Soybeans: Planting and Crop Development
| Corn |
Soybeans | Forages
| Cereals | Dry
Edible Beans |
| Spring and Winter Canola
| Other Crops | Soil
Management |
| Soil Fertility and Nutrient
Use | Field Scouting
|
| On-Farm Stored Grain Management
| Weed Control |
| Insects and Pests of Field
Crops | Diseases of Field
Crops | Appendices |
Pub 811:
Agronomy Guide > Soybeans
> Planting and Crop Development
Order OMAFRA Publication
811: Agronomy Guide for Field Crops
Seed Quality
It is important to know the quality of the seed being planted. Certified
seed must meet purity and germination standards. The quality of common
seed is not known unless the germination is tested at an accredited seed
lab prior to planting (see Ontario Laboratories
Offering Custom Seed Germination Testing).
Viability and Deterioration
Germination is the major quality consideration used in grading seedlots.
It is the ability of a seedlot to produce normal seedlings under favourable
conditions of 95%-100% humidity and 25°C. Stress conditions in the
field following planting often reduce field emergence compared to that
in the lab.
A better measure of the ability of seed to emerge rapidly and uniformly
under a wide range of conditions is the vigour rating of the seed, called
the vigour test, or more appropriately referred to as a stress test. Certified
seed standards require that seed be tested for germination. In addition
to germination, many seed distributors routinely test and report seed
vigour.
Figure 2-1, The Relationship Between Seed Vigour,
Viability and Deterioration, illustrates the relationship between germination
and vigour. As seed deterioration increases, germination drops slowly,
whereas vigour drops very rapidly.
With Lot A, deterioration is minimal and germination and vigour are similar.
On the other hand, Lot B has excellent germination but low vigour.
A number of factors can contribute to loss of seed vigour, including genetics,
disease, mechanical seed damage, and deterioration in storage and weather
conditions prior to harvest. The most important factor affecting vigour
appears to be environmental. Time-of-harvest studies conducted by the
University of Guelph suggest that vigour is lost if there is a delay between
physiological maturity and harvest. Timely harvest is important if soybeans
are being grown for seed.
Figure 2-1. The Relationship
Between Seed Vigour, Viability and Deterioration
Source: Delouche and Caldwell, 1960.
Inoculation
Biological N fixation converts gaseous nitrogen in the air (N2) to a
form of nitrogen the plant can use, namely ammonium (NH4+). In legumes,
symbiotic nitrogen fixation occurs when rhizobia bacteria invade the root
hair and form a nodule. The process of adding soybean rhizobia (Bradyrhizobium
japonicum) to the soil is called "inoculation." The rhizobia
receive a protected growing environment, carbohydrates, and minerals from
the plant and in turn provide the plant with nitrogen. A 3.4 t/ha (50
bu/acre) crop of soybeans will remove 210 lb/acre of nitrogen. Some of
this nitrogen comes from residual nitrogen in the soil, but between 40%-75%
will come from the nodules depending on how much soil N is available.
Inoculants can be applied "on farm" at planting time or as "pre-inoculants."
Pre-inoculants are formulated to allow the bacteria to survive on the
seed, making it possible to inoculate the seed well before planting. These
products are usually applied by a commercial seed treater and are compatible
with many fungicide and insecticide seed treatments. In trials conducted
by the University of Guelph, pre-inoculants show similar efficacy to inoculants
applied at planting time.
The majority of products now available use a sterile peat-based carrier
or a liquid formulation. Sterile-carrier inoculants use a powdered peat
base that is sterilized prior to the addition of the inoculant strain.
These inoculants carry much higher numbers of rhizobia than the older,
non-sterile powdered peat. Non-sterile powdered peat often contains microbial
contaminants, which may compete with the rhizobia.
When soybeans are grown on land for the first time, inoculation with soybean
rhizobia is essential for high yields. The use of two different products
or at least two different lots of the same product can improve the chances
of good inoculation.
Good seed coverage is required for maximum efficacy of any inoculant.
When applying "on farm," apply inoculants at the base of a brush
auger when loading the planter. Kits that hang on the side of a truck,
tote or gravity wagon are available from dealers. Occasionally, some growers
have experienced bridging in the planter or build-up in augers from over-application
of liquid seed treatments or inoculants. Simultaneous application of a
low rate of peat is one option to reduce bridging.
