Average Illinois soybean yield first exceeded 50 bushels per acre
in 2004, when it was 50.5 bushels. It was 51.5 bushels in 2010, and 50
bushels in 2013. Over the five years beginning in 2014, it was 56, 56,
59, 58, and, in 2018, an astonishing 65 bushels per acre. Yield in each
of the past five years was above trendline, which is a first—the longest
stretch of above-trendline yields in the previous 30 years was for
three years. In each of the three years 2004, 2014, and 2018, soybean
yield exceeded the previous record by about 10 percent. Illinois corn
yields were record-high in each of these three years as well, showing
that both crops tend to respond similarly to unusually good growing
Variety selection and maturity
Most people have already selected varieties for 2019. There are a lot of good varieties and a lot of information available on their performance. The University of Illinois soybean variety trials are only a small part of this information, but there are comparisons there that might not be available from other sources. Still, seed companies remain the primary source for information about the varieties they sell, and finding topnotch genetics shouldn’t be too difficult.
One issue that continues to attract interest is how varietal maturity
affects yield. In the UI variety trials, entries at each location are
separated in roughly equal numbers into two sets: one with longer- and
one with shorter-maturity varieties. These sets are planted in separate
trials, mostly so the early-maturing ones can be harvested first if a
long delay will compromise their performance. Averaged across 5 regions
and 13 sites in 2018, the “early” varieties averaged 74.8 bushels per
acre, and the “late” ones averaged 73.5 bushels per acre. So across a
range of maturities within and among regions running from north (average
MG 2.7) to south (average MG 4.2) in Illinois, maturity was not
consistently related to yield. We might want to choose a mixture of
earlier and later varieties to spread harvest some, but should
concentrate more on yield potential than on maturity.
We continue to hear a great deal of talk about the need to plant
soybeans early in order to get high yields. This is hardly a new
discovery: ever since we saw major improvements in seed quality and seed
treatments several decades ago, we have known that early-planted
soybeans were capable of emerging without the need to wait until soils
had warmed up to 55 or 60 degrees before planting.
In recent years some have taken “early” planting to an extreme,
however, with claims that soybean planting should some before corn
planting, as early in March if possible. Figure 1 summarizes the results
of 26 planting-date trials conducted in central and northern Illinois
between 2010 and 2016. Our target planting dates were in mid-April and
then about every two weeks to early June. Planting dates were converted
to days after April 1, and yields within each trial to percent of
maximum yield for that trial.
Figure 1. Combined results over 26 soybean planting date trials in central and northern Illinois from 2010 through 2016.
We saw little yield decrease when planting was done by May 1 (day
31), about 7% lower yield if planting was on May 15 (day 46), and 14%
lower yield if planted on June 1 (day 62). While we did not plant before
April 10 in any of these trials, the fact that yields were no higher
from planting on April 15 than on April 30 shows that the “early
planting” advantage is generally maximized if planting can be done by
the end of April.
It’s not clear what advantage there might be in planting soybeans in
March, or even, as some did in 2017, in February. Emerged soybean plants
are can tolerate low temperatures, with the exception of the few days
when the “hypocotyl hook” appears above-ground but before it straightens
(in response to light hitting its upper surface) to pull the cotyledons
above the soil surface. If frost hits at this point, the exposed hook
(stem tissue) can be killed, which kills the seedling. Seedlings are
usually in slightly different stages down the row, so frost at this
stage will seldom kill all of the seedlings, but it can certainly thin
Soybeans planted in March 2018 encountered cold, wet conditions,
including several snow events, during the month after planting. While
any emergence under such conditions testifies to the toughness of
soybeans, it’s likely that many of these were replanted. Besides stand
loss, soybean plants exposed to low temperatures early in the season
typically stay short, and often do not yield as much as later-planted
soybeans. This shortening might have been partially reversed by
increased internode elongation during very warm May weather in 2018.
Still, soybeans planted in late April in 2018 also made rapid growth in
May and had better stands, so probably yielded more than those that
survived March planting. The goal of planting early is not to have the
crop survive, but to have it yield more. Low stands and short plants
aren’t generally conducive to highest yields, and issues with crop
insurance coverage may be another disincentive. There certainly seems to
be little reward for taking the risk of planting very early.
Should later-maturing varieties be planted first in order to take
maximum advantage of the longer time in the field? There’s no problem
with doing that, although early planting moves up harvest date some, so
works counter to the goal of spreading harvest time by using different
maturities. In 2018 we ran a trial at Urbana, supported by a seed
company, to see how varietal maturity affected response to planting
date. The first planting date was April 26, the last was June 6, and
varieties ranged in maturity from MG 2.3 (very early for this location)
to MG 3.6, which is a little later than average for this location.
2. Response to planting date by five soybean varieties differing in
maturity. Data are from a trial conducted at Urbana, Illinois in 2018.
For all but the earliest-maturing variety in this trial, the planting
date response was almost perfectly linear, with the loss of nearly
7/10ths of a bushel per day of planting delay—a total of more than 27
bushels—over the 41 days from the first to the last date (Figure 2).
