**14. Weed management**

Compared to several other crops, the initial growth of cassava is slow. As a result of this and the wide spacing between planted stakes, weed emergence and competition with the crop for available soil nutrients and sunlight is rife. In the first 4 months after planting, cassava can easily be overwhelmed by competition from weeds, and other leguminous plants. In East Africa, weeds are often a more serious production constraint than insect pests or diseases and may reduce yields by about 50% [24]. In Nigeria, farmers expend more resources controlling weed than other aspect of crop production. Once the cassava canopy has closed, it'll shade out most weeds and keep the sector almost completely weed-free (**Figure 5**). Six to eight months after planting, when cassava starts to shed many leaves (especially during the dry season), weeds may reappear, but this generally does not seriously affect yields. Excessive late weed growth may make harvesting harder, but also can protect the soil from erosion if postharvest rains are heavy.

While cultural controls might not perfectly control weed, they are effective in reducing weed competition, and thus the necessity for mechanical or chemical weeding [25]. Cultural control begins with selection of high-quality planting material from varieties with vigorous early growth and tolerance or resistance to diseases and pests. High planting density and therefore the correct type and rate of fertilizer can stimulate early crop growth and rapid canopy closure. Planting within the season under drip irrigation also can encourage the expansion of cassava but not that of weeds. The soil should be covered with a thick layer of mulching material such as rice straw or maize residues to stop weed. Also, intercropping cassava with fast-growing plants, like melons, squash, pumpkins, common beans, groundnuts, soybeans, mungbeans and cowpeas proved to be effective in controlling weeds. Since those are short-duration crops, they will be harvested after about 3 to 4 months,

**Figure 5.** *Cassava canopy checks weeds.*

#### *Cassava Production Enterprise in the Tropics DOI: http://dx.doi.org/10.5772/intechopen.104677*

when the cassava canopy closes and weeds are shaded out. While intercrops may reduce cassava root yields, they markedly reduce weed growth, and offer an ecofriendly and fewer expensive alternative to spraying with herbicides. A study in Nigeria of legume cover crops during a mixed cassava/maize system reported significant improvements in cassava root yields when velvet beans were grown to suppress weeds [26]. Common among the smallholder cassava farmers is mechanical control measures–by hoeing, starting after emergence. Research in Colombia found that with hand-weeding at 15, 30, 60 and 120 days after planting, cassava root yields were 18 tonnes per ha compared with only 8 tonnes/ha were obtained when weeds were controlled with herbicides. When weeds were not controlled in the least, yields fell to only 1.4 tonnes.

Weeds are often controlled with herbicides. Although many herbicides are highly toxic and, being water soluble and protracted within the environment, are often washed away to contaminate ground and surface water. Farmers got to exercise care within the choice of the herbicide to be used and follow the recommendation of local plant protection specialists. Pre-emergence herbicides do not kill existing weeds. Instead, they prevent weed seeds within the soil from emerging or, at least, reduce their rate of growth. Pre-emergence herbicides are either incorporated into the soil before planting or applied on the soil surface with a knapsack sprayer immediately after planting. Pre-emergence herbicides that are selective for cassava are often applied over the vertically planted stakes without affecting cassava sprouting or yield. The application of pre-emergence herbicides can maintain a cassava field almost weed-free for 6–8 weeks after planting. Farmers may apply a mix of two herbicides; one that controls the grassy weeds and the other on the broad-leaf weeds. A lower dosage is suggested on light-textured soils, while a higher dosage could be needed in heavy soils, like clay-loamy. Special care must be taken when cassava is grown in association with other crops, because the pre-emergence herbicides normally used for cassava may harm the intercrop. At about 2 months after planting, weeds may have to be controlled again to scale back competition with cassava. This is often usually done by hoeing or using an animal or tractor-mounted cultivator, counting on the peak of the growing cassava plants and therefore the extent of cover closure. When most of the weeds are grassy species, it's also possible to use a selective post-emergence herbicide, which kills grasses but does not affect the cassava plant. Post-emergence herbicides are often used about 4–5 months after planting, when some bottom leaves start to drop off. It is best done on a windless days and with a nozzle shield to stop spray from reaching the cassava stems or leaves.
