Soybeans

Land Preparation

Clear all vegetation before land preparation. The seedbed may be prepared manually with a hoe. Prior sowing, first and second ploughing are needed to ensure good germination and also to reduce weed infestation. When the slope is high, it is important to put in place the soil erosion control system.

Planting

It is recommended to use high quality seeds of the selected variety. Before planting, it is advisable to check germination rate to make sure that the seeds don’t lose viability when seed origin is not sure. It is recommended to avoid using seeds which are more than 12 months old. It is also recommended to sort out the good seeds for planting to ensure that they are free from insects, disease infestation, and weed seeds. If possible, make sure that seeds are from Rwanda Agriculture Board or a Seed Company/Cooperative. Suitable period for planting is September-October for Season A and early March for Season B. Before planting, seeds are inoculated with Rhizobia inoculants. Seeds are coated with inoculants just before planting and covered shortly after planting to protect rhizobia from the sun and drying. The quantity of seeds per hectare is about 50-70 kg depending on grain size. Soybean is planted using a spacing of 40 cm between rows and 5 cm within rows with one seed per hole.

Fertilization

Logically, a single fertilizer recommendation must be avoided for soybean production in the country. A good fertilizer recommendation for soybean production should depend on a soil test. However, under normal conditions, soybean as a legume should provide itself with nitrogen through biological nitrogen fixation. Phosphorus is often the most deficient nutrient in tropical soil and the most required macro-element for the success of the biological nitrogen fixation, as a starter to stimulate the vigor of seedlings at early stage. Currently, Di-ammonium phosphate (DAP) is recommended at a rate of 100 kg/ha (46 kg of P/ha). The advantage of DAP is that in addition to P supply, it also provides a small amount of nitrogen which is among limiting factors to soybean production, DAP is supplying 18 kg/ha  of N to ensure plant growth before BNF taking place.  DAP is incorporated into the soil at owing time in addition to farmyard manure which is applied at a rate of 5 Tons/ha while ploughing.

Weeding is carried out to avoid competition but also to increase soil aeration required to improve plant growth. It is organized two times, the first weeding take place 2 weeks after planting while the second is undertaken 5-6 weeks after planting. Poor weeding or delay in weeding affects seriously soybean yields. It is not recommended to make weeding immediately after a rainfall as it would be as transplanting weeds.

Pests’ control

Different insects occur in soybean fields but few are normally of any economic importance, and the species that cause damage are usually not abundant enough to warrant control measures.

However, a kind of butterfly lays eggs which give caterpillars with green color and black lines in their back often hiding under soybean leaves and devour them. Caterpillars’ attacks inflicted

important damages at flowering stage and during dry season. Insect pests can be controlled with a single spray of Cypermethrin, deltamethrine or fenvalerate.

· Soybean rust disease 

Soybean diseases normally result in major yield losses in Rwanda.

The most devastating disease remains the asian soybean rust caused by Phakopsora pachyrhizi, which is also the most important foliar disease in the country. Severe infection leads to premature defoliation and yield losses up to 80%. The disease is of great economic importance when rainfall and humidity are high. Plant resistant varieties use is the best option to control disease. The unique disadvantage of using rust resistant varieties is that it is increasing inoculums levels in the field. Foliar fungicide spraying such as Benlate allows controlling disease when weather is highly humid and with heavy rains thus favorable for disease development. It is also recommended to proceed to crop rotations with maize in order to prevent the increase of inoculums levels in a field.

· Bacterial diseases 

The main bacterial disease is caused by Pseudomonas syringae pv. glycinea . Even the disease is not severe; it appears sometimes for non-resistant varieties. Symptoms appear as specks to large, irregular spots with raised light-colored pustules in the elevated centers of the spots on the lower surface. The elevated pustules sometimes have cracks in them. Later lesions join together and the dead areas tear away to give a ragged appearance to the leaves. The best option to control the disease is the use plant resistant varieties.

· Virus diseases

Soybean is also susceptible to viruses; however it is urgent to investigate more to know exactly which viruses are prevailing in Rwanda. Most of the virus infection results in foliar symptoms such as mosaic and mottling, thickening/brittling of older leaves, puckering, leaf distortion, severe reduction in leaf size, and stunting of plants. The cultivation of virus disease-resistant

varieties is the most convenient, economical, and effective approach for controlling soybean virus diseases. As many viruses involved virus disease are seed transmitted in soybean, strategies to control the diseases include the use of free seed-borne diseases, uprooting and destruction of diseased plants, eradication of weeds and voluntary plants, seed treatment with systemic insecticides and spraying foliar insecticide to reduce insect vector activity during preflowering stage which is the most vulnerable period to virus infections of the plant.

Harvesting

It is recommended to harvest soybean when about 85% of the pods have turned brown for a non-shattering variety while it is 80% for shattering varieties. Actually, most of the varieties in Rwanda are not shattering as non-shattering is one of the criteria in variety selection. It is recommended to harvest on time in order to avoid losses. Harvesting can be done with a cutlass or sickles. Mature plants are cut plants at ground level and sun-dried or air-dried before threshing. Soybean harvesting by hand pulling is not recommended as it may remove the nutrient added to the soil by the crop.

Threshing is executed either manually or mechanically when soybean plants are dried. When soybean plants are dried, it is recommended to proceed to threshing as soon as possible. Manual threshing is the common method in the country. After threshing, the material is then winnowed to remove the seeds from the debris. However, mechanically threshing in large scale production may be more interesting as it is fast and cost effective.

Soybean should be stored at a moisture content of 10% or less. A soybean seed is sufficiently dry when it cannot be dented with the teeth or fingernails. Soybean grains usually contain about 14% moisture; long and short storage require respectively decreasing moisture content to 13% and 10–11%. The most practical way to protect soybean in storage is open-air drying by placing 50 kg or 100 kg bags of clean soybean on a rack in the cold room or in shade. High moisture content affect stored soybean encouraging the development of various agents of deterioration, such as insects and microorganisms. Good storage management influences the storability of soybean and subsequent seed germination when planted. It is not recommended to expose soybean grains to high temperatures, as it will increase deterioration and reduce seed viability.