4. Water management
Once established, cassava can grow in areas that receive just 400 mm of average annual rainfall. But much higher yields can be obtained with higher levels of water supply.
Delaying planting can lead to drastic yield reductions
Although cassava can withstand periods of drought, it is very sensitive to soil water deficit during the first three months after planting. Water stress at any time in that early period reduces significantly the growth of roots and shoots, and impairs subsequent development of the storage roots.
Once established, cassava can grow in areas that receive just 400 mm of average annual rainfall. But higher yields have been obtained with much higher levels of water supply. Research in Thailand indicates that maximum root yields are correlated with rainfall totalling about 1 700 mm during the fourth to eleventh month after planting. Cassava also responds well to irrigation. In trials in Nigeria, root yields increased sixfold when the quantity of water supplied by supplementary drip irrigation matched that of the season’s rainfall.
In most parts of the world, cassava is almost exclusively a rainfed crop. In areas with only one rainy season per year, farmers usually plant as soon as the rains start. Delaying planting can lead to drastic yield reductions. In areas with two relatively short rainy seasons per year, cassava can be planted in the early or middle part of either rainy season and harvested after 10 to 14 months, preferably during the dry season.
Planting early in the rainy season will generally produce the highest yields because the plants have adequate soil moisture during the most critical part of their growth cycle. However, research has shown that yields may vary according to the variety used, the soil type, the plant’s age at harvest, and the rainfall intensity and distribution during any particular year.
Planting methods need to be tailored to soil moisture conditions under rainfed production. When the soil is not well-drained and too wet owing to heavy rains, it is better to plant stakes on the top of ridges or mounds to keep the roots above standing water.
However, where cassava is planted during dry periods, the rates of stake sprouting and plant survival are significantly higher when cassava stakes are planted on the flat, owing to the slightly higher soil moisture content of the topsoil. Stakes should be planted at a shallow depth, of 5 to 10 cm, in heavy and wet soils, but slightly deeper in light-textured and dry soils to avoid surface heat and lack of moisture.
If the first rains are intense, the risk of waterlogging is greatest in shallow soils, and in poorly drained soils compacted by heavy tilling equipment. The risk of waterlogging can be reduced with zero tillage, which improves internal drainage. Where tillage is practised, soil should be prepared when it is not too dry or too wet. If necessary, a subsoiler can be used to break up the compacted soil layer.
Planting towards the end, rather than at the beginning, of the rainy season usually results in lower yields, but it has some advantages: less weed competition and – if the crop is harvested in the off-season – the possibility of higher market prices. Another advantage is that the late planting of cassava does not coincide with other major agricultural activities, so there is less competition for labour.
Cassava benefits from supplemental irrigation during rainless periods. Research in India found that during periods of drought, yields increased with increasing amounts of surface irrigation water applied. Full irrigation, at 100 percent of crop water requirements, doubled the root yield obtained without irrigation.
Drip irrigation makes more efficient use of water by providing small, frequent applications, which saves water while maintaining soil moisture at a level that is highly favourable to crop growth. In trials in India, drip irrigation produced about the same root yields as flood irrigation – around 60 tonnes per ha – using 50 percent less water. When the water applied through drip irrigation was equal to that used in flood irrigation, yields continued to increase substantially, to 67.3 tonnes per hectare.
Similar results were reported from experiments in south-western Nigeria. In plots under supplemental drip irrigation, yields rose sharply with increasing levels of water applied. Yield increases at low application rates were significant – irrigation that boosted the water supply by 20 percent almost doubled yields.