Some seed treatments and liquid fertilizers can negatively impact inoculant
performance. When using an inoculant, check the label to confirm how long
the inoculant will be viable on the seed if applied with a seed treatment
or mixed with a liquid fertilizer.
Inoculant is not essential where a well-nodulated, dark-green soybean
crop has been grown in the past. Exceptions are acid soils (pH below 6.0),
sandy soils and fields with poor drainage that have been flooded for an
extended period of time. Under these conditions, inoculation is recommended
for each soybean crop. A grower who is not certain that previous soybean
crops were well nodulated should inoculate to avoid the possibility of
poor nodulation. Ontario trials indicate a 0.1 t/ha (1.5 bu/acre) yield
increase by inoculating soybeans planted into fields that have previously
grown well-nodulated soybeans. Even in the absence of a soybean crop,
soybean rhizobia will survive in most soils for 7-10 years and in some
fields for over 50 years.
Studies have shown little success in attempting to replace existing strains
of rhizobia in the soil with newer, more-effective strains. Once a strain
of rhizobia has become established in the soil, it will out-compete any
new strain that is introduced on the seed.
Manure or commercial nitrogen fertilizer applied to soybean fields supplies
a readily available supply of nitrogen, which soybeans will use prior
to that provided by the rhizobia. In these fields, nodulation may be delayed,
but yields are not generally reduced. On first-time soybean ground where
manure or commercial N is applied, nodulation may not occur, and unless
soil nitrogen is abundant, nitrogen deficiency may be observed late in
the season.
Table 2-7. Effect of Planting Date on
Yield and Plant Height
| Planting Date |
Yield
t/ha (bu/acre)
|
Percent of Full Yield (%) |
Plant Height
cm
|
|
May 10
|
3.23 (48)
|
100
|
104
|
|
May 24
|
2.96 (44)
|
92
|
93
|
|
June 7
|
2.75 (41)
|
85
|
89
|
| Source: Bohner, OMAFRA, 2004-06. |
Soybean roots normally become infected with Bradyrhizobium japonicum shortly
after emergence. Nodulation of soybeans may be observed 2-3 weeks after
planting. Checking fields at this point will allow time for nitrogen application,
should an inoculant failure occur. In first-time fields, nodules will
be located on the taproot. In previous soybean fields, nodules will also
be found along lateral roots
Seven to 14 nodules per plant indicate adequate nodulation at first flower.
Soybeans often go through a period when leaves are light green or even
pale yellow. This is the period just before the nodules start to supply
adequate nitrogen to the leaves and is an important phase in the development
of a healthy crop. Once the nodules have established and start providing
nitrogen, the leaves will turn a dark-green colour. If proper nodulation,
sufficient nutrients and moisture are present, soybeans will remain yellow
for only 7-10 days.
Planting Date
Planting date is an important management tool to maximize yield potential.
The highest yields of soybeans are obtained from early plantings, generally
the first 10 days of May. Later plantings are likely to incur significant
reductions in yield as shown in Table 2-7, Effect
of Planting Date on Yield and Plant Height.
Soybeans are more sensitive to soil temperature than corn. However, if soil
temperature and moisture conditions are suitable for planting corn, they
are generally also suitable for soybeans.
A hard spring frost can kill early-planted soybeans, since the growing point
of the emerged seedling is above the soil surface. However, soybean plants
can withstand temperatures as low as -2.8°C for a short period of time,
while corn experiences tissue damage at -2°C.
Table 2-8. Row Spacing vs. Days to Full
Canopy (May Planting)
|
Row Spacing
|
Days to Full Canopy |
Planting
Before May 15
|
Planting
After May 15
|
|
18 cm (7 in.)
|
30
|
25
|
|
38 cm (15 in.)
|
45
|
40
|
|
51 cm (20 in.)
|
55
|
50
|
|
76 cm (30 in.)
|
70
|
65
|
Delayed Planting
When planting is delayed, fewer days are required for the plant to reach
maturity. A one-month delay in planting results in a 9-day delay at maturity.
Delayed planting can reduce the vegetative growth period. This results
in shorter plants with lower pods. Late planting also reduces the number
of pods per plant because of the shorter flowering period. Planting date
also has some effect on the duration of the pod-filling period.
A 3-day delay in planting date generally results in a 1-day delay in maturity.
