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Vitamin A deficiency, drought, low soil nitrogen (low‐N), and Striga hermonthica parasitism of maize (Zea mays L.) cause malnutrition and food insecurity in sub‐Saharan Africa. The objectives of this study were to determine combining abilities of extra‐early provitamin A (PVA) lines, classify them into heterotic groups (HGs), identify testers, and determine yield stability of hybrids under contrasting environments in two trials. In Trial 1, 190 F1 hybrids plus six checks were tested under Striga‐infested, drought, and stress‐free environments in Nigeria from 2015–2017. In Trial 2, 35 extra‐early yellow hybrids were evaluated under low‐N, Striga‐infested, and stress‐free environments in 2018. TZEEIOR 202 and TZEEIOR 205 had PVA concentrations of 23.98 and 22.56 μg g−1. TZEEIOR 197 × TZEEIOR 205 (20.1 μg g−1) and TZEEIOR 202 × TZEEIOR 205 (22.7 μg g−1) contained about double the PVA level of the commercial check, TZEEI 58 × TZEE‐Y Pop STR C5 (11.4 μg g−1). Both general (GCA) and specific (SCA) combining ability variances were significant for most agronomic traits, although GCA was larger than SCA effects, indicating GCA effects primarily controlled the inheritance of those traits. TZEEIOR 97 and TZEEIOR 197 were identified as inbred testers. TZEEIOR 197 × TZEEIOR 205 was identified as a single‐cross tester and the most stable and highest‐yielding hybrid across environments. TZEEIOR 202 and TZEEIOR 205 should be invaluable resources for breeding for high PVA. Provitamin A level was independent of hybrid yield potential, indicating that selection of superior hybrids with elevated PVA levels should be feasible. [ABSTRACT FROM AUTHOR]

Identification of outstanding maize (Zea mays L.) hybrids for target environments is complicated by genotype x environment interactions. Thirty-two early-maturity maize hybrids were evaluated at eight locations in Nigeria and six locations in Ghana for 2 yr to (i) identify highyielding, stable hybrids across locations and/or hybrids specifically adapted to different locations and (ii) identify ideal test sites for selection of superior hybrids in the two countries. Genotype, country, year, location (country) and their interactive effects were significant (P < 0·01) for grain yield, days to anthesis and silking, anthesis- silking interval, plant and ear aspects and ears per plant. Mean grain yield of the hybrids ranged from 3177 kg ha-1 for EWH-5 to 4596 kg ha-1 for EWH-29. The genotype main effects plus genotype x environment interaction (GGE) biplot analysis revealed that EWH-29, EWH-8 and EWH-30 did not differ significantly in grain yield and were the most stable hybrids in both countries, whereas EWH-26 and EWH-32 were the most stable hybrids only in Ghana. The GGE biplot analysis identified Samaru, Kafin Soli and Minjibir in Nigeria and Nyankpala, Damongo and Fumesua in Ghana as the most discriminating locations. Minjibir (Nigeria) and Nyankpala (Ghana), being most discriminating and representative locations, were considered the ideal testing sites for the respective countries. The type of cultivars evaluated determined the most suitable locations for multilocation testing within Ghana and Nigeria. Whereas hybrids EWH-29, EWH-8 and EWH-30 should be suitable for production in Nigeria, EWH-26 and EWH-32 should be promoted for production in Ghana. [ABSTRACT FROM AUTHOR]

Availability of extra-early maize cultivars has facilitated the expansion of maize production into savannas of West and Central Africa (WCA). Fifty-six extra-early maize cultivars of three breeding eras;1995 to 2000, 2001 to 2006, and 2007 to 2012 were evaluated for 2 yr under 24 multiple-stress and 28 non-stress environments in WCA. Objectives of the study were to determine genetic improvement in grain yield of cultivars developed during the breeding eras, and identify high-yielding and stable cultivars across multiple-stress and non-stress environments. Average increase in yield of 44 kg ha-1 yr-1 (2.72%) and 67 kg ha-1 yr-1 (2.28%) were obtained under multiple-stress and non-stress environments. Yield gains from era 1 to era 3 under multiple stresses was associated wiThincreased days to anthesis, reduced stalk lodging, and improved husk cover. Cultivars 2004 TZEE-Y Pop STR C4, TZEE-W Pop STR QPM C0, and TZEE-W Pop STR BC2 C0 of era 2; and TZEE-W STR 107 BC1, TZEE-W Pop STR C5, and 2012 TZEE-Y DT STR C5 of era 3 were high-yielding and stable across multiple-stress environments while 98 Syn EE-W from era 1, FERKE TZEE-W STR, TZEE-W Pop STR C3, and TZEE-Y Pop STR QPM C0 from era 2, and TZEE-W Pop STR C5, 2009 TZEE-OR2 STR QPM, 2009 TZEE-W STR, TZEE-Y STR 106, and TZEE-W DT C0 STR C5 from era 3 were outstanding across non-stress environments and should be tested extensively and commercialized. Considerable improvement has been made in breeding for multiple-stress tolerant extra-early maize cultivars. [ABSTRACT FROM AUTHOR]

Drought and low soil nitrogen (low-N) constitute major constraints to maize (Zea mays L.) production in West Africa (WA). Th e International Institute of Tropical Agriculture (IITA) and International Center for Maize and Wheat Improvement (CIMMYT) have developed drought and low-N tolerant inbreds for Africa. Combinations between selected IITA and CIMMYT inbreds could produce outstanding hybrids. Diallel crosses of 12 IITA and five CIMMYT early yellow inbreds plus four checks were evaluated under drought, low-N and optimal conditions at four locations in Nigeria for 2 yr. The objectives were to examine the combining ability of the inbreds for grain yield and other traits, classify the lines into heterotic groups, identify best testers under the contrasting environments, assess genotype × environment interactions and evaluate the yield performance and stability of single-cross hybrids. Grouping of the inbred lines was done with heterotic group's specific combining ability (SCA) and general combining ability (GCA) (HSGCA), single nucleotide polymorphism based genetic distance (SNP-GD) and GCA effects of multiple traits (HGCAMT) methods. Th e GCA mean squares for all traits were greater than SCA mean squares in all environments. The inbreds were classified into four heterotic groups each across environments by HSGCA and SNP-GD while HGCAMT method placed them into three groups. HGCAMT was the most efficient followed by HSGCA and then SNP-GD method. The HGCAMT identified inbreds ENT 17, ENT 15, and ENT 8 as best testers for heterotic Groups 1, 2, and 3. Hybrids TZEI 17 × ENT 15, and TZEI 149 × ENT 15 were outstanding across environments. [ABSTRACT FROM AUTHOR]

Food insecurity and malnutrition are major challenges facing rural populations in sub-Saharan Africa. A total of 150 quality protein maize (Zea mays L.) (QPM) hybrids generated from 30 earlymaturing QPM inbreds plus six checks were evaluated under drought, low soil N, and Striga [Striga hermonthica (Delile) Benth.]-infested environments in Nigeria for 2 yr. The objectives were to (i) examine the gene action conditioning the traits in the inbreds, (ii) classify them into heterotic groups using two methods, (iii) identify the best QPM inbred testers across environments, and (iv) identify stable and high-yielding hybrids. General and specific combining ability (GCA and SCA, respectively) mean squares were significant (P < 0.01) for grain yield and other traits across environments, indicating that additive and nonadditive gene actions were important in the inheritance of most traits of the inbreds. Preponderance of SCA sum of squares over GCA for most measured traits across environments indicated that nonadditive gene action largely modulated inbred trait inheritance. The GCA effects of multiple traits (HGCAMT) method classified the inbreds into three heterotic groups each under drought and across environments and four groups under low N and Striga-infested environments. Single nucleotide polymorphism (SNP)-based method placed the inbreds into three groups across environments and was more efficient. TZEQI 6 and TZEQI 55 were identified as testers across environments. TZEQI 44 × TZEQI 4, TZEQI 35 × TZEQI 39, TZEQI 35 × TZEQI 59, TZEQI 6 × TZEQI 35, and TZEQI 45 × TZEQI 33 were the most stable and highest-yielding hybrids across environments and should be commercialized for improved nutrition and food security in sub-Saharan Africa. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.), an important staple crop in West and Central Africa (WCA), has enormous potential to reduce food insecurity in this subregion. Research covering three periods or eras of breeding has been conducted to develop cultivars resistant/tolerant to three maize stress factors: Striga parasitism, drought, and low soil nitrogen. A study was conducted under optimal or natural growing environments at 35 locations in WCA for 2 yr to determine genetic improvement in grain yield of the maize cultivars developed during the three breeding periods: 1988-2000 (period 1), 2001-2006 (period 2), and 2007-2010 (period 3). The average rate of increase in grain yield under optimum growing conditions was 40 kg ha-1 yr-1 with a genetic gain of 1.3% yr-1, which was slightly higher than the gain of 30 kg ha-1 yr-1, an annual genetic gain of 1.2% across 16 stress environments. It was concluded that substantial improvement in the yield potential of early maize under relatively nonstress environmental conditions has been made in this subregion by breeding for stress tolerance during the past three decades. The varieties EV DT-W 2008 STR, 2009 DTE-Y STR Syn, and TZE-W DT C2 STR, all from the latest era of improvement, were identified as the highest yielding and most stable cultivars and should be promoted to contribute to food security in this subregion. [ABSTRACT FROM AUTHOR]

Maize ( Zea mays L.) is an important staple food crop in West and Central Africa ( WCA). However, its production is constrained by drought. Knowledge and understanding of the genetics of hybrid performance under drought is invaluable in designing breeding strategies for improving maize yield. One hundred and fifty hybrids obtained by crossing 30 inbreds in sets using the North Carolina Design II plus six checks were evaluated under drought and well-watered conditions for 2 years at three locations in Nigeria. The objectives of the studies were to (i) determine the mode of gene action controlling grain yield and other important agronomic traits of selected early inbred lines, (ii) examine the relationship between per se performance of inbreds and their hybrids and (iii) identify appropriate testers for maize breeding programmes in WCA. General combining ability ( GCA) and specific combining ability ( SCA) mean squares were significant (P < 0.01) for grain yield and other traits under the research environments. The GCA accounted for 64.5 % and 62.3 % of the total variation for grain yield under drought and well-watered conditions, indicating that additive gene action largely controlled the inheritance of grain yield of the hybrids. Narrow-sense heritability was 67 % for grain yield under drought and 49 % under well-watered conditions. The correlations between traits of early-maturing parental lines and their hybrids were significant (P < 0.01) under drought, well-watered and across environments. Mid-parent and better-parent heterosis for grain yield were 45.3 % and 18.4 % under drought stress and 111.9 % and 102.6 % under well-watered conditions. Inbreds TZEI 31, TZEI 17, TZEI 129 and TZEI 157 were identified as the best testers. Drought-tolerant hybrids with superior performance under stress and non-stress conditions could be obtained through the accumulation of favourable alleles for drought tolerance in both parental lines. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) is a major staple crop in West Africa and has the potential to mitigate the food insecurity in the subregion. However, maize grain yield is severely constrained by drought. A study was conducted at 13 locations in West Africa for 2 yr to determine genetic gains in yield of cultivars developed during three eras, 1988 to 2000 (first-generation cultivars), 2001 to 2006 (second-generation cultivars), and 2007 to 2010 (third-generation cultivars) under drought and optimal conditions. Under drought, yield ranged from 1346 kg ha-1 for first-generation cultivars to 1613 kg ha-1 for third-generation cultivars with a genetic gain of 1.1% yr-1. Under optimal conditions, yield gain ranged from 3363 kg ha-1 for first-generation cultivars to 3956 kg ha-1 for third-generation cultivars with genetic gain of 1.3%. The average rate of increase in yield was 14 and 40 kg ha-1 yr-1 under drought and optimum conditions. Genetic gains in yield from first- to third-generation cultivars under drought was associated with improved plant aspect and husk cover, whereas under optimum conditions it was associated with plant and ear aspects, increased ears per plant, plant and ear heights, and improved husk cover. Cultivars TZE-W DT C2 STR, DTE-W STR Syn C1, DT-W STR Synthetic, 2009 DTE-W STR Syn, and EV DT-W 2008 STR were high yielding and stable across drought environments. Substantial progress has been made in breeding for drought tolerance during the last three decades. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) has the greatest yield potential in savannas of West and Central Africa (WCA). However, production in this agroecology is constrained by low levels of N, drought, and parasitism by striga [Striga hermonthica (Del) Benth.]. Diallel crosses involving 17 extra‐early white maize inbreds were evaluated under drought, striga, and optimal environments at three locations in Nigeria for 2 yr. The objectives of this study were to estimate general and specific combining ability for grain yield, classify the inbreds into heterotic groups, and identify inbred testers. Gene action controlling grain yield and most other traits was predominantly nonadditive. TZEEI 14, TZEEI 29, TZEEI 21, and TZEI 13 were the best testers across environments. Four heterotic groups were identified based on the heterotic group's specific and general combining ability (HSGCA) method and five groups based on microsatellite markers. TZEEI 29 × TZEEI 14 was the second highest yielding and most stable across environments. Fifty‐nine alleles with an average of 2.57 alleles per locus were detected among the inbreds. Correlation between genetic distance estimates of parental lines and their F1 hybrid means were not significant for grain yield and most other traits. There was close correspondence between heterotic groups identified by the HSGCA method and microsatellite markers. Extra‐early inbreds and hybrids with drought tolerance and striga resistance genes identified should be used to reduce instability of maize yields in WCA. [ABSTRACT FROM AUTHOR]

