When to plant soybeans

November 2015

Soybeans and maize react in more or less that same way to heat units. The more heat units that are available during the growing season, the later you can plant. In cooler areas you should plant earlier to obtain the best results.

The other factor to be taken into account is that soybeans must experience a certain number of hours of darkness every day before they progress from the vegetative to the reproductive stage. As the blossom stage in soybeans is controlled by the number of hours of darkness, the harmful effect of a midsummer drought cannot be avoided by changing the planting date. These two factors, together with the growth class of the cultivar being planted, determine the optimum planting date for each area.

Advantages of an early planting date

• Soybeans have a need for hours of darkness that is affected by the number of calendar days. That is why an early planting date will lead to a bigger plant. A bigger plant has more internodes and therefore also more spikelets, which lead to bigger harvests.
• In the cooler eastern areas an earlier planting date leads to earlier maturation, therefore the risk of early frost damage in autumn is limited.
• More rapid growth leads to earlier closure of the canopy over the rows, which facilitates weed control.
• Early closure of the canopy leads to increased transpiration and reduced evaporation. Most of the available water now goes towards increasing the yield.
• An early planting date results in bigger plants that bear pods higher above the ground, which makes the harvesting easier and reduces wastage.
• An early plant date has a bigger effect on yield than row width, and special planters are therefore not essential.

Risks of an early planting date

• Cold, wet soil early in the season can lead to serious plant establishment losses.
• The extended time it takes seed to emerge extends the period for which herbicides must work to successfully control weeds.
• Late frost can cause great damage.
• In warmer areas an early planting date can result in excessive vegetation growth, which can cause lodging.

The interaction between length of daylight and temperature on blossom stimulation has a major effect on the ideal planting date. The impact is much bigger in the moderate and warm production areas than in the cool areas. In the cool areas the 4,5 to 6,5 growth classes area the best adapted ones. The ideal planting date for the areas is from the beginning of October to the beginning of November. The shorter growth classes (4 - 5) usually have a smaller need for hours of darkness than the taller growth classes, and it is therefore better to plant the six growth class cultivars very early and the 4 - 5 growth class cultivars after the middle of October.

Should weather conditions not allow a normal planting date and force a late planting date, it is better to start with the quicker growth classes and then plant the tallest ones. Any planting after the end of November is regarded as late.

Growth classes from 5 to 7 are adapted the best to the moderate areas. The western part of the moderate areas usually receives rain later in the season and it is better to plant 6 to 7 growth class cultivars here. In the warm production areas any growth class can be planted. However, the full-season cultivars usually provide the best yield here.

With earlier planting dates soil temperature can play a major role in effective germination. Soybeans can germinate at 10°C, but the ideal soil temperature is 13°C for strong germination. If you plant early in the cool production areas, it is better to measure the soil temperature before starting to plant. Take the temperature at 07:00 at a depth of 5 cm, and if it is warmer than 13°C for at least three consecutive days, the planting process can start. Just keep in mind that cold fronts or a big hail storm can drastically reduce the soil temperature.

There are a few management aspects that should be addressed to make the best of a late planting date. Yield is determined by the amount of sunlight that is intercepted. Soybeans that are planted late, have a shorter period to absorb sufficient sunlight. This is applicable in the grain filling stage in particular, when the days become shorter. The plants themselves are also smaller and have fewer nodes where pods can develop. Soybeans that are planted late should therefore be managed so that they can absorb more sunlight to produce maximally.

Management practices for late planting dates

• Narrower rows catch more sunlight for vegetative growth at the beginning of the growing season, and mode internodes per hectare are produced.
• Together with the narrower rows, the plant establishment should be increased by 25%.
• Late planting dates are usually associated with wetter soil. Prevent compaction and make sure that seedlings can emerge quickly and grow strongly.
• Pythium is a major destroyer of plant establishment in warm, wet conditions. Make sure that seed has been treated with a fungicide to protect the arrangement.
• Plant the growth class recommended for a normal planting date for the area concerned, as a shorter growth class cultivar goes through the experimental stages more quickly, which reduces the number of nodes.

Article submitted by Nico Barnard: Research Agronomist, Pannar Seed.
For more information, send an email to Nico.barnard@pannar.co.za.

Publication: November 2015

Section: Pula/Imvula