When planting is delayed beyond June 15, reduce the estimate of heat
units available for crop growth by 100-200 CHUs. Planting after July 1
has largely been unsuccessful in Ontario. If planting must be delayed
past July 1, select a full-season, light-hilum variety. An early frost
may cause dark hilums to "bleed" into the soybean. Planting
a short-day variety late in the season will result in extremely short
plants with few pods.
Improve vegetative growth of late plantings by selecting taller varieties
and planting in narrow rows. Using wide rows when planting late will lead
to reduced yield potential. Increase seeding rates by 10%. This will increase
the height of the lowest pods as well as the number of pods per acre.
Double Cropping Soybeans
Occasionally, a small number of soybean growers in the southernmost regions
of Ontario have attempted to grow soybeans immediately following the harvest
of their winter cereal or pea crop. Unfortunately, double cropping of
soybeans in Ontario is often unsuccessful. Do not take out a good red
clover stand to double crop. The benefits from the clover stand will outweigh
the risk involved in a double crop venture. Do not attempt double cropping
if soybean cyst nematode is a problem in the field. The soybean crop will
reduce the benefits of the non-host (winter cereal) crop and increase
cyst populations.
Table 2-9. Effect of Row Width on Yield
| Soybean Row Width |
Yield1 |
|
18 cm (7 in.)
|
3.3 t/ha (49 bu/acre)
|
|
36 cm (14 in.)
|
3.2 t/ha (47 bu/acre)
|
|
53 cm (21 in.)
|
3.0 t/ha (45 bu/acre)
|
|
71 cm (28 in.)
|
2.7 t/ha (40 bu/acre)
|
1 Values are based on research on clay loam soils in the 3,250-CHU
area. Greater response would be anticipated in shorter season regions.
The response to row-width reductions is reduced under stressful growing
conditions.
The following management tips will increase the chances of a successful
double crop:
- At harvest, leave approximately 20 cm (8 in.) of stubble to promote
soybean stem elongation and higher pod set.
- Plant immediately after a timely cereal or pea harvest.
- Plant 1 cm (1?2 in.) into moisture, but do not plant deeper than 7.5
cm (3 in.).
- Select tall, clear-hilum, full-season varieties if possible.
- Plant in narrow rows using high seeding rates.
Row Width
Soybeans grow well under a wide range of row widths in the long-season
regions of Ontario. The choice of row width depends on factors such as
tillage system, equipment suitability, weed problems, soil conditions,
white mould pressure and planting date. Most soybeans grown in Ontario
are solid seeded (19 cm or 7.5 in. spacing) or intermediate row widths
(38-56 cm or 15-22 in.).
Wide rows allow for inter-row cultivation and are less affected by soil
crusting. Narrow rows allow the crop canopy to fill in more quickly, providing
maximum light interception Table 2-8, Row Spacing
vs. Days to Full Canopy (May Planting), shows relative time differences
to canopy cover. Quick canopy development also contributes to weed suppression.
On heavier soil types such as clay, wider row widths increase the number
of seeds per foot of row, which can aid in emergence. On clay soils prone
to crusting, a minimum row width of 38 cm (15 in.) has shown better emergence
than solid seeded beans. Improved air movement in wider rows will also
help reduce the severity of white mould.
The increase in yield potential from growing soybeans in narrow rows is
greatest in the short-season areas. The yield advantage decreases towards
Southwestern Ontario. Row widths of 18 cm (7 in.) are recommended in short-season
areas (less than 2,800 CHUs).
Table 2-10. Solid Seeded vs. Planter
Unit Yields
Seeding Rate
Seeds/ha
(seeds/acre)
|
19-cm (7.5-in.) Drill
t/ha (bu/acre)1
|
38-cm (15-in.) Planter
t/ha (bu/acre)
|
|
370 500 (150 000)
|
2.92 (43.4) b
|
3.03 (45.0) ab
|
|
494 000 (200 000)
|
3.11 (46.2) a
|
3.12 (46.4) a
|
| Least Significant Difference (P = 0.05) = 2.2 bu |
| Source: Bohner, OMAFRA. |
1 Values followed by the same letter are not significantly
different.
Grow fields prone to white mould in row widths of 38 cm (15 in.) or greater,
even in short-season areas. In Southwestern Ontario, there may still be
some yield advantage in reducing row widths to less than 53 cm (21 in.),
as noted in Table 2-9, Effect of Row Width
on Yield, although this effect is less consistent than it is further
north. Row widths of 38 cm (15 in.) have gained popularity because they
allow a reduction in seeding rates compared to 19-cm (7.5-in.) rows but
still provide excellent yield potential.
