Soybean Crop ManagementWritten by Alejandro Lehmann. Agronomical Engineer – University of Buenos Aires

Undoubtedly, there are unique challenges to growing soybeans in South Africa. In no other country in the world can so many hectares of crop production be found at altitudes higher than 1200 m above sea level. Although soybean production practices and experiences used by the largest producers (USA, Brazil and Argentina) cannot be applied completely in the South African context, a great deal can be learned from them. To attain a profitable soybean crop, South African farmers are challenged with adapting existing technology and management skills to their specific local conditions. Clearly understanding the physiology and biology of the soybean crop is essential to finding truly South African production practices.

Let´s go through some general concepts that can be helpful in deciding the right combination of the different management tools available in order to increase the chances of a higher yielding crop.

Some definitions and statements:

The rate of plant development for any variety is directly related to temperature; therefore, the time between stages will vary as temperature varies. Soybean is a short-day plant and genotypes differ in photoperiod requirements for flowering induction.

The staging system employed divides plant development into vegetative stages (V) and reproductive stages (R). Subdivisions are designated numerically as follows:

Soybeans planted at high densities tend to grow taller and produce fewer branches, pods and seeds per plant compared to soybeans planted at low densities. High-density soybeans also set pods higher off the ground and have greater tendency to lodge.

The indeterminate growth habit is characterized by a continuation of vegetative growth after flowering begins, while the determinate growth habit finishes most of its vegetative growth before flowering begins.

Growth rate is related to the solar radiation interception. To be able to capture 95% of the solar radiation, a soybean crop needs a Leaf Area Index of 4, meaning that 4 m² of leaf area for each square meter of soil is required. Plant structure, plant population and row width are variables which influence this value.

Regardless of the variety, the better the environment, the taller the plants will grow (more bio-mass).

EmergenceUnrolled unifoliolate leavesFirst-trifoliolate (completely unrolled)Second-trifoliolate (completely unrolled)Third-trifoliolate (completely unrolled)Fourth-trifoliolate (completely unrolled)Sixth-trifoliolate (completely unrolled)

VEVCV1V2V3V4V6And so on...

Beginning bloomFull BloomBeginning podFull podBeginning seedFull seedBeginning maturityFull maturity

R1R2R3R4R5R6R7R8

Yield construction:

To maximize the chance of attaining a high yielding crop, the highest possible number of internods per square meter is required. If growing conditions are good, a high number of seeds per nod will be fixed (through a high number of pods per nod and seeds per pod). Ultimately the amount of seeds per square meter will have the greatest impact on yield. See figure 2.

Plants/m²

Nods/Plant

Grain/Pod

Pod/Nod

Number of Nods/m²

Number of Grain/Nod

Grain Number

Grain Weight

YIELDFig. 2

Environment and Yield Potential:

In order to understand the relationship between different environmental situations and yield, planting date can be assumed as an environment indicator. So, optimum planting date equals a good environmental and an early or late planting date equals a lower environmental quality.

Figure 3 shows the effect of different environmental conditions (shown by different planting dates) on the final yield of several types of varieties. Short growers (blue dotted line) require good growing conditions to outperform longer varieties. On the other hand, longer indeterminate varieties are more stable under less than optimal growing conditions (red line).

Fig

. 3

There are several contributing factors to final yield at harvest:

The defining factors that establish POTENTIAL YIELD are: solar radiation, temperature and genotype. On the other hand, the ACHIEVABLE YIELD is determined by the limiting factors, which are: water and nutrients. But the REAL YIELD is finally influenced by the reducing factors such as: weeds, insects and diseases.

The most important parameters about yield are: potential and stability. Yield potential is a genetic attribute strongly influenced by the environment. Usually shorter Maturity Groups have a higher yield potential, but require better environmental conditions during the critical period. (Baigorri, 1997). Yield stability is directly related to the length of the growth cycle hence, longer Maturity Group varieties are more stable. (Baigorri, 1997)

A special comment on nutrition is important at this point. Soybean is a very demanding crop when it comes to Nitrogen. The requirements are close to 70 Kg of N per ton of grain produced so, to attain a yield of 3 t/ha, 210 kg of N (equivalent to 450 Kg of Urea) have to be made available. The good news is that Biological Nitrogen Fixation can provide, in a very cost effective way, most of the Nitrogen requirement of the crop. High qual i ty and concentrated inoculants are the easiest way to assure N supply (Fig. 4).

Cultivar Choice:

Environmental quality is a key element in the choice of cultivars and when and how to plant. It can be evaluated by analyzing the following attributes: water situation (rainfall, water table, water storage capacity, irrigation system, etc.), temperature and solar radiation, tillage system, soil (texture, structure and nutrient availability), root exploration capacity, and of course, all the biotic factors such as: insect, disease and weed pressure.

Next, the decision falls to choosing the most convenient planting date and maturity group. Soybean planting date has to be decided upon so that the critical period (R4 to R6) enjoys the best growing conditions possible. To do so, two key elements are decisive: growth cycle and planting date. (Andrade et al, 2000). Local information about variety behavior comes into play at this moment. Phenology charts provided by seed companies are important in making the right decisions.

Taking all these factors into consideration, the variety options can be narrowed down. The final decision has to include a comprehensive evaluation of: cycle length, growth habit (determinate or indeterminate), yield potential and stability, reaction to different planting dates, health, lodging risk, seed quality, among others.

Spatial plant distribution is the next item to consider. It is important to remember that shorter growers require higher population, better distribution, high quality environment and better insect, disease and weed management. Short growers are less prone to lodging and respond better to high fertility and water supply.

Cultivar SpatialDistribution

The beauty of planting soybean is its great adaptability and flexibility to various growing conditions. Farmers all around the world have been taking advantage of these characteristics to increase yields, thus profitability. It is essential to work towards finding the right combination of all variables for each specific situation. Unfortunately, there is no recipe to success. The only way of increasing future yields is through adapting the already existing knowledge and experiences to the different South African scenarios.

References:ARC-LNR. 2012. Soybean production manual. ARC-Grain Crops Institute

García, Fernando.2006. Soja: Nutrición del Cultivo y Fertilización en la Región Pampeana Argentina. INPOFOS/PPI/PPIC Cono Sur.Kantolic, A, y E. Satorre, 2004. Elementos centrales de ecofisiología del cultivo de soja. Manualpráctico para la producción de soja.

1ra edición. Ed: M. Díaz Zorita y G. Duarte, Buenos Aires.Lafuente, Francisco. 2014. Soybean Crop in South Africa.

Pedersen, Palle. 2004. Soybean Growth and Development. Iowa State University, University Extension.Toledo, Rubén. 2008. Cultivo de Soja. Cereales y Oleaginosas, FCA-UNC

Planting DateMaturity Group

Environment (Description)

ControlFig. 4