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The two most important activities in maize breeding are the development of inbred lines with high values of general combining ability (GCA) and specific combining ability (SCA), and the identification of hybrids with high yield potentials. Genomic selection (GS) is a promising genomic tool to perform selection on the untested breeding material based on the genomic estimated breeding values estimated from the genomic prediction (GP). In this study, GP analyses were carried out to estimate the performance of hybrids, GCA, and SCA for grain yield (GY) in three maize line-by-tester trials, where all the material was phenotyped in 10 to 11 multiple-location trials and genotyped with a mid-density molecular marker platform. Results showed that the prediction abilities for the performance of hybrids ranged from 0.59 to 0.81 across all trials in the model including the additive effect of lines and testers. In the model including both additive and non-additive effects, the prediction abilities for the performance of hybrids were improved and ranged from 0.64 to 0.86 across all trials. The prediction abilities of the GCA for GY were low, ranging between − 0.14 and 0.13 across all trials in the model including only inbred lines; the prediction abilities of the GCA for GY were improved and ranged from 0.49 to 0.55 across all trials in the model including both inbred lines and testers, while the prediction abilities of the SCA for GY were negative across all trials. The prediction abilities for GY between testers varied from − 0.66 to 0.82; the performance of hybrids between testers is difficult to predict. GS offers the opportunity to predict the performance of new hybrids and the GCA of new inbred lines based on the molecular marker information, the total breeding cost could be reduced dramatically by phenotyping fewer multiple-location trials.

The use of temperate maize (Zea maysL.) inbreds with expired Plant Variety Protection in tropical maize breeding programs could enhance the combining ability for grain yield among tropical heterotic groups. We used DNA markers from the DArTseq genotyping‐by‐sequencing platform to investigate the genetic structure of lines with expired U.S. Plant Variety Protection (ex‐PVP) relative to the International Maize and Wheat Improvement Center's (CIMMYT's) maize heterotic groups. Neighbor‐joining cluster analysis revealed two major groups: CIMMYT and ex‐PVP. The CIMMYT lines clustered according to their pedigree relationships and adaptation, but not according to their heterotic groups. In contrast, ex‐PVP lines clustered according to the Stiff Stalk Synthetic (BSSS) and non‐Stiff Stalk Synthetic (NSSS) heterotic groups, except for a few lines that were considered to be mixed. The genetic divergence, estimated as Wright's fixation index (FST), between BSSS and NSSS (FST= .053, P< .01) was four times as large as the divergence between CIMMYT Tuxpeño and non‐Tuxpeño heterotic groups (FST= .013, P= .068). Estimates of genetic divergence marginally favored breeding with BSSS in Tuxpeño and NSSS in non‐Tuxpeño. However, CIMMYT breeders may still exploit the ex‐PVP heterotic structure fully only by ensuring that the temperate heterotic groups are placed on opposite sides of the Tuxpeño and non‐Tuxpeño heterotic pattern. We also showed how estimates of admixture from model‐based clustering could be used to avoid ex‐PVP lines of mixed heterotic background when selecting lines to maximize the genetic divergence and combining ability of CIMMYT heterotic groups.

Maize (Zea maysL.) hybrids with enhanced provitamin A (proVA) content have been deployed in sub‐Saharan Africa, where low soil nitrogen and drought stress are common. The objectives of this study were to assess: (i) the effects of drought and low‐N stress on grain proVA content of hybrids with enhanced proVA content, and (ii) the inheritance of proVA carotenoids under these stress conditions. An 11‐line diallel cross (55 F1crosses) was evaluated for carotenoid content and grain yield under optimum conditions, drought, and low‐N stress. Compared with the optimum treatment, mean proVA was lower under both stress treatments. The consistency of genetic effects across stress treatments suggested that hybrids with improved proVA content can be developed for a broad range of environments, provided they are sufficiently adapted. General combining ability (GCA) was significant (P< 0.01), and accounted for >85% of the variation among hybrids, whereas specific combining ability (SCA) effects were generally weak (P< 0.05), accounting for 5 to 15% of hybrid sums of squares across the three treatments. These results indicated that the inheritance of proVA was not affected by stress treatments. A negative correlation between grain yield and proVA carotenoids was detected, but the data suggested that it was caused by the genetic background of the germplasm used rather than pleiotropy. Our results provide insights that may help breeders design effective breeding strategies to develop proVA‐enriched cultivars for resource‐limited farming systems.