Table 2-10, Solid Seeded vs. Planter Unit
Yields, shows the yield impact of drilled, solid-seeded stands versus
wider rows using planter units. The negative yield impact associated with
slightly wider rows is offset by the more accurate seed placement when
using a 38-cm (15-in.) planter compared to a drill. When using very low
seeding rates, the planter will outperform the drill.
Seeding Rates
Soybeans will yield well over a wide range of seeding rates. Plants will
compensate considerably for differences in stands without impacting yield.
Too high a seeding rate adds unnecessary seed costs and may increase lodging
and disease. Soybeans should be planted based on seeds/ha (seeds/acre)
not simply by the kg/ha or lb/acre. For most soil types, there is no significant
yield advantage to seeding rates over 494,000 seeds/ha (200,000 seeds/acre)
as is shown in Figure 2-2, Soybean Yield Response
to Seeding Rates. Higher seeding rates (10%) are required for maximum
yield potential on heavy clay soils or when using poor quality seed.
Table 2-11. Recommended Soybean Seeding
Rates
Seeds/
Kilogram
|
Seeds/
Pound
|
Row Width cm (in.) |
| 19 (7.5) |
38 (15) |
56 (22) |
76 (30) |
| Seeds/hectare (seeds/acre)
|
| 480,000 (194,000) |
437,000 (177,000) |
425,000 (172,000) |
400,000 (162,000) |
| Number of Seeds/m of
row (per ft of row) |
| 9 (2.8) |
17 (5.1) |
24 (7.2) |
30 (9.3) |
| Seeding Rate
kg/ha (lb/acre)1 |
|
4,400
|
2,000
|
109 (97)
|
99 (89)
|
98 (86)
|
91 (81)
|
|
4,900
|
2,200
|
98 (88)
|
89 (80)
|
88 (79)
|
82 (74)
|
|
5,300
|
2,400
|
91 (81)
|
82 (74)
|
82 (72)
|
76 (68)
|
|
5,700
|
2,600
|
84 (75)
|
77 (68)
|
76 (66)
|
70 (63)
|
|
6,200
|
2,800
|
77 (69)
|
70 (63)
|
70 (62)
|
65 (58)
|
|
6,600
|
3,000
|
73 (65)
|
66 (59)
|
65 (58)
|
61 (54)
|
|
7,100
|
3,200
|
68 (61)
|
62 (55)
|
61 (54)
|
57 (51)
|
|
7,500
|
3,400
|
64 (57)
|
58 (52)
|
58 (51)
|
53 (48)
|
1 These seeding rates are based on having a germination of
90% and an emergence of 85%-90% (plant stand of 76%-81% of seeding rate).
Recommended seeding rates are listed in Table 2-11,
Recommended Soybean Seeding Rates. The wider the row width, the
lower the seeding rate required. These seeding rates are adequate for
both conventional and no-till production. Rates can be reduced by 5%,
when using precision seeding equipment compared to a seed drill. An emergence
rate of 75%-80% is considered normal. Full yield potential is achieved
in Ontario with final plant stands 309,000-370,000 plants/ha (125,000-150,000
plants/acre), depending on row width. Seeding rate must be adjusted upward
for seed with a lower germination or vigour rating or for soils that tend
to crust.
Figure 2-2. Soybean Yield Response
to Seeding Rates
Values based on results from 45 Ontario trials in 38-cm. (7.5-in.) rows.
Source: Earl, Bohner, University of Guelph, OMAFRA (2005-07).
Give special consideration to fields prone to white mould. Variety selection,
wider rows and lower plant populations are the main tools available to
minimize disease damage. Although wider rows and lower seeding rates will
give up some yield in years when no white mould develops, this strategy
can significantly reduce white mould severity during wetter summers. Grow
fields prone to white mould with a minimum row width of 38 cm (15 in.)
at 370,000 seeds/ha (150,000 seeds/acre). In fields with a severe history
of white mould, consider using 76-cm (30-in.) rows.
