**2.8 Maize-legume rotation and intercropping**

Growing maize and grain legumes together as intercrops or in rotation offers many advantages compared to growing maize continuously as a monocrop [46]. Legumes increase nitrogen (N) in soils through biological nitrogen fixation and subsequent

**Figure 8.** *An innovative, staggered maize-soybean intercrop.*

mineralization and can be used to offset the N requirements of the maize crop. Rotation and intercropping legumes with maize (**Figure 8**) improve the efficiency of land, nutrient, and water use due to synergistic effects between the crops [47]. Mixing maize and legumes also reduces the infestation of weeds, pests, and diseases in farmers' fields. Intercropping is crucial for small and marginal farmers in numerous countries as it diversifies and mitigates risks, improves the efficiency of land utilization, enhances soil fertility, and boosts economic returns, particularly in unpredictable weather conditions [48].

Large numbers of farmers in major maize production areas across Sub-Saharan Africa practice maize and legume rotation and intercropping, substantially increasing maize and legume yields and total harvests from a given land area. Growing a highenergy crop such as maize with high-protein legume results in improved diets among small-scale farmers and mitigates the risk of a hunger season when one of the two crops may fail because of drought or pest attacks. Biological nitrogen fixation in the root nodules of legumes benefits the productivity of maize crops rotated in the same field because part of the assimilated nitrogen is transferred between the crops through the decomposition of legume residues [49]. Mineral fertilizer application in mixed cropping systems is used very efficiently since either of the crops can benefit from residual nutrients that might have otherwise been lost due to the different root depths and distribution of maize and legumes [50]. Maize and legume intercropping are beneficial by reducing weed infestation, soil erosion, and run-off. This plant arrangement increases crop coverage and protection throughout the growing season.

Legumes can offer other advantages to maize crops. For example, soybeans and cowpeas can help to reduce parasitic striga weed infestations. This is because these legumes induce the germination of Striga seeds, but the weed does not infect them. Taller-statured maize benefits the legumes by better-regulating soil temperature soil through shading. However, understory legumes compete with maize for light, water, and nutrients. Intercropping can be a good way to increase maize yields and generate larger returns to labor. However, some challenges are associated with intercropping, such as careful crop selection and spacing. Additionally, some field

operations, such as mechanization and chemical weeding, can be more difficult with intercropping systems [51].

### **2.9 Pre-emergent herbicides for weed management**

Weeds can compromise maize croplands by competing for limited soil water and nutrients. Uncontrolled weeds can reduce yields and limit returns on agro-input investments. Controlling weeds in maize is critical, particularly during its early establishment and vegetative growth phases that extend to 10 weeks or so after planting. Without effective weed control, maize yields can be reduced by up to 50% on average [52], and losses can reach 80% if no measures are taken. In Africa, most smallholder farmers weed their maize crops by hand, a labor-intensive practice that must be repeated 2 or 3 times to be effective. This is because shallow hoeing can agitate the soil and promote the germination of weed seeds. Pre-emergent herbicides can help to reduce labor requirements by eliminating the need for hand weeding and help to improve soil quality by reducing the need for tillage, which can damage soil structure.

Pre-emergence herbicides prevent weeds from developing and allow fields to remain weed–free through the critical stages of crop establishment (**Figure 9**). This is important because it prevents weeds from competing with the maize crop for water, nutrients, and sunlight, which can help to reduce crop losses. This effect continues until the maize canopy shades the ground and weeds become suppressed [53]. This class of herbicides is applied shortly before or when planting maize and after the soil has been tilled. This technology prevents weed seedlings from establishing but requires that the proper chemicals are affordable, and that application equipment and safety gear are available. Some weeds emerge in most maize fields after crop establishment during the latter vegetative stage. These late-season weeds are effectively controlled by spraying recommended post-emergence herbicides to keep the fields

**Figure 9.** *Weedy (left) and weed-free maize understory.*

clean until harvest, further enhancing maize productivity. Maize is very sensitive to competition from weeds between the emergence to the unfurling of six leaves. During this time, maize's fibrous root system is under development, and its shoots may become outcompeted by faster-growing plants. Maize gains the upper hand against weeds with pre-emergent herbicides. These herbicides remove the competition for light, nutrients, and moisture during maize's vulnerable initial growth phase. This, in turn, speeds up the growth of both roots and shoots.
