April 22, 2004
From:
The
Bulletin
Integrated Pest Management
University of Illinois Extension
The news this week is that an
astounding 40% of the corn in Illinois was planted by April 18.
Given that April 19 (Monday) was a good planting day over
virtually the entire state and that many people report being
finished with planting corn, it is likely that more than 50% was
planted before rain on Tuesday, April 20 slowed progress in some
(but not all) areas.
One of the recurring questions
is whether or not to rework fields before planting if rainfall
prevents planting after a field was tilled and ready to plant.
We don't have current research on this, but we expect that the
answer is difficult to know with certainty, even if research
results were available. If the soil was in good shape and the
rainfall was such that the surface structure of the soil is more
or less unchanged, then tilling again before planting is
probably not necessary. One of the main problems with this is
that many people want to plant after the surface has dried but
before the soil beneath the surface is dry enough to plant into.
The result can be a lot of soil compaction, and even poor
seed-soil contact if the soil doesn't "flow" well enough back
into the planting furrow. In fact, the impulse to till soils
before planting is often to aid in surface drying, but even in
such cases, driving on soil that is too wet can damage the soil
structure and may disguise seed placement problems more than
prevent them. Whether you till or not, be sure to plant only
when you can do a good job of seed placement, without excessive
sidewall smearing and with good seed coverage. Tilled soils
often dry more slowly because tillage breaks capillarity, making
it harder to move water to the surface for evaporation.
Soybean planting has gotten
under way in some areas, with 1% listed as having been planted
by April 18. If current weather patterns hold, we should be able
to finish up by mid-May in most areas. This is an improvement
over the late planting of recent years; with the recent fall in
soybean prices, the market seems to see this as a positive for
production. As I indicated last week, our recent data show no
dropoff in soybean yields until planting is delayed past
mid-May. We also saw that later-maturing varieties were not
favored by earlier planting in most cases. Given the photoperiod
response in soybean, planting early does not translate directly
into earlier maturity at the end of the season, but it does
contribute; planting 10 days later delays maturity by about 3
days or so, though this varies some by actual planting date and
by maturity of the variety.
The growing conditions of 2003
resulted in variable, but often small, soybean seed size for the
2004 growing season. While newer planters may allow better
calibration of seeding rate by seed number rather than weight
per acre, setting exact seed drop rates is still a challenge.
The percentage of emergence and plant establishment also varies
considerably with different planting conditions, crop residue
amounts, soil temperature, and seed quality. Using 80% plant
establishment for row units and 70% for drill units has worked
for some people. Our research usually shows higher establishment
percentages than this, so it may be helpful to view these as
"safe" numbers, appropriate when emergence conditions are not
very good. I would suggest that individual producers start to
assess this a little closer for individual planters, and perhaps
to raise these percentages if plant counts versus dropped seed
numbers justify that.
Though seed number per pound
listed on bags tends to be somewhat approximate, it should be
accurate enough to establish dropped seed number per acre;
simply multiply seeds per pound times pounds of seed used per
acre. Take plant counts after emergence (waiting until about the
2-leaf stage to allow for loss of small, emerging plants); then
divide plants per acre by number of seeds dropped per acre, and
multiply by 100 to give emergence percentage. It is helpful to
record general emergence conditions and days to emergence, as
things that might influence emergence percentage. If you find
that plant counts are higher than you really need, even under
less than ideal emergence conditions, you might consider
"dialing back" seeding rates in the future, especially when
planting conditions are good. In fact, it can be useful to
adjust dropped seed numbers based on current planting
conditions. For example, if the soil temperature is above 60 and
soil moisture is ideal, it may be appropriate to use 90%, as the
expected establishment percentage, and to drop seeding rate
accordingly.
How many soybean plants do we
need per acre? This is an area where research findings often
seem to make little sense in production fields, but that's
mostly due to differences in approach to risk. That is, the
researcher may have an inadequate concept of establishment risk
and so might feel free to say that research shows that 100,000
plants per acre is adequate for maximum yields, then wonder why
the producer still plants 180,000. Part of the problem is also
that the 100,000 plants that the researcher establishes are
usually distributed uniformly, often in small plots, while the
producer has to plant whole, and often variable, fields. In
addition, low plant stands usually look bad early in the season,
only to expand out with extra space and yield as much as higher
stands. That's especially true of drilled soybean plants, where
looking down the row early in the season gives the "feel" of
inadequate stands compared to looking down rows with higher
plant counts per foot of row. We also have a tendency to worry
about low plant counts and more weed problems, though glyphosate
use has (or should have) diminished this concern.
Regardless of planting method
or what the early stand looks like, it is clear that stand
counts are often higher than they need to be. Given the fact
that yield usually does not decrease until soybean plant stands
exceed 250,000 or more plants per acre, especially in row widths
less than 20 inches, it is doubtful that most stands are high
enough to actually reduce yields. The unneeded plants do
represent extra seed cost, however, and can increase lodging in
some cases. In general, aiming for 150,000 established plants is
a "generous" goal and one that should provide adequate stands
even if emergence suffers, regardless of row spacing. That means
dropping 150,000 divided by expected emergence percentage,
including an adjustment for germination percentage. So, if we
expect 85% establishment and 92% germination, we need to drop
150,000 divided by (0.85 times 0.92) = 192,000 seeds. Data show
that 100,000 or even fewer plants will often maximize yields, so
those who consider themselves "lucky" can drop seeding rates. On
the other hand, dropping more than 225,000 seeds per acre is
probably never justified; if seed quality or planting conditions
are that poor, then planting ought to be delayed or different
seed located.
For a handy soybean seed drop
calculator, go to the "Soybean Seed Drop" decision aid at
http://www.ag.uiuc.edu/iah/index.php?ch=ch3/ in the online
Illinois Agronomy Handbook. This calculator asks the user
to consider some of what we've talked about here, and also
provides cost and number of units needed to plant individual
fields.--Emerson D. Nafziger |