Increase planting rates by 10% with late plantings into mid-June. Varieties
respond similarly to changes in seeding rate. The formula for determining
seeds needed per foot of row is:
Seeds needed per m (ft) of row
= Desired final plant population per m (ft) of row
% germination x % expected emergence
Seed size differences affect seeding rates. For each variety, seed size
and seed quality are influenced by growing and harvest weather of the
previous year. There can be as much as 20% variation in the seed size
of a variety from one year to the next.
Table 2-12. Soybean Plant Stand and Yield
Response to Seed Treatments
|
Seed Treatment1
|
Plants/ha (plants/acre)2 |
Yield3
t/ha (bu/acre)
|
|
Control
|
345 000 (139 700)
|
3.3 (49.2) b
|
|
Fungicide
|
355 700 (144 000)
|
3.4 (50.4) a
|
|
Fungicide + Insecticide
|
369 500 (149 600)
|
3.4 (51.2) a
|
| Least Significant Difference (P = 0.05) = 1.0 bu |
| Source: Hooker, Bohner, University of Guelph, OMAFRA. |
1 Values based on 35 seed treatment trials in Ontario.
2 Plant stands taken at 30 days after seeding.
3 Values followed by the same letter are not significantly
different.
Seed Treatments
Soybean seed treatments have been shown to increase plant stands and
improve yields in some situations. They can be an important tool in establishing
a uniform plant stand especially in no-till, clay soils or early planted
fields. Stand and yield response are dependent on the weather conditions
following seeding and the level of disease and insects pressure. Of 35
Ontario trials conducted, 23% showed an increased plant stand with the
use of a fungicide + insecticide seed treatment, while 17% of the trials
increased yields. Table 2-12, Soybean Plant
Stand and Yield Response to Seed Treatments, shows average trial results.
When conditions were favourable for quick emergence and little disease
or insect pressure was evident, no yield benefit was found to soybean
seed treatments. For more details on specific pests and control measures,
see OMAFRA Publication 812,
Field Crop Protection Guide.
Planting Depth
A seeding depth of 3.8 cm (1.5 in.) is generally adequate for soybeans.
Early planting into no-till conditions can often be reduced to 2.5 cm
(1 in.) if there is sufficient soil moisture. However, due to the high
water demand for germination, plant 1 cm into moisture (approximately
1/2 in.), but never deeper than 6.4 cm (2.5 in.). A newly planted soybean
seed is completely dependent on its reserve of energy to push through
the soil. In general, larger seeds contain more energy and can be planted
slightly deeper than small seed. Precise seed placement is difficult to
achieve with some seed drills, especially in reduced or no-till fields.
Adequate down pressure, ballast and the use of a coulter cart can help
achieve proper seeding depth. It is important to have good seed-to-soil
contact and a closed seed slot. The key is to plant into adequate soil
moisture with a properly adjusted planter or drill. If seeding into moisture
with a drill cannot be achieved, consider seeding with the planter, rather
than waiting for rain.
Varieties differ in their ability to emerge from planting depths greater
than 5 cm (2 in.). Seed companies can provide an "emergence score"
or hypocotyl length rating, which rates the ability of the seedling to
emerge from unusually deep planting.
Rolling
Rolling helps conserve moisture and prepare the field for harvest. Rolling
can help level the soil and push rocks into the ground, making it possible
to do a better job combining. Some producers roll immediately after planting,
while others wait until the soybeans have emerged. Rolling immediately
after planting provides improved seed-to-soil contact and reduces the
likelihood of plant injury. However, it also increases the chance of soil
crusting, which hinders soybean emergence. Soybean fields that are not
rolled after the drill often emerge more quickly and uniformly. If rainfall
occurs after seeding, rolled fields are more prone to crusting. However,
if conditions are very dry, rolling can improve emergence because moisture
is conserved.
Rolling soybeans after emergence does not reduce yields if:
- fields are rolled during the heat of the day to ensure that soybeans
are limp. Soybeans are the most turgid (stiff) during the morning hours
and rolling during that time will result in more plant injury.
- soybeans that are just emerging are left to grow until at least the
unifoliate stage since seedlings are vulnerable to being broken off
at emergence.
Soil Crusting
Crusting of the soil surface following a driving rain or ponding water
can inhibit soybean emergence. The crust can break the hypocotyl arch
(the portion of the plant that lifts the cotyledons above the soil surface).
If soil is prone to crusting, plan to break the crust before the seedlings
are attempting to break through.
Light tillage with a rotary hoe, harrows, coulter cart or even the planter
or seed drill can help break the soil crust and aid bean emergence.