Maize stalk rot reduces grain yield and quality. Information about the genetics of resistance to maize stalk rot could help breeders design effective breeding strategies for the trait. Genomic prediction may be a more effective breeding strategy for stalk-rot resistance than marker-assisted selection. We performed genome-wide association study (GWAS) and genomic prediction of resistance in testcross hybrids of 677 inbred lines from the Tuxpeño and non-Tuxpeño heterotic pools grown in three environments and genotyped with 200,681 single-nucleotide polymorphisms (SNPs). Eighteen SNPs associated with stalk rot shared genomic regions with gene families previously associated with plant biotic and abiotic responses. More favorable SNP haplotypes traced to tropical than to temperate progenitors of the inbred lines. Incorporating genotype-by-environment (G × E) interaction increased genomic prediction accuracy.

We studied the interrelation among time to anthesis, physiological maturity, and grain yield in tropical maize (Zea maysL.) facing terminal drought and nitrogen deficiency. Sixty-one hybrids were grown under optimum, nitrogen-deficient (LN), and drought-stressed (DS) conditions. Grain yield was negatively correlated with time to anthesis under DS (r= -0.4*) and positively correlated with the grain-filling period (optimum: r= 0.54**; LN: r= 0.61***; DS: r= 0.69***) across treatments. These results suggest that coselecting for grain yield, early anthesis, and a long grain-filling period within a maturity group could help increase grain yield. A second experiment involving eight hybrids contrasting for grain yield and time to anthesis was conducted under similar stresses to identify secondary traits associated with grain yield and time to physiological maturity: early vigor (measured as normalized differential vegetation index) correlated with grain yield under optimum (r= 0.77*) and LN conditions (r= 0.72*). Time to physiological maturity was associated with senescence across treatments (optimum: r= -0.97***; LN: r= -0.82*; DS: r= -0.81*), making senescence suitable to approximate physiological maturity. Repeatability for secondary traits (r2= 0.79 on average) was higher than for grain yield (r2= 0.68) across treatments. In light of the association of these traits with grain yield and their high repeatability, grain yield could be increased within a maturity group by simultaneously selecting for high normalized differential vegetation index during vegetative growth and early anthesis and low senescence during grain filling, in addition to grain yield.

Maize (Zea maysL.) cultivars with increased concentrations of provitamin A (proVA) carotenoids can improve the health of millions of consumers who are vitamin A deficient and rely on maize as a staple food. Three open‐pollinated maize cultivars (OPC) were subjected to three cycles of S1 recurrent selection for increased proVA concentration. Agronomic performance of Cycles 0, 1, 2, and 3 for each OPC was evaluated using three replications at 10 locations, and changes in proVA concentration were assessed for hand‐pollinated grain produced at two sites. Selection resulted in significant (P <0.01 for 11, and P <0.05 for 1 occurrence) linear increases of 25 to 67% per cycle for total proVA, 28 to 60% for β‐carotene, 18 to 70% for β‐cryptoxanthin, and 11 to 46% for zeaxanthin. These findings are especially significant because, in contrast to recent trends, they demonstrate the feasibility of developing proVA‐enhanced maize while meeting nutritionists’ recommendations not to sacrifice β‐cryptoxanthin and zeaxanthin to increase β‐carotene concentration in grain. Grain yield increased in one but decreased (P <0.01) in two of the OPCs, and we hypothesize that linkage drag associated with proVA‐enhancing genes from exotic donor lines may be responsible for the negative trends. We conclude that breeding proVA‐enriched maize without sacrificing β‐cryptoxanthin and zeaxanthin concentrations is feasible, but that (i) it remains unknown whether such approaches can achieve the high concentrations of proVA reported elsewhere by using marker‐assisted selection for genes that favor β‐carotene accumulation, and (ii) that grain yield and agronomic performance should be simultaneously selected if useful cultivars are desired.

