M. A. B. Fakorede

Agronomic performance of maize cultivars representing three decades of breeding in the Guinea Savannas of West and Central Africa

Maize improvement at the International Institute of Tropical Agriculture (IITA), which began in the 1970s, built on the germplasm and experience of earlier years. The main breeding emphasis was to develop maize cultivars and hybrids with high yield potential and durable resistance to diseases and pests with specific adaptation to the different agro-ecological zones of West and Central Africa. Over the years, open-pollinated cultivars have been developed with different levels of resistance to biotic and abiotic stresses. Identification of the factors that contributed to improvements in the maize cultivars developed during the past decades may be useful to sustain the genetic gain from selection in the future. A study was conducted to quantify genetic gains in yield and associated traits of open pollinated maize cultivars released from 1970 to 1999 in the West African savannas. The genetic gain in grain yield was 0.41 % per year and seems to be associated with increases in total biomass and kernel weight, and reductions in plant height and days to flowering (anthesis and silking). There was no significant change in harvest index of the cultivars.

Genetic gains in grain yield under nitrogen stress following three decades of breeding for drought tolerance and Striga resistance in early maturing maize

Breeding for resistance to Striga hermonthica Del. (Benth) and tolerance to drought has been a major strategy to improve maize ( Zea mays L.) production and productivity in West and Central Africa during the last three decades. The three decades consisted of three breeding periods or eras based on the germplasm and methodologies used; that is, 1988–2000, 2001–06 and 2007–11. A total of 50 early maturing cultivars, combining Striga resistance with drought tolerance were developed, including 15, 16 and 19 cultivars for the three periods, respectively. Although the cultivars were not selected intentionally for low-nitrogen (N) tolerance, it was hypothesized that tolerance to low-N had been significantly improved while selecting for drought tolerance and Striga resistance. This hypothesis was tested by evaluating the 50 cultivars in 2010 and 2011 in Nigeria at Mokwa and Ile-Ife under both low-N (30 kg N/ha) and high-N (90 kg N/ha) levels. Under low-N conditions, grain yield improved from 2280 kg/ha during the first period to 2610 kg/ha during the third period, an increase of 165 kg/ha per period with r 2 of 0·99. Under high-N, yield increased from 3200 to 3650 kg/ha, an increase of 225 kg/ha and r 2 of 0·93. Relative gain per period was 30 kg/ha for the two N rates with r 2 values of 0·99 and 0·94 respectively. Grain yield performance of the 50 cultivars under low-N conditions adequately predicted their performance under high-N. Selection for Striga resistance and drought tolerance in early maturing maize populations enhanced low-N tolerance in the maize cultivars derived from the populations. The improvement was higher in later than earlier breeding periods.

Introgression of Alleles from Maize Landraces to Improve Drought Tolerance in an Adapted Germplasm

Maize (Zea mays L.) landraces in the northern Guinea savanna and Sudan savanna in West and Central Africa appear to have some drought-adaptive traits. This study was initiated to assess the level of improvement in yield potential and other agronomic traits achieved under drought stress (DS) and in multiple locations (ML) after introgression of alleles from maize landraces into an elite maize variety (AK9443-DMRSR) via backcrossing. Six backcross (BC) populations together with recurrent parent (AK9443-DMRSR), a commercial hybrid (Oba Super-II), and an improved variety (TZLCOMP4C1) were evaluated under controlled DS and full irrigation (FI) during the dry seasons of 1999 and 2000, as well as in seven ML trials. No significant differences were observed among genotypes for grain yield and most of the traits measured under DS and FI. Significant differences were recorded among genotypes for grain yield and other agronomic traits measured in ML and across 11 environments. Drought stress reduced grain yields of the BC1F2 populations by 64% and recurrent parent by 71%. In ML trials, at least half of the populations were better than recurrent parent. The top three BC1F2 populations produced more grains than the recurrent parent (258–360 kg/ha) and Oba Super-II (555–657 kg/ha) with introgression of only 25% genome of the landraces. We concluded that backcross procedure was able to transfer a quantitative trait of grain yield of an elite recurrent parent into maize landraces. Additional backcross generations are needed for improved performance of the BC1F2 populations in drought-prone environments.