Typically these operations can cause a 10% loss of emerged beans. A
higher stand loss can occur when the hypocotyl arch is breaking the
surface. "Crust-busting" may not be necessary in thin stands
(e.g., 60%) where full yield potential already exists. See Table
2-13, Expected Yield of Soybeans in Optimum and Reduced Stands,to
determine yield potential.
Table 2-13. Expected Yield of Soybeans
in Optimum and Reduced Stands
% of
Full
Stand
|
Expected
Final
Yield as
% of
Optimum
|
Plants per Hectare |
18-cm rows
(7-in.)
|
36-cm rows
(14-in.)
|
53-cm rows
(21-in.)
|
76-cm rows
(30-in.)
|
|
100
|
100
|
553,300
|
402,600
|
392,700
|
405,100
|
|
80
|
100
|
442,100
|
323,600
|
313,700
|
323,600
|
|
60
|
100
|
331,000
|
242,100
|
237,100
|
244,500
|
|
40
|
87
|
222,300
|
160,600
|
158,100
|
163,000
|
|
20
|
62
|
111,200
|
81,500
|
79,000
|
81,500
|
| Divide plants/ha by 2.47 to calculate plants/acre. |
Source: University of Guelph, Huron Research Station and Kemptville College
Replant Decisions
Soybeans are more prone to poor stand establishment than corn or wheat,
because the seedling must pull the cotyledon seed leaves through the ground
to emerge. Deciding whether it is worth replanting a poor crop can be
difficult. One of the main challenges in making a decision is that stand
reductions are rarely uniform, so parts of a field may need to be treated
separately. Do not assess a poor soybean stand too quickly, since more
seedlings may still emerge. Fields with a plant reduction of 50% do not
need replanting if plant loss is uniform and the stand is healthy. Numerous
studies and field experience have demonstrated that keeping an existing
stand is often more profitable than replanting. Replanting gives no guarantee
of a perfect stand.
Every replant decision is based on factors surrounding the individual
field. Information needed to make a replant decision includes:
- the population and health of existing stand. Normal seeding rates
include a margin of safety to ensure emergence of an adequate stand.
- the cause of the low plant population. A number of factors can cause
reduced soybean stands. These include soil crusting, herbicide injury,
frost, hail, insects and diseases. For instance, in a wet year, damping-off
is likely to be caused by two fungal classes - Pythium and Phytophthora.
In this situation, if the stand is to be replanted, consider the use
of a variety resistant to Phytophthora plus a seed treatment.
- the uniformity of the remaining plant stand
- the yield potential of the existing stand compared to the yield potential
of the replanted stand. Yield potential begins to decline after the
optimum planting date and declines throughout June.
- the cost of replanting and possibly additional weed control costs
in thin stands
Compensation and Plant Spacing (Gaps)
Soybean plants have an amazing ability to compensate for thin stands.
Soybean plants can fill interplant spaces up to about 30 cm (12 in.) within
or between rows without any yield loss, provided weeds do not compete
for this space. Ontario research has found that a 33% reduction in the
stand, distributed uniformly over the field, will not significantly affect
yield.
Plants in thin stands branch profusely, making them heavy and more prone
to lodging. Branched plants tend to bear more of their pods near the ground.
Consequently, harvest losses can be slightly higher in these stands. In
trials with thin stands, lodging did not become a problem until populations
dropped below 60% of a full stand.
Evaluating Stand Reductions
Accurately assess the stand for the population, spacing and health of
the remaining plants. To determine plant population, see the hula-hoop
method in Appendix K, Hula Hoop Method
for Determining Plant and Pest Populations.
Table 2-13, Expected Yield of Soybeans in
Optimum and Reduced Stands,provides an estimate of the yield potential
compared to a full stand, based on research conducted in Ontario. It is
important to note that Table 2-13 is based on the number of healthy plants
remaining in a thin stand spaced uniformly and kept free of weed competition.
Do not replant a plant stand of more than 222,000 plants/ha (90,000 plants/acre),
in 19-cm (7.5-in.) row spacings on most soil types. Very heavy clay soils
need a minimum of 250,000 plants/ha (110,000 plants/acre) before a replant
is worthwhile.
Calculating Returns From Replanting
- Estimate the yield of a full stand with the original planting date.
- Determine the population of the existing stand. See the hula-hoop
method described in Appendix K, Hula
Hoop Method for Determining Plant and Pest Populations.