Maize (Zea maysL.) breeding programs are increasingly using molecular tools to enhance the efficiency and speed of developing productive cultivars. As a preliminary step toward using each other's germplasm, 20 elite inbred lines, 10 each from CIMMYT and IITA, were crossed according to a modified North Carolina Design II and were genotyped using 62 simple sequence repeat (SSR) markers. The design used six sets of 25 hybrids, including all combinations among groups of five lines each from CIMMYT and IITA's respective heterotic groups A and B. The 150 single‐cross hybrids were evaluated in six environments in Zimbabwe and Nigeria during 2003 and 2004, with average grain yield ranging from 5.3 to 11.9 Mg ha−1Combining ability analysis indicated that CML444 and CML312 can contribute to the breeding program of IITA, and that TZMI702 should be useful to CIMMYT. The SSR markers identified 209 alleles with an average 3.4 alleles locus−1and revealed moderate genetic distances among the parent lines. Genetic distances were not significantly associated with hybrid performance, specific combining ability effects, or midparent heterosis for grain yield, but they effectively grouped the lines according to known pedigree relationships and approximately according to heterotic patterns used by CIMMYT and IITA. Although SSR‐determined genetic distances were not useful to predict hybrid grain yield, they could be helpful for preliminary classification and use of exotic or poorly characterized germplasm.

Maize (Zea maysL.) improvement involves formation, evaluation, selection, and recombination of genetically variable families or inbred lines, and because cultivars must combine many desirable traits, the process can be complicated and lengthy. This study compared 12 experimental open‐pollinated varieties (OPVs) developed from maize population Pool 9A by two approaches, full‐sib (FS) selection alone and FS selection combined with inbreeding with selection for Maize streakvirus(MSV) resistance. The experimental OPVs were evaluated in 13 environments in Africa, including three sites with artificial MSV inoculation. Full‐sib selection combined with inbreeding and selection for MSV resistance [with mild selection for gray leaf spot resistance (GLS, caused by Cercospora zeae maydisTehon & Daniels)], reduced ear height and reduced days to flowering] resulted in OPVs with similar grain yield but superior MSV resistance (P< 0.01), fewer days to anthesis (P< 0.01), and lower ear height (P< 0.05) than OPVs developed by FS selection alone. Each generation (from S0to S3) of inbreeding with selection resulted in OPVs that were 16, 8, 2, and 1% improved for MSV and GLS resistance, ear height, and days to anthesis, respectively. Our results demonstrate improvement of a maize population for MSV resistance and other traits by selection during inbreeding (from S0to S3), without negative impact on gains for grain yield achieved by evaluation and selection among the progenitor FS families.

Maize weevil, Sitophilus zeamaisMotschulsky, is an important pest of stored maize (Zea maysL.) in the tropics. We used 14 southern African maize inbred lines to assess (i) combining ability for resistance to maize weevil in F2and F2–Syn 1 grain, (ii) the importance of maternal effects for resistance of F2and F2–Syn 1 grain to maize weevil, and (iii) combining ability for grain yield of F1hybrids. Maize weevil resistance of F2grain was evaluated in 2000 for a 14‐parent diallel, and in 2002 for F2and F2–Syn 1 grain of a 10‐parent subset diallel. Fifty‐gram grain samples of each hybrid were infested with 32 weevils for a 10‐d oviposition period, after which the samples were incubated in a laboratory. Grain yield was evaluated for the 14‐parent diallel during the summer of 1999–2000 at four locations in Zimbabwe. General combining ability (GCA), specific combining ability (SCA), and reciprocal effects were highly significant (P< 0.01) for number of F1weevils emerged from F2grain samples for the combined 2000 and 2002 analysis for the 10‐parent subset diallel. For weevil resistance of F2–Syn 1 grain, GCA and SCA effects were significant (P< 0.01), while reciprocal effects were not important. The GCA was more important than SCA for grain yield, indicating that yield was controlled mainly by additive gene action among these lines. There was no significant relationship between grain yield and weevil resistance. Although inheritance of weevil resistance was complex, our results suggest using inbred lines with good GCA for weevil resistance as female parents for hybrids or as components of synthetic (open‐pollinated) cultivars for regions where farmers often store maize grain without chemical protection against weevils.

Maize weevil (Sitophilus zeamaisMotschulsky) is an important pest of maize (Zea maysL.) in the tropics, causing serious losses for many resource‐poor farmers who store grain on‐farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by S1and S2selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated S1and for replicated and unreplicated S2lines by infesting 50‐g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated S2selection was successful for both populations where it was applied, resulting in an average of 16% (P< 0.01), 49% (P< 0.05), and 20% (P< 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. S1unreplicated selection was successful for two of the six populations, while S2unreplicated selection was not successful. Reciprocal effects were significant (P< 0.01) for weevil resistance of F1varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations.