Soybean response to planting date is important not only in years when planting is delayed by inclement weather, but also when weather does not disrupt the normal planting season.
Early season freezes, hail storms, flooding, and other situations can reduce crop stands to a point where late planting is necessary.

The Photoperiod Effect

The concept of planting shorter season corn hybrids as planting date is delayed is a proven practice in most management systems. However, this concept needs modifying when applied to soybeans.

Soybean varieties respond much differently to delayed planting than corn hybrids.

This is because soybean flowering is more closely related to photoperiod (the length of the daily light and dark periods) than corn.
The shift from the vegetative to the flowering stage in soybeans is caused mostly by changes in the length of darkness.
Adapted varieties flower soon after the dark period begins to lengthen in late June.

Soybean flowering is also influenced to some extent by temperature.

High temperatures hasten flowering. Given a very warm vegetative period, flowering can start before the dark period begins to lengthen.
Since flowering response of corn is more temperature dependent than that of soybeans, accumulated growing degree days are reliable for estimating corn growth stages but not for estimating soybean growth stages.

Planting Date Considerations

Soil temperature is an important aspect.

The optimum temperature for soybean germination is 86°F.
Seed planted into soil that is 50°F germinates slowly, and emergence will probably be reduced.
Planting into seedbeds that are in the low fifties is not advisable unless soil temperatures are rising rapidly.

Sixty degrees Fahrenheit is a good target at which to begin planting.

Soybeans have a unique ability to yield well when planted over an extended time period. This permits them to complement other crops in cropping systems.

Soybeans planted in May are the most productive. Yields were considerably lower after mid-June.

Plant heights were greatest from mid-May to mid-June and are shorter with earlier and later planting dates. Podding height dropped off considerably in July.
Earlier planting may reduce stands because of the inability of emerged beans to tolerate freezing temperatures. If you intend to plant soybeans after mid-June, your best variety choice is an early to mid-season, adapted variety. Non-adapted varieties do not have the yield potential given a later than average freeze date, and later varieties might not fully mature.
Indeterminate varieties are much more suited to the stressful conditions associated with late plantings and have greater yield potentials than determinate varieties for late plantings. These recommendations apply to double crop situations as well.

Late-April to early-May planting dates are more beneficial to corn than to soybeans. Planting soybeans in mid-May after corn provides the best results.

Late Planting Cultural Practices

When soybeans are planted later than mid-June, vegetative growth is reduced.

Without changes in planting patterns a large portion of the available light energy is lost, evaporative water losses are greater, and weeds are more competitive.
Row widths less than 20 inches, combined with plant populations 20 to 25 percent higher than normal will provide a more rapid canopy closure and will maximize yields.
Late-planted soybeans are shorter and sometimes have lower podding heights. Narrow rows and slightly higher planting rates provide a better chance of maximizing yields.

ROW SPACING

Reduction of row spacing to between 7 to 15 inches will increase yields and reduce competition due to weeds.

Skip row soybeans

narrow row concept with cultivation.
spaces are left during planting for tractor tires 15 inch row width and 30 inch skips.
primary advantage is weed control by both cultivation and chemicals.
provides flexibility for controlling other pests without damaging plants.
have yield advantage of narrow rows while having tracks to work fields.
skip row soybeans may yield slightly less than solid seeded soybeans if chemical weed control in solid seeded soybeans is successful.

PLANT DENSITY

Soybeans do not have a large yield response to a wide range in seeding rates because soybeans can compensate by branching.

For this reason, most soybeans in the Midwest are seeded at a rate of 50 to 70 lbs of seed per acre.
An adequate seeding rate, rather than a high seeding rate, reduces the possibility of lodging and makes more efficient use of the seed that is planted.
Because soybean varieties differ substantially in seed size, many growers determine seeding rate in number of seeds/acre or number of seeds/foot of row. This number can be converted to pounds of seed/acre for calibrating the planter.
Increase seeding rates on light colored soils and decrease seeding rates on dark colored soils. For rows wider than 20 inches, use tall, bushy soybean varieties. Regardless of planting date and row spacing, select varieties with good tolerance and/or resistance to Phytophthora root rot. Research indicates seed treatments aid in Phytophthora control and are highly economical.

A commonly recommended rate in the Midwest is 150,000 plants/acre

40 inch rows = 10 to 11 seeds/foot of row
30 inch rows = 8 to 9 seeds/foot of row
20 inch rows = 5 to 6 seeds/foot of row

The following equations can be used to calculate seeding rate in pounds/acre:

43,560 sq ft/acre times plants/ft of row = seeds/acre divided by row width in feet times % germination
Seeds/acre = seeding rate in pounds/acre divided by seeds/pound
Example: suppose that Hardin soybeans will be planted in 30 inch rows at 150,000 seeds/acre. Let's say that Hardin has 80% germination and has 2870 seeds/pound. What is the seeding rate in pounds/acre?
- 43,560 X (8.5 / ((30/12) X 0.80)) = 187,308 seeds/acre
- 187,308 / 2870 = 65.3 pounds/acre

Seeding rate adjustments

Use 175,000 plants/acre as a base if planting in 10 inch row width or less.
Add 10% if planting very early or very late.
Add 10% if planting short, narrow canopy varieties or early maturing varieties.
Add 10% if planting in a poor seedbed.
Subtract 10% if planting in deep, fertile soils where lodging is a likely problem.
Subtract 10% if planting in ideal conditions for the area (planting date, seedbed, soil moisture and temperature).

SEEDING DEPTH

Depth is more critical than with wheat and corn.

Depth should be about 1 inch.