Enhancing the capacity of national scientists to generate and transfer maize technology in west and central Africa : Research implementation, monitoring and evaluation

The West and Central Africa Collaborative Maize Research Network (WECAMAN) was established in 1987 to strengthen the capacity and capability of national programmes of West and Central Africa (WCA) to tackle regional constraints to maize production. The Network created several mechanisms for implemenling, monitoring and evaluating maize research and development activities, including research project development and implementation, attendance and quality of paper presentation at techanical conferences organized by the Network, scientific monitoring tours, consultation visits, mid-term reviews, and end-of-project reviews and impact assessment. WECAMANs approach to the system of allocating research responsibilities and competitive grants resulted in increased research efficiency and the generation of sustainable technologies that have catalysed increased maize production in the region.

Maize in Sub-Saharan Africa: Importance and Production Constraints

Maize (Zea mays L.), an important food, feed, and industrial crop in sub-Saharan Africa (SSA), has been researched extensively for genetic enhancement for more than a half century in the subregion. One aspect that received intensified research attention in the last three or four decades is the genetic enhancement of early and extra-early maturing germplasm for resistance or tolerance to drought, Striga, and low soil nitrogen. Improved open-pollinated varieties (OPVs) and hybrids with tolerance/resistance to the stresses and/or high levels of micronutrients such as beta carotene along with elevated levels of lysine and tryptophan are now available in SSA. A brief description of the achievements and the challenges still confronting maize breeders and geneticists is presented in this chapter.

Genetic analysis of drought-adaptive traits at seedling stage in early-maturing maize inbred lines and field performance under stress conditions

Maize hybrids that are tolerant to drought at the seedling stage are needed to boost productivity in the rainforest agro-ecology of West Africa. Genetics of tolerance of maize seedling to drought stress is not well understood and is poorly documented. The objectives of this study were to screen early-maturing maize lines for seedling drought tolerance, determine the inheritance and the combining ability of selected inbred lines, and evaluate the performance of seedling drought-tolerant hybrids under field conditions. Forty-nine early maize lines were screened for drought tolerance at the seedling stage. Ten drought-tolerant and two susceptible inbred lines were selected and used in diallel crosses to generate 66 hybrids. The twelve inbred lines and their hybrids were evaluated under induced drought at seedling stage in the screen house and under marginal growing conditions on the field for two seasons. Data collected were subjected to analysis of variance using the DIALLEL-SAS program. Mean squares for both GCA and SCA were significant for most traits in all research environments, indicating that additive and non-additive gene actions are controlling seedling traits under stress conditions. However, for most traits, SCA was preponderant over GCA in all environments, indicating overdominating effect of non-additive gene action. Which in turn implied that the best improvement method for the traits is hybridization. Inbred TZEI 7 had the best GCA effect for seedling traits under screenhouse conditions and for grain yield and other agronomic traits under drought conditions in the field. Hybrids TZEI 357 × TZEI 411 and TZEI 380 × TZEI 410 showed superior SCA effects under screen house conditions. In conclusion, the study established wide genetic variability for drought tolerance at seedling stage among tropical early-maturing maize germplasm however, the non-additive gene action was more important for most seedling traits.

Selection Indices and Use of Secondary Traits

Yield is controlled by many genes, and it is highly influenced by the environment along with often high genotype × environment interaction (GEI), resulting in low heritability estimates. Secondary traits are under the control of fewer genes, have lower GEI, and are characterized by higher heritability estimates. Therefore, traits that show consistent genotypic and phenotypic relationships with yield may be more effective as selection criteria than direct selection for grain yield alone. Breeders commonly use three methods to select for several traits, including tandem selection, independent culling, and index selection. In tandem selection, individual traits are improved successively. In independent culling, individuals must surpass a certain minimum value for each trait to be selected. The plants that are outstanding in certain traits may not be selected if they do not meet the minimum standards for other traits, whereas individuals that are relatively mediocre in some traits are selected as long as they meet the cutoff value in the others. In index selection, each trait is weighted by a score, and the individual scores are summed to give a total score, which is referred to as the index value (I) for the genotype. This is an attempt to correct the weaknesses in tandem selection and independent culling methods. For example, selection for maize grain yield under severe drought stress or low-N has often been considered inefficient because the estimates of heritability of grain yield has been observed to decline with reduced yield levels characteristic of stressed environments. Under these conditions, secondary traits may increase selection efficiency provided they have adaptive value, relatively high heritability, significant genetic correlation with grain yield, and are easy to measure. Based on this, base indices for selecting for Strigaresistance, drought, low-N, and multiple stress tolerance have been developed by IITA and CIMMYT scientists for selecting for tolerance to the individual stresses and/or multiple stress tolerance. Ensuring that greater response to selection is achieved using secondary traits in combination with grain yield in a selection index rather than the primary trait per se is an important strategy in the enhancement of early and extra-early maize in SSA.

