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Recommendations for soybean plant establishment

December 2015

Soybeans have a smaller plant than most other grain crops and therefore have a smaller leaf area. A higher population is planted in order to intercept the maximum amount of sunlight.

The ability of soybeans to branch out and produce more pods if there is enough room (phenotypic plasticity) explains the minor reaction soybeans display to different plant establishments. At a high plant establishment soy plants will branch little, grow straight up and bear pods high above the ground. With a decrease in plant population, the inner-row space plants increases and the plant produces more lateral branches and pods.

With very low plant populations some of the lateral branches and pods will be borne close to the ground. Although yield is retained, some of the lateral branches will lodge and bear pods on the ground, which reduces the harvestable yield.

Recommendations for establishment

Recommendations for soybean establishment differ considerably and are complex due to the variation with respect to row widths, tillage practices and planting date differences. In normal spring plantings in October, maximum yields are obtained with 25 to 50 plants per square metre. In practice, the ideal establishment for 0,5 m rows is 14 to 18 seeds per metre, for 0,76 m rows it is 22 to 26 seeds per metre, for 0,91 m rows it is 24 to 28 seeds per metre and for 1,52 m rows 28 to 32 seeds per metre.

Adjustments in plant population must be made on the basis of the interaction between the production potential of the field, phenotypic plasticity and lodging of the cultivar on the basis of the proposed planting date. With early plant dates cultivars that tend to lodge or produce more lateral branches can be planted in a lower population. This is not the case for a late planting date. Soybeans that are planted early in fields with a high potential tend to grow very lushly. In such a case the plant population can be reduced.

The above points to final plant populations of 180 000 to 400 000 plants per hectare. The higher populations in narrower rows are a result of more space in the rows and the interplant competition being less, which reduces the risk of thin stems and lodging.

Early planting dates

Early planting dates in the cooler eastern production area have a very beneficial effect on yield, but this is accompanied by greater risk. This period is characterised by regular cold fronts that cause the soil temperature to drop considerably. This exposes the seed to fungi for longer in the cold, wet conditions, which can lead to seedling wilting. If you have to plant in these conditions, the plant population should be increased by 15% to 25% to make provision for establishment losses, particularly with no-till systems.

If the planting date has to be postponed to late November and December due to weather conditions, the vegetative development is limited considerably. The smaller plants have less room to produce sufficient pods and tend not to close up the space between the rows. The plants then cannot utilise all the sunlight. To compensate for this, the plant density must be increased by 40% to 60%. A better option would be to accommodate the high plant establishment in narrower rows.

Plant population

Accurate plant densities must be based on seed quantities per hectare and not on seed mass. Seed is sold according to weight, which makes it difficult to calculate seed requirements. The labels on the seed package are supposed to indicate the number of seeds per kilogram. This can then be used as a basis for calculating the relative quantity of seeds required.

Most planter tables that are used for calibration also use relative data and the plant population should then be regarded as estimated. The only way in which planters can be calibrated accurately is to count the number of seeds that are placed over a specific distance by the planter.

Seed size

The seed sizes of different cultivars vary considerably. They will probably vary between 5 000 and 8 000 seeds per kilogram. Some cultivars tend to produce bigger seeds more than others, but the influence of environmental conditions during the grain-filling period have the biggest effect.

If drought or any other stress factor is experienced during the grain-filling period, the seeds will be smaller than normal. A good blossom period that leads to a lot of pods can also result in smaller seeds. The reverse can also be true. Very small seeds will not have an effect on the yield of the crop, provided the germination of the seed occurs correctly. It is not uncommon to get a large variation in seed sizes in one production unit. Planters that use volumetric seed measurement must be calibrated regularly – particularly if you change cultivars.

Damage to seeds

The number of seeds that survive is always smaller than that planted. Some seeds simply will not germinate. Diseases, insects and other factors damage the seeds even before they germinate. After emergence some seedlings will be damaged by herbicides, equipment, insects, rodents, diseases and other factors. Seed is also damaged when treated with vaccines.

To compensate for the losses, more seed should be planted than the intended final plant establishment. The total loss can amount to 40%, depending on the practices that are followed. For practical reasons the figure of 20% should be used in calculations.

The following formula can be used to calculate the plant population:

Plant population = (expected plant establishment)/(germination percentage x expected survival)

Expected plant establishment =
final establishment that should be in the field (240 000)

Germination percentage
= read this on the package label (90%)

Expected survival
= unknown but 80% is commonly used for soybeans

Plant population
= 240 000 / (0,9 x 0,8)
= 333 333 seeds per hectare.

Article submitted by Nico Barnard: Research Agronomist, Pannar Seed.
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Publication: December 2015

Section: Pula/Imvula