Alpha Y. Kamara, email@example.com
In the past two decades, maize has spread rapidly into the moist savannas of West Africa, replacing traditional cereal crops such as sorghum and millet, particularly in areas with good access to fertilizer inputs and markets.
In the West African moist savannas, higher radiation levels, lower night temperatures, and a reduced incidence of diseases and insect pests have helped to increase maize yield potentials compared with traditional areas for maize cultivation (Kassam et al. 1975). Because of the availability of short-season early maturing varieties, cultivation has gradually spread to the Sudan savanna where the growing period is 90â€“100 days. Despite the expansion in these production areas, maize yields in farmersâ€™ fields average from 1 to 2 t/ha in contrast to the higher yields of about 5 to 7 t/ha reported on breeding stations in the region (Fakorede et al. 2003).
Maize production in the savannas is faced with several production constraints which limit productivity. Poor soil fertility, drought, and Striga hermonthica parasitism combined can reduce on-farm yield by over 70% even with the use of high-yielding varieties. Land-use intensification in the northern Guinea savanna has resulted in serious land degradation and nutrient depletion (Oikeh et al. 2003). Nitrogen (N) is the nutrient most deficient in the soils and it most often limits maize yield (Carsky and Iwuafor 1995). Unfortunately, due to high cost and poor infrastructure, the availability of N fertilizers is limited.
The problem of poor soil fertility in the Guinea savanna is compounded by recurrent drought at various stages of crop growth. For maize, drought at the flowering and grain-filling stages can cause serious yield losses (Grant et al. 1989). This indicates that farmersâ€™ fields are rarely characterized by only one biotic stress. It would, therefore, be desirable to increase the tolerance of crops to several stresses that occur in the target environment (BaÃ±ziger et al. 1999).
Surveys in the northern Guinea and Sudan savannas of Nigeria showed that Striga has remained a serious problem, attacking millet, sorghum, maize, and upland rice (Showemimo et al. 2002). In northern Nigeria, over 85% of fields planted to maize and sorghum were found to be infested (Dugje et al. 2006). Grain yield losses ranged from 10 to 100% for these crops (Oikeh et al. 1996). In addition to the damage from parasitic weeds, significant losses occur in maize if other weeds (grassy and broadleaf) are left to compete freely with the crop.
To maintain a good crop and increase grain yield/unit area, agronomic best practices should be undertaken to address these constraints. Appropriate soil fertility management, drought adaptation, and proper weed management can help to close the yield gap for maize in the West African savannas.
Soil fertility management
Maize is a heavy feeder particularly in terms of mineral N. Because soils in the West African savannas are low in plant nutrients, the crop cannot be grown without the application of some form of mineral and/or organic inputs. Farmers often see a dramatic increase in the response of maize to mineral N. If the fertilizer is applied wrongly, however, use efficiency will be reduced and the benefit will be minimal. For optimum economic yield, we recommend 50 kg/ha each of N, P, and K in the form of NPK 15:15:15 at planting if moisture is sufficient or at one week after planting (WAP), and 50 kg N/ha in the form of urea at 3â€“4 WAP. Increased use of organic and mineral fertilizers, together with diversification in cropping to include legumes grown in rotation is an important tool in restoring or sustaining soil fertility of the intensifying cropping systems of the dry savannas (Sanginga et al. 2003).
These so-called â€œbalanced nutrient management systemsâ€ can be further enhanced through the use of improved cultivars that are drought tolerant and can use available nutrients efficiently, such as maize cultivars developed at IITA. This approach that has come to be known as integrated soil fertility management (ISFM) is not characterized by unique field practices, but is a fresh approach to combining available technologies in ways that preserve soil quality while promoting its productivity (Sanginga et al. 2003).
Agronomic practices for drought adaptation
Agronomic practices that enable farmers to adapt to the effect of mid- and end-of-season drought will increase maize productivity in the West African savannas. Several strategies have been developed for the conservation of soil and water to maintain productivity including rainwater harvesting, live barriers, supplementary irrigation, minimum tillage, mulching, bunded basins, and tree planting (Drechsel et al. 2004).
A central approach to increasing crop production in the dry savannas is the planting of well-adapted cultivars at the optimum date. The short growing season and frequent droughts require early and extra-early maturing crop cultivars with drought tolerance. Late- and medium-maturing cultivars, should also be drought tolerant and planted by mid-June after the rains have established. Breeders at IITA and partner institutions have developed cultivars that are early maturing, tolerant of drought, high temperatures, and low contents of soil nutrients and resistant to pests and diseases. These early maturing cultivars can be planted between mid-June and 25 July in the Guinea savannas and between the first week of July and mid-July in the Sudan savanna.
Different approaches are recommended in managing parasitic weeds on the one hand and grassy and broadleaf weeds on the other. An integrated approach is recommended for the control of parasitic weeds. Because Striga attacks the plant underground and causes damage before emergence on the host, the use of postemergence herbicides and hand pulling cannot be recommended. Damage in maize can be reduced by growing varieties that are tolerant of or resistant to Striga or by planting trap crops such as varieties of groundnut (Arachis hypogaea), soybean (Glycine max), cowpea (Vigna unguiculata), and sesame (Sesamum indicum) that stimulate the Striga seeds to germinate without providing a viable host (Carsky et al. 2000).
Some studies have shown that applying N fertilizer reduces Striga emergence and numbers, and boosts cereal grain yield (Showemimo et al. 2002; Kamara et al. 2009). Applying N fertilizer may not be feasible as a stand-alone solution to managing Striga in maize because of the high cost but the combined use of N fertilizer and Striga-tolerant/resistant varieties has shown promise in the West African savannas (Showemimo et al. 2002; Kamara et al. 2009). However, control is most effective if a range of practices is combined into a program of integrated Striga control (ISC) that can provide sustainable control over a wide range of biophysical and socioeconomic environments. Ellis-Jones et al. (2004) showed that growing Striga-resistant maize after a soybean trap crop more than doubled economic returns compared with continuous cropping with local (nonresistant) maize. Kamara et al. (2008) showed that these practices reduced Striga infestation and damage on farmersâ€™ fields and increased productivity by more than 200%.
Although manual weeding is an age-old practice in West Africa, it is no longer sustainable because of high labor costs and the aging farming population. Judicious use of herbicides is recommended to control weeds effectively and increase maize productivity. We normally recommend the use of postemergence herbicides to kill weeds before land preparation and planting. Two common types are Glyphosate and Paraquat. Glyphosate (Round-up, Glycel, Force-up) is usually strictly applied before planting, whereas Paraquat (Gramazone) can be mixed with Pendimenthalin (Stomp, Pendilin) and applied immediately after planting. Paraquat kills any live weeds in the field; Pendimenthalin kills preemerging weeds.
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