This loss rate accelerated a little for the latest-maturing variety
between May 24 and June 6. The earliest-maturing variety lost only 17
bushels from first to last planting, but only because its yield at the
earliest date was so much lower than yields of the later-maturing
The month of May 2018 was much warmer than normal, and this got the
soybean plants off to a very fast start. Warm nights are conducive to
early flowering, and this was especially notable in 2018. In the
early-planted crop, first flowers appeared in early June, well before
the longest day of the year, and unlike the interruption of flowering
that often takes place under normal night temperatures for about a week
before and after the longest day, flowering was early and continuous in
2018. As a result, nearly half of the Illinois soybean crop was
flowering by July 1. The warm May probably affected the yield response
to planting date as well; with warm temperatures, early-planted soybeans
as fast as late-planted ones, and this widened the developmental gap
between the different plantings.
Planted on April 26, the earliest variety reached first flower on
June 9 and matured on August 28, compared to June 15 and September 17
for the latest-maturing variety. When planted on May 24, the earlier and
later varieties flowered on June 15 and July 2, and matured on
September 12 and September 25, respectively. So when planted late, both
varieties flowered very early in their life cycles, both spent less time
in reproductive stages than when they were planted early, and they
ended up yielding about the same. While in this case it’s accurate to
say that the later-maturing variety benefitted more from early planting,
that’s only because the early-maturing one was physiologically less
able to use the longer growing period allowed by early planting to
produce high yield.
Seed and seeding rate
Seed quality as measured by the standard warm germination results is
probably good in most cases this year, but there were enough issues
related mostly to harvest time weather last fall that vigor—the ability
to produce good stands under stressful soil conditions—may be lower than
in some previous years. Vigor is commonly measured by the “cold test”,
which consists of keeping the seed in soil at 50 degrees for several
days followed by a standard warm germination test. Cold test results can
be somewhat hard to interpret, but most of the time, actual emergence
is higher than the cold test score. Many companies take cold scores and
might be willing to share them for seed lots this year.
Although promoting low soybean seeding rate for high yields seems to
have cooled a little, some still contend that planting 100,000 seeds per
acre is enough. We have conducted small-plot seeding rate trials each
of the past four years at six or seven sites per year around Illinois.
We plant rates of 50, 100, 150, and 200 thousand seeds in replicated
trials, and report yields by actual plant stands. Figure 3 below shows
the results from the 2018 trials. Plant stands as percentage of dropped
seeds were high in 2018, and yields were high to very high. Optimum
stands ranged from 86,000 to 201,000 plants per acre.
seeding rate responses at seven Illinois sites in 2018. The yellow
triangle on each curve marks the optimum stand, where added seed just
paid for itself in higher yield.
The wide ranges of optimum plant stands and yields in 2018 were not
unusual; across all 27 trials over the past four years, optimum stands
ranged from 50,000 to 201,000, and yields at optimum stands from 53 to
90 bushels per acre, with no discernible correlation between yields and
stands (Figure 4). How do we deal with such variability across sites? We
certainly can’t plant enough seeds to make sure we never have too few
plants—that would take planting more than 200,000 seeds per acre. With
no correlation between yield and seeding rate, we also can’t reasonably
adjust seeding rate by expected yield, and we can’t know actual yield. A
reasonable approach is to cover most cases by planting to produce
stands of about 120,000 plants per acre, which is close to the stand
needed to maximize return to seed (gross return minus seed cost) in the
majority of responses.
Figure 4. Optimum plant stands and yields at those stands across 27 sites in Illinois, 2015-2018.
How many seeds do we plant if we want to end up with 120,000 plants?
Over the trials reported here, plant stand as a percentage of dropped
seeds averaged in the low 80s for the first three years, and was close
to 100% in 2018. We normally divide the desired stand by the warm
germination (percentage expressed as a fraction, as in 0.95), and then
by a percentage that, based on soil conditions and seed placement, we
reasonable expect to produce a stand. Planting seed with high
germination percentage into good soil conditions with a planter that
places seed well, it may be reasonable to expect 80% establishment. If
we want 120,000 plants, that would mean planting 120,000 ÷ 0.80 =
Should we raise seeding rates to avoid having to replant if we’re
planting into cool soils the last week of April? We may want to raise
them a little if the cold test percentage is marginal. But the most
common cause of having to replant is heavy rains after planting that
leaves seeds without oxygen in the soil, causing seed death and very low
stands. In few cases like this does raising the seeding rate increase
the stand enough to avoid replanting.
On soils heavier than loam in texture, it is unlikely that adding nitrogen fertilizer at planting, or during the season, will increase yields. On lighter soils, adding urea at planting might increase early growth and yields, but the chances of this happening are not high. There has been some data from Iowa and Indiana suggesting that adding sulfur might help soybean yields, especially (but not only) in lighter soils. As with N, we don’t have good guidelines on when S might be needed, but we do not think that the need is routine. Ammonium sulfate broadcast at planting at a rate of about 20 lb. of S (83 lb AS) can supply both N and S, but is not inexpensive. Keep in mind that most of the outstanding soybean yields in 2018 were produced without adding N, S, or other products not known to be needed for high soybean yields.
Source: University of Illinois