Cassava agronomy research and adoption of improved practices in Indonesia: Major achievements during the past 20 years

In Indonesia, annual growth rates for cassava production and yield from 1961 to 2000 were 0.81% and 1.35%, respectively. However, the harvested area decreased at an average annual rate of 0.61%. The average yield of about 12 tonnes of fresh roots/ha is much below the potential yield of 20 to 40 t/ha obtained in experiments, indicating that cassava yields could be further increased by the adoption of improved practices.
Soil preparation using minimum tillage reduced erosion effectively and had no significant effect on root yield compared to that of complete tillage, but the control of weeds was much more difficult. Therefore, most cassava farmers prepare the soil using complete tillage. The quality of planting material used influences the final population and yield. Cuttings of 15-25 cm length planted vertically is used by most farmers even though no significant differences in sprouting capacity and root yield were observed compared to shorter cuttings of 2 to 3 nodes. It means that reducing the stake length from 25 cm to 2 nodes is a way to get more high-quality cuttings when planting material is limited. Maintaining only two stems per plant, as farmers do, produced good planting material and high root yields. Cassava planting time is affected by the cropping system, soil type and water availability. Planting cassava on medium to light textured soils could be done from the beginning to the end of the rainy season without any significant effect on root yield when plants were harvested at 8 to 12 months, since water availability of 35 to 60 mm/10 days could be maintained during the first five months.
Intercropping of cassava can result in a decrease in root yield, but this is generally compensated by the yield of the interplanted crops; therefore, intercropping cassava did not affect total crop value. Most farmers plant intercropped cassava in the early rainy season, whereas monoculture cassava is planted from the early rainy season to the early dry season, especially in areas surrounding cassava factories and near big cities. Plant spacing of cassava was determined by soil fertility, plant type, cropping system and expected yield. The optimum plant population for monoculture cassava using non- or late-branching varieties on poor and better soils are 12,000- 14,000 and 10,000 plants/ha, respectively. The best plant population of branching varieties under monoculture on both poor and better soils is 10,000 plants/ha. For monoculture, farmers often use a plant spacing of 100-125 cm between rows and 75-100 cm in the row, while for intercropped cassava they plant at 200-300 cm between rows and 50-75 cm in the row. Intercropping systems of cassava with upland rice and other secondary food crops increased LER to 1.59, increased net income 15%, reduced soil erosion 20% and resulted in a B/C ratio of about 2.80. Therefore, an intercropping system of cassava + maize + upland rice or grain legumes followed by grain legumes is often practiced by farmers which have limited land and capital.
The soil fertility of cassava production areas is rather low; therefore, annual fertilization to increase soil fertility and crop productivity is generally needed. A recommended fertilization to produce 25-35 t/ha of fresh roots for monocropped cassava is 60 kg N+40 P2O5+60 K2O/ha, while that for intercropping systems to produce 20-30 t/ha fresh roots, 2 t/ha dry grain of maize and rice as well as 1 t/ha of legumes is 180 kg N + 90 P2O5 + 180 K2O/ha. When fertilizers were not applied annually, cassava yields of 25 t/ha during the first year decreased to 5 t/ha in the 8th year. Applying organic matter annually or every two years could maintain root yields of 20 t/ha, improve both soil fertility and physical conditions and increase fertilizer use efficiency. Annual fertilization of cassava is practiced by most farmers surrounding cassava factories and near big cities who grow cassava commercially, while most other farmers apply a combination of small amounts of inorganic fertilizers and farm-yard manure (FYM).