- Estimate the yield potential of the reduced stand. See Table
2-13.
- Estimate the yield potential of the replanted full stand. The later
date will reduce the yield potential. See Table
2-7, Effect of Planting Date on Yield and Plant Height.
- Estimate the cost of replanting
- Compare the value of reduced stand to replanted stand. (see Figure
2-3, Reduced Stand in the Field).
Figure 2-3. Reduced Stand in
the Field
Example
A field planted on May 12 is estimated to have a yield potential of 45
bu/acre if there were a full stand. On June 6, a reduced stand of solid-seeded,
18-cm-row (7-in.-row) soybeans has an average population of 222,220 plants/ha
(90,000 plants/acre). The yield potential of this stand is 87% (39 bu/acre)
of a full stand (Table 2-13). Yield expectation
from replanting on June 6 would be about 38 bu/acre because of the later
planting date (45 bu x 85% - from Table 2-7).
Replanting would not be justified in this situation
Patching or Thickening Thin Stands
In cases of poor stand establishment, replanting alongside the established
seedlings to patch up or thicken the existing stand seldom improves yields
unless the stand is very poor. If patching is contemplated, use the same
variety and do not destroy the original stand. Repair planting often leads
to timing difficulties with weed control and harvest date.
Plant Development
Table 2-14, Vegetative Growth Stages in Soybean,
and Table 2-15, Reproductive Growth Stages
in Soybean, show the growth stages of the soybean plant from emergence
to full maturity.
The system used to describe soybean growth stages divides plant development
into vegetative (V) - leaves and nodes - and reproductive (R) - flowers,
pods and seeds - stages. The V stage refers to the number of nodes on
the main stem with fully developed leaves, beginning with the unifoliate
node. A leaf is considered fully developed when the leaflets on the next
node have unrolled far enough so that their edges are not touching. For
example, V1 refers to the stage when the unifoliate node has a fully developed
leaf, meaning that the leaf above (first trifoliate) is unrolled. This
stage is commonly referred to as the "first trifoliate" because
the first trifoliate is unrolled. The node is the place on the stem where
the leaf is or was attached. Trifoliate leaves on branches are not counted
when determining V stages.
The first two leaves of the soybean plant are unifoliates (single leaflets)
occurring opposite each other at the first node above the cotyledons.
Subsequent leaves are trifoliate (three leaflets) and are on alternate
sides along the stem. When the plant has 2-3 trifoliates, the nodules,
which are important for the fixation of atmospheric nitrogen, become visible
on the roots.
When planted at the optimal time, soybeans will develop 5-7 trifoliates
before flowering begins. Flowering is triggered mainly by day length and
temperature changes. Very early-maturing soybeans are nearly insensitive
to day length. Instead, flowering is controlled mainly by accumulated
heat units. Later-maturing varieties are influenced more by day length.
Therefore, late-planted, long-season soybeans take fewer days to mature
than those planted early.
Germination and Emergence
Germination begins with the seed absorbing soil moisture until it reaches
a moisture content of about 50%. The first external sign of germination
is the emergence of the radicle (primary root), which grows downward and
anchors itself in the soil. Shortly after, the hypocotyl (the section
of the stem above the radicle) starts growing upwards, pulling the cotyledons
(seed leaves) with it. Once emerged, the hook-shaped hypocotyl straightens
out, the cotyledons fold down and the growing point is exposed to sunlight.
Emergence normally occurs about 5-21 days after planting, depending on
soil moisture, soil temperature and planting depth.
Most commercial soybean varieties in Ontario are indeterminate. Indeterminate
varieties continue to grow taller and produce new leaves after flowering
has commenced. Tall-determinate varieties grow to their full height before
flowering begins. The flowering process occurs over a shorter period of
time. Tall-determinate varieties characteristically have their lowest
pods higher off the ground than indeterminate varieties.
Table 2-14. Vegetative Growth Stages
in Soybean
| |
|
|
|
|
Stage Abbreviated1
|
VE
|
VC
|
V1
|
|
Stage Title
|
Emergence
|
Unifoliate
|
First Trifoliate
|
|
Trifoliate Leaves
|
0
|
0
|
1
|
|
Days to Achieve Stage2
|
12
|
5
|
~ 5 days/fully expanded trifoliate leaf
|
|
Range in Days3
|
5-21
|
3-10
|
3-10
|
|
Notes
|
- Seedlings emerge from soil, and cotyledons are above soil surface.