Grain yield of maize (Zea mays L.) has low heritability under low soil nitrogen (low N) and drought, necessitating the use of secondary traits with strong associations with yield for selection. A base index involving anthesis-silking interval, plant and ear aspects, ears per plant, and stay green characteristic is used for selection for yield under drought and low-N stresses. Reports are contradictory on the reliability of stay green characteristic for selecting for yield under drought stress and of ears per plant and anthesis-silking interval in selecting for low-N tolerance. Ninety extra-early inbreds were evaluated for 2 yr at three locations in Nigeria under low N and drought to confirm reliability of stay green characteristic for selecting for drought tolerance and of ears per plant and anthesis-silking interval for low N. Plant aspect and plant and ear heights were identified as the most reliable traits for simultaneous selection for yield under low N and drought in the extra-early inbreds. Stay green characteristic was unreliable for selecting drought tolerant genotypes while ears per plant and anthesis-silking interval were not among the reliable traits for selecting low-N tolerant genotypes. Ear height, plant and ear aspects, and stay green characteristic were identified by both path-coefficient and genotype main effect plus genotype × environment interaction (GGE) biplot analyses as reliable for selecting for low N and ear aspect, plant height, and anthesis-silking interval for drought tolerance. [ABSTRACT FROM AUTHOR]

Secondary traits can improve precision with which drought or low soil nitrogen (low-N) tolerant maize (Zea mays L.) genotypes are identified, compared to measuring only grain yield under each stress. The objective of this study was to identify secondary traits for indirect selection for yield improvement under drought and/or low-N. Two sets of experiments were conducted at four locations in Nigeria for 2 yr. In the first experiment, cultivars were evaluated under low (30 kg ha-1) and high (90 kg ha-1) N and drought and well-watered environments in the second experiment. Most reliable traits for selection for yield under drought were ear aspect (EASP), plant aspect (PASP), anthesis-silking interval (ASI), and number of ears per plant (EPP). Days to anthesis (DYA) and silking (DYS) were the most reliable under well-watered environments. Under low-N, most reliable traits were DYA, DYS, stay green characteristic (LDTH), ASI, plant height (PLHT), EPP, EASP, and PASP whereas PLHT, EPP, PASP, and EASP were most reliable traits under high-N environments. The ASI, EPP, EASP, and PASP were identified as most reliable traits for simultaneous selection of drought and low-N tolerant genotypes. Under drought, cultivars TZE-Y DT STR C4, TZE-W DT STR C4, and TZE Comp 3 DT C1F2 were outstanding and closest to the ideal cultivar. Seven cultivars were closest to the ideal under low-N, two of which were among the three selected under drought Stress. Faster progress in selecting for improved grain yield is possible using EPP, EASP, PASP, and ASI under either drought or low-N. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) hybrids resistant to Striga hermonthica (Del) Benth and tolerant to drought are vital to the sustainability of productivity of the maize-based farming systems of West and Central Africa (WCA). Thirty-six diallel crosses derived from nine early maturing white maize inbreds were evaluated under drought, well-watered, striga-infested, and striga-free conditions at five locations in Nigeria between 2007 and 2009. The objective was to examine the combining ability, performance and stability of the inbreds, and to identify the heterotic groups. Additive and nonadditive gene actions were important in the control of the inheritance of drought tolerance and striga resistance in the inbreds. General combining ability (GCA) mean squares of grain yield and other traits were larger than those of specific combining ability (SCA), indicating that additive gene action was more important in the inheritance of drought tolerance. The GCA mean squares for striga damage rating at 8 and 10 weeks after planting (WAP) were about six and five times greater than those of the SCA, indicating that additive gene action played a major role in the inheritance of the striga traits. The GGE biplot analysis revealed that TZEI 4 and TZEI 5 were the most promising inbreds in yield performance and stability across the test environments. Two heterotic groups were identified; [TZEI 7, TZEI 19, TZEI 2, TZEI 4] and [TZEI 5, TZEI 3]. Tester TZEI 3 was the closest to the ideal tester. Entry TZEI 7 had the highest GCA effects and TZEI 2 the lowest. [ABSTRACT FROM AUTHOR]

Appropriate testers are needed as well as information on heterotic patterns of tropical lowland early-maturing germplasm. Diallel crosses involving nine tropical early maturing maize (Zea mays L.) inbreds were evaluated at five locations in Nigeria from 2007 to 2009 under drought stress, when Striga infested, and in optimal growing conditions to examine the combining abilities and heterotic patterns and identify appropriate testers. Results revealed that TZEI 17 was the closest to the ideal tester and may be considered as the best under drought stress, TZEI 23 and TZEI 9 under artificial Striga infestation, and TZEI 13 across growing conditions. No appropriate tester was identified for the optimal growing conditions. Inbreds were placed in three contrasting tester groups under drought stress, and four groups each were placed under Striga infestation and optimal growing conditions and across research environments. Hybrids between inbreds in contrasting tester groups of each research environment are expected to show heterosis. TZEI 23 x TZEI 13 had a superior performance under all research conditions, suggesting that it has a broad adaptation. Classification of inbreds into distinct tester groups was not possible for 33% of the inbreds under drought stress and for 22% when Striga infested, under optimal growing conditions, and across the test environments. [ABSTRACT FROM AUTHOR]