Soybeans: Planting and Crop Development
| Soybeans | Forages
| Cereals | Dry
Edible Beans |
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.
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.
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 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.
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.
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:
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).
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.
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.
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.
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
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.
1 Values based on 35 seed treatment trials in Ontario.
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.
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 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:
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.
Source: University of Guelph, Huron Research Station and Kemptville College
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:
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
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.
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.
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
1 R refers to the various reproductive stages of soybean development.
For more information:
Toll Free: 1-877-424-1300