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 early‐maturing 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.

a b s t r a c tFood insecurity and malnutrition are two major challenges facing rural populations in sub-Saharan Africa (SSA). Hybrids of quality protein maize (QPM) have a crucial role here to play because QPM contains increased lysine and tryptophan concentrations and has a higher biological value than the normal maize. Information on the combining ability and heterotic patterns of QPM inbreds is crucial for the success of hybrid programs in the sub-region. Ninety-one diallel crosses derived from 14 early maturing yellowendosperm QPM inbreds were evaluated from 2010 to 2012 under Striga infested, drought, low-N and optimal environments in Nigeria. The objectives were to (i) examine the combining ability of the set of early yellow QPM inbreds, (ii) classify the inbreds into heterotic groups and identify the best testers (iii) compare the efficiencies of the heterotic grouping methods in classifying the inbreds and (iv) determine the grain yield and stability of the inbreds in hybrid combinations under the research environments. General (GCA) and specific (SCA) combining ability effects were important in the inheritance of grain yield and other traits of the inbreds. However, GCA was more important than SCA under each contrasting environment and across environments suggesting that the additive gene action was more important than the non-additive in the set of inbreds. The SCA effects of grain yield and the heterotic group's SCA and GCA of grain yield (HSGCA) methods classified the inbreds into three groups each, while the heterotic grouping based on GCA of multiple traits (HGCAMT) and the SNP-based genetic distance (GD) methods had two groups each across research environments. There was close correspondence among the classifications of all the grouping methods in terms of placement of inbreds into the same heterotic groups. The SNP-based method was the most efficient and was used to identify TZEQI 87 and TZEQI 91 as the best testers for the SNP-based heterotic groups 1 and 2. The hybrids, TZEQI 87 × TZEQI 93, TZEQI 77 × TZEQI 91 and TZEQI 80 × TZEQI 91 were identified as the most stable and high yielding across research environments and should be commercialized.

Two major constraints militating against the achievement of food security in West Africa (WA) are recurrent drought and poor soil fertility. Seventeen early maturing maize inbreds from IITA and CIMMYT were used as parents to produce 136 diallel crosses which were evaluated along with four checks in contrasting environments at four locations for 2 year in Nigeria. The objectives of the study were to (i) examine the combining ability of the lines under drought, low soil nitrogen (low N), optimal and across environments; (ii) classify the inbreds into heterotic groups using the specific combining ability (SCA) effects of grain yield, heterotic group's specific and general combining ability (HSGCA), the heterotic grouping based on general combining ability (GCA) of multiple traits (HGCAMT) and the molecular-based genetic distance methods; (iii) compare the efficiencies of the four heterotic grouping methods in classifying the inbreds and identifying the best testers; and (iv) examine the performance of the inbreds in hybrid combinations across environments. Sum of squares for GCA of inbreds for grain yield and other measured traits were larger than those of the SCA in all environments. The relative importance of GCA to SCA effects for grain yield and other traits increased from stress to nonstress environments with the additive genetic effects accounting for the major portion of the total genetic variation under all research environments. The HSGCA method classified the lines into three groups and was the most efficient because it had the highest breeding efficiency (40 %) in the test environments followed by the HGCAMT, SNP marker-based and the SCA effects of grain yield methods. Inbred TZEI 19 was identified as the best tester across research environments based on HSGCA method. Hybrids ENT 11 9 TZEI 19 and TZEI 1 9 TZEI 19 were the most outstanding and should be tested extensively in on-farm trials and commercialized.

SUMMARYMaize (Zea maysL.), a major staple food crop in West and Central Africa (WCA), is adapted to all agro-ecologies in the sub-region. Its production in the sub-region is greatly constrained by infestation ofStriga hermonthica(Del.) Benth. The performance and stability of the extra-early maturing hybrids, which are particularly adapted to areas with short growing seasons, were assessed underStriga-infested andStriga-free conditions. A total of 120 extra-early hybrids and an open-pollinated variety (OPV) 2008 Syn EE-Y DT STR used as a control were evaluated at two locations each underStriga-infested (Mokwa and Abuja) andStriga-free (Ikenne and Mokwa) conditions in 2010/11. TheStriga-resistant hybrids were characterized by higher grain yield, shorter anthesis–silking interval (ASI), better ear aspect, higher numbers of ears per plant (EPP), lowerStrigadamage rating, and lower number of emergedStrigaplants at 8 and 10 weeks after planting (WAP) compared with the susceptible inbreds. UnderStrigainfestation, mean grain yield ranged from 0·71 to 3·18 t/ha and 1·19 to 3·94 t/ha underStriga-free conditions. The highest yielding hybrid, TZEEI 83×TZEEI 79, out-yielded the OPV control by 157% underStrigainfestation. The hybrids TZEEI 83×TZEEI 79 and TZEEI 67×TZEEI 63 were the highest yielding under bothStriga-infested andStriga-free conditions. The genotype main effect plus genotype×environment interaction (GGE) biplot analysis identified TZEEI 88×TZEEI 79 and TZEEI 81×TZEEI 95 as the ideal hybrids across research environments. Twenty-three pairs of simple sequence repeat (SSR) markers were used to assess the genetic diversity among the inbred lines. The correlations between the SSR-based genetic distance (GD) estimates of parental lines and the means observed in F1hybrid underStrigainfestation and optimum growing conditions were not significant for grain yield and other traits except ASI under optimum conditions. Grain yield of inbreds was not significantly correlated with that of F1hybrids. However, a significant correlation existed between F1hybrid grain yield and heterosis underStrigainfestation (r=0·72,P<0·01). These hybrids have the potential for increasing maize production inStrigaendemic areas in WCA.