Molecular Approaches to Maize Improvement

The presence of quantitatively inherited traits in a genotype can make the testing procedure complex, unreliable, and expensive. Under such circumstances, indirect selection using molecular markers becomes an efficient complementary breeding tool. When target traits can be easily identified and linked with one or more markers, the marker loci can be used as a surrogate for the trait, resulting in greatly enhanced breeding efficiency. Marker-assisted selection (MAS) involves the use of DNA markers instead of phenotypic selection to speed up the process of development and release of cultivars. Marker-assisted selection is useful for selecting quantitatively inherited traits that are difficult or expensive to measure, exhibit low heritability, and/or are expressed late during the developmental process. The approaches to MAS include marker-assisted backcrossing (MABC) and marker-assisted recurrent selection (MARS). Marker-assisted selection exploits the tight linkage between QTL and nearby markers and was proposed as easier, faster, and more efficient selection method. However, the use of marker-assisted selection has been limited in complex traits because of low power to detect QTL and bias in the estimated marker effects. The use of markers to track transgenes or pyramid favorable alleles determining a significant proportion of the phenotypic variance is possible for many crops, including maize. It is now generally accepted that the role of MB goes beyond the manipulation of elite alleles at a few loci in biparental segregating populations. There is a need for validation of genetic gain of favorable alleles so that markers could be developed and employed.

Breeding for Disease Resistance in Maize

The maize production environments in sub-Saharan Africa (SSA) are conducive to the development of many disease-causing organisms, which have infected the crop, causing serious reduction in maize production and productivity. Some maize diseases infect maize in all SSA ecologies, while some others are specific to some environments and absent from others. The prominent maize diseases in SSA changed over time, and presently, the most devastating are gray leaf spot (GLS), incited by Cercospora zeae-maydis; northern corn leaf blight, incited by Exserohilum turcicum; southern corn rust (Puccinia polysora); maize streak virus (MSV) disease, transmitted by the leafhopper, Cicadulina mbila; downy mildew, by Peronosclerospora sorghi; ear rots, incited by several fungi, including Aspergillus sp. and Fusarium sp.; and the recently discovered disease that is spreading fast across SSA, the maize lethal necrosis (MLN), a disease incited by the synergistic effect of maize chlorotic mottle virus (MCMV; Tombusviridae: Machlomovirus) and several other potyvirus, such as maize dwarf mosaic virus (MDMV), sugarcane mosaic virus (SCMV), and wheat streak mosaic virus (WSMV). There are several methods used for disease control, but the most effective and economic method is breeding for host plant resistance. In this chapter, four case studies of this breeding approach are presented and their effectiveness in SSA clearly highlighted.

Breeding for Striga Resistance

Striga hermonthica(L.) Kuntze, a parasitic weed, is endemic in a large part of the Guinea savanna of West and Central Africa (WCA). Strigaplants do much damage underground before the parasitic plants appear on top of the soil around the maize plant. The weed, which could cause 100% yield loss in the maize crop, has defied all control efforts at the national level of WCA countries thereby forcing farmers to abandon their farmlands. Teaming up with the national research scientists, IITA initiated a massive control effort on Striga. Screening and breeding were done for tolerance, the ability of the host to perform well regardless of the number of Striga plants parasitic on the host plant, as well as resistance, the ability of the host plant to reduce the number of Striga plants surviving on it. An efficacious manual infestation method was established and has been in use since its discovery. Genetics of tolerance/resistance was investigated, along with combining ability of inbred lines in hybrid seed production. Early and extra-early maize populations have been subjected to recurrent selection under Striga infestation, and the response to selection along with correlated responses in other nonselected traits has been evaluated under Striga-infested and Striga-free conditions. Early and extra-early Striga-tolerant/Striga-resistant populations, inbred lines, and hybrids are now available to farmers in the Striga endemic areas of sub-Saharan Africa (SSA). The materials also have value addition of good performance in non-Striga environments.