- Emergence can be hindered by soil crusting.
|
- Hypocotyl straightens, cotyledons unfold.
- Unifoliate leaves unroll so that leaf edges are not touching.
- Growing point is above soil surface.
- Frost can kill the plant.
- Stem severed below the cotyledons will kill the plant.
|
- First trifoliate has emerged and opened (unifoliate leaves are
now considered fully developed).
- Start of critical weed-free period.
|
| |
|
|
|
|
Stage Abbreviated1
|
V3
|
V5
|
Vn
|
|
Stage Title
|
Third Trifoliate
|
Fifth Trifoliate
|
Nth Trifoliate
|
|
Trifoliate Leaves
|
3
|
5
|
n
|
|
Days to Achieve Stage2
|
|
|
~3 days/trifoliate leaf (V6-Vn)
|
|
Range in Days3
|
3-10
|
3-10
|
|
|
Notes
|
- 3 trifoliate leaves emerged and opened (3 nodes on main stem
with fully developed leaves, starting with the unifoliate node).
- End of critical weed-free period.
- Nitrogen fixation has begun.
|
- 5 trifoliate leaves emerged and open (5 nodes on main stem with
fully developed leaves, starting with the unifoliate node).
- 50% leaf loss has little impact on final yield.
- Early maturity soybeans reach R1 at approx. V4.
|
- n = number of nodes on the main stem with fully developed leaves,
beginning with the unifoliate node.
- The number of nodes is a function of maturity rating, planting
date and climatic conditions.
|
1 V refers to the vegetative stages of soybean development.
Vn = number of nodes on the main stem with fully developed leaves beginning
with the unifoliate node. A fully developed leaf is defined as one that
has a leaf above it (at the next node) with an unrolled leaf.
2 An estimate of the number of days required to move from one
stage to the next.
3 Range is an estimate of days within a specific stage of development
and is influenced by planting date, maturity rating and climatic conditions
and can vary considerably within and between seasons.
Table 2-15 Reproductive Growth Stages
in Soybean
| |
|
|
|
|
|
R Stage1
|
R1 - Beginning Bloom
One open flower visible from any node on stem.
|
R2 - Full Bloom
Open flower on one of the top 2 nodes of main stem.
|
R3 - Beginning Pod
Short pods visible at top 4 nodes of main stem with fully developed
leaves.
|
R4 - Full Pod
Pods 2 cm (.75 in.) long at top 4 nodes of main stem.
|
|
Target Event
|
Flowering |
Flowering |
Pod Development |
Pod Development |
|
Notes
|
- Triggered by changing day length and temperature.
- Flowering begins near node 5 (V4) and moves up and down the
stem.
- Root growth rates increase.
- Extreme heat (i.e., over 32°C) can reduce growth, flowering
and pod development.
|
- 50% height and
dry weight accumulation.
- Stress does not usually reduce yield.
- Nitrogen fixation increasing rapidly.
|
- Look for 2-3 seeds per pod.
- Flowering peaks.
|
- Stress occurring between R4-R6 can result in significant yield
loss.
|
| |
|
|
|
|
|
R Stage1
|
R5 - Beginning Seed
Seed 0.3 cm long within upper (top 4) pods.
|
R6 - Full Seed
Seeds within top 4 pods fill cavity in the upper pods.
|
R7 - Beginning Maturity
One major pod has changed to brown colour on the main stem.
|
R8 - Full Maturity
95% of pods have changed to brown colour.
|
|
Target Event
|
Seed Development |
Seed Development |
Plant Maturity |
Plant Maturity |
|
Notes
|
- Flowering completed except for some branches.
- Plant reaches max. height, nodes and leaf area.
- Nitrogen fixation rates reach maximum and begin to decline.
- Rapid nutrient uptake and redistribution to pods.
|
- Pods reaching full length.
Root growth slows substantially.
- Above-ground dry weight accumulation slows.
- Rapid leaf yellowing begins.
- Leaves in lower canopy begin to fall.
|
- Moisture begins to decline in seeds.
- Physiological maturity reached, maximum dry weight.
- Seed moisture is about 60%.
|
- Harvest moisture reached in 1-2 weeks after R8.
|
1 R refers to the various reproductive stages
of soybean development.
