Soybean (Glycine max [L.] Merrill) is a legume that grows in tropical, subtropical, and temperate climates. It has 40 chromosomes (2n = 40) and is a self-fertile species with less than 1% out-crossing. Soybean was domesticated in the 11th century BC around northeast of China. It is believed that it might have been introduced to Africa in the 19th century by Chinese traders along the east cost of Africa. Reports indicate that soybean has been under cultivation in Tanzania in 1907 and Malawi in 1909.
Soybean is an important source of high quality but inexpensive protein and oil. It has an average protein content of 40% and oil content of 20%. It also has a superior amino acid profile. Soybean protein has great potential as a major source of dietary protein. The oil produced from soybean is highly digestible and contains no cholesterol. A by-product from the oil production (soybean cake) is used as a high-protein animal feed in many countries. Soybean also improves soil fertility by adding nitrogen from the atmosphere. This is a major benefit in African farming systems, where soils have become exhausted by the need to produce more food for increasing populations, and where fertilizers are hardly available and are expensive for farmers.
According to FAO data of 2005, total land area under soybean in the world was 95.2 million ha and total production was 212.6 million tonnes. The three major soybean- producing countries were USA (29 million ha), Brazil (23 million ha), and Argentina (14 million ha). In terms of total production, USA produced 83 million tones, followed by Brazil (51 million tonnes) and Argentina (38 million tons). In relation to Africa, the same source showed that, soybean was grown on an average of 1.16 million hectares with an average production of 1.26 million tonnes in 2005. African countries with the largest area of production were Nigeria (601 000 ha), South Africa (150 000 ha), Uganda (144 000 ha), Malawi (68 000 ha), and Zimbabwe (61 000 ha). Other countries with sizeable areas are Rwanda (42 160 ha), DRC (30 000 ha), and Zambia (15 000 ha). Soybean is also grown in small scale in more than 10 other African countries.
How soybean is grown
Soybean grows from sea level, up to 2 000m; from equator to latitudes 55 o N and 55 oS. It grows under a wide range of temperature, but the optimum for growth and development is 30 oC whilst for proper emergence of seedlings, a seedbed temperature of 25–33 oC is optimal. The crop requires 500–850 mm water during the growing season. A seed rate of 40–120 kg/ha is used under various planting conditions. Soybean can be planted in rows 20–40 cm apart and in some cases as wide as 75 cm and 7–10 cm between plants within a row at a depth of 2–5 cm. Normally, sowing rates are low with intercropping as compared to sole planting. Phosphorous fertilization at the rate of 20–25 kg/ha is advisable to help the plants fix atmospheric nitrogen. Depending on the level of weed infestation, one to three times weeding in the first 6–8 weeks after planting is recommended to increase yield. Soybeans can mature in 65–150 days after planting depending on the varieties and their maturity group.
Constraints to soybean production
Average grain yield of soybean is low (<1 t/ha) in tropical Africa. Dual-purpose improved varieties of soybean have not reached many soybean growers to increase production. Seed production and distribution is also a major impediment in soybean– growing countries of Africa. Pod shattering especially in the hot dry savanna environments, reduces seed longevity, and lodging are still constraints in soybean production. Diseases such as rust, red leaf blotch, frog-eye leaf spot, bacterial pustule, bacterial blight, and soybean mosaic virus are problems to be resolved in soybean. Soybean rust (Phakopsora pachyrhizi) particularly is the most destructive foliar disease of soybean in recent times, and can cause 50–60% yield loss. It is a major disease worldwide. This disease was first reported in 1998 in Uganda and Zimbabwe; and in 1999, its existence was reported in Nigeria, Cameroon, and Benin Republic. In Nigeria, the disease kept on recurring every season since 1999. Among insect pests, pod sucking and defoliating insects are major constraints.
Lack of varieties tolerant to midseason moisture stress and high yielding varieties tolerant to low phosphorus are among the abiotic constraints. In some African countries, farmers lack interest in the production of soybean because there is no market for the grain, and many people do not know how to prepare the crop for home consumption. Besides, equipment for processing soybean is also unavailable in many African countries. Research on seed quality such as protein, oil, carbohydrate, and antinutritional factors is lacking. Moreover, lack of emphasis on using molecular markers as aid to conventional breeding is also worth mentioning.
Our work on soybean improvement
Our scientists have developed early, medium, and late maturing soybean varieties which combine high grain and stover yields with good promiscuous nodulation, enhanced nitrogen fixation, low pod shattering, resistance to frog-eye spot, bacterial pustule, and bacterial blight. Several varieties have been developed in the past but recent promising varieties include TGx 1910-11F (early), TGx 1905-2F (medium), and TGx 1910-8 (late). These varieties produce about 2 t/ha per ha grain and 2–3 t/ha stover yields. Our scientists have been developed efficient and rapid methods of evaluating soybean lines for rust resistance. Among the rust-resistant lines identified from adapted materials, TGx 1740-2F and TGx 1903-3F combine high yield with moderate resistance to rust in Nigeria. Other lines resistant to rust including PI594538A and UG5 are being used as sources of resistance alleles for breeding. Mapping populations and recombinant inbred lines are being developed for identifying molecular markers associated with resistance that can aid in resistance breeding.
The breeding strategy pursued at our institute focuses on developing soybean lines that are capable of nodulating with native rizobium strains. Our team of scientists has developed soybean lines that are capable of forming effective nodules over the years. Several studies have demonstrated the presence of significant genotypic differences in nitrogen fixation among soybean lines within different maturity groups. Recently, our scientists at IITA developed three new soybean lines, TGx 1909-7F, TGx 1910-10F, and TGx 1910-11F, that could fix 10% more nitrogen than the current best widely grown soybean variety, TGx 1448-2E.
Our scientists have researched and adapted various soybean-processing machines for use in sub-Saharan Africa. These machines have been widely adopted by both small-scale and medium-scale processors. Over 100 food products with good nutritive value and consumer acceptability have been developed. Soybean products are also being used in hospitals for treating sick people and malnourished children.
To improve soybean in tropical Africa, our institute works in collaboration with national agricultural research and extension systems (NARES) in Africa, USDA-ARS at the National Soybean Research Laboratory in the University of Illinois to develop soybean lines that are resistance to soybean rust, and TSBF-CIAT.