Maize (Zea mays L.), a food security crop in West Africa (WA) is threatened by Striga hermonthica (Del.) Benth. infestation. A study was conducted at five locations in WA for 2 yr to determine genetic gains in grain yield of 56 extra‐early maturing cultivars developed during three breeding eras, 1995 to 2000 (Era 1), 2001 to 2006 (Era 2), and 2007 to 2012 (Era 3) under Striga‐infested and Striga‐free conditions. Yield ranged from 1827 kg ha−1 for Era 1 cultivars to 2292 kg ha−1 for Era 3 cultivars under Striga infestation with average rate of increase in grain yield of 42 kg ha−1 yr−1 corresponding to 2.56% annual genetic gain. Under Striga‐free conditions, yield ranged from 2939 kg ha−1 for Era 1 cultivars to 3549 kg ha−1 for Era 3 cultivars, but the average rate of increase in grain yield of 54 kg ha−1 yr −1 corresponding to 1.3% annual genetic gain was not significant. The increase in yield under Striga was associated with increased plant height. Although there was no significant gain in yield under Striga‐free conditions, significant increase in plant height and decrease in root lodging were observed. Cultivars 53, 17, and 45 from Era 3 and 23, 21, 34, and 28 from Era 2 were the highest yielding and most stable under Striga infestation. Cultivars 53 and 55 from Era 3 were the most outstanding under Striga‐free conditions. Considerable progress has been made in breeding for high yielding and Striga resistant or tolerant extra‐early maize cultivars during the last three decades.

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 fi ve CIMMYT early yellow inbreds plus four checks were evaluated under drought, low-N and optimal conditions at four locations in Nigeria for 2 yr. Th e 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 specifi c combining ability (SCA) and general combining ability (GCA) (HSGCA), single nucleotide polymorphism based genetic distance (SNP-GD) and GCA eff ects of multiple traits (HGCAMT) methods. Th e GCA mean squares for all traits were greater than SCA mean squares in all environments. Th e inbreds were classifi ed into four heterotic groups each across environments by HSGCA and SNP-GD while HGCAMT method placed them into three groups. HGCAMT was the most effi cient followed by HSGCA and then SNP-GD method. Th e HGCAMT identifi ed 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.

Core Ideas The study determined genetic improvement in grain yield of the cultivars during the breeding eras, investigated trait associations, and identified high‐yielding and stable cultivars across multiple‐stress and non‐stress environments. The study revealed an annual genetic gain of 2.72 and 2.28% for the cultivars under multiple‐stress and non‐stress environments. Cultivars 2004 TZEE‐Y Pop STR C4, TZEE‐W Pop STR QPM C0, 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 the highest yielding and stable across multiple‐stress environments while 98 Syn EE‐W from era 1, FERKE TZEE‐W STR, TZEE‐W Pop STR C3, 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, TZEE‐W DT C0 STR C5 from era 3 were the most outstanding across non‐stress environments. We conclude that substantial progress has been made in breeding for multiple‐stress tolerant extra‐early maize cultivars in West and central Africa. 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 s multiple‐stress and non‐stress environments. Yield gains from era 1 to era 3 under multiple stresses was associated with increased 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.

Striga hermonthica can cause as high as 100% yield loss in maize depending on soil fertility level, type of genotype, severity of infestation and climatic conditions. Understanding the mode of inheritance of Striga resistance in maize is crucial for introgression of resistance genes into tropical germplasm and deployment of resistant varieties. This study examined the mode of inheritance of resistance to Striga in early‐maturing inbred line, TZdEI 352 containing resistance genes from Zea diploperennis. Six generations, P1, P2, F1, F2, BC1P1 and BC1P2 derived from a cross between resistant line, TZdEI 352 and susceptible line, TZdEI 425 were screened under artificial Striga infestation at Mokwa and Abuja, Nigeria, 2015. Additive‐dominance model was adequate in describing observed variations in the number of emerged Striga plants among the population; hence, digenic epistatic model was adopted for Striga damage. Dominance effects were higher than the additive effects for the number of emerged Striga plants at both locations signifying that non‐additive gene action conditioned inheritance of Striga resistance. Inbred TZdEI 352 could serve as invaluable parent for hybrid development in Striga endemic agro‐ecologies of sub‐Saharan Africa.