now that most areas are no longer dry," said Nafziger. Hence, the advantage
that wheat following corn can retrieve some of the leftover soil nitrogen is not
an important consideration.
In some cases, planting wheat following soybean may be more
advantageous than wheat following corn. If wheat follows corn, Nafziger advises
using normal monoammonium phosphate
(MAP) or diammonium phosphate (DAP)
applications to supply some nitrogen and phosphorus, especially in fields that
show low or medium phosphorus levels.
"If the winter is dry, we might be able to adjust spring
nitrogen applications, perhaps based on soil-test nitrate in March," Nafziger
said. "Chances of that would appear to be small at this point, and with more
rain will get even smaller."
In short, because of the rain, normal procedures
should be used to manage wheat: approximately 1.4 to 1.6 million seeds per
acre, planted after October 3 to 5 in south-central and southwestern Illinois
and after October 7-8 in the southernmost parts, with serious attention to
uniformity of seed placement regardless of tillage or previous crop.
Source: Emerson Nafziger, Extension Specialist, Crop Production, email@example.com
Pull date